ArticlePDF Available

Abstract and Figures

Heteromorphs are ammonoids forming a conch with detached whorls (open coiling) or non-planispiral coiling. Such aberrant forms appeared convergently four times within this extinct group of cephalopods. Since Wiedmann's seminal paper in this journal, the palaeobiology of heteromorphs has advanced substantially. Combining direct evidence from their fossil record, indirect insights from phylogenetic bracketing, and physical as well as virtual models, we reach an improved understanding of heteromorph ammonoid palaeobiology. Their anatomy, buoyancy, locomotion, predators, diet, palaeoecology, and extinction are discussed. Based on phylogenetic bracketing with nautiloids and coleoids, hetero-morphs like other ammonoids had 10 arms, a well-developed brain, lens eyes, a buccal mass with a radula and a smaller upper as well as a larger lower jaw, and ammonia in their soft tissue. Heteromorphs likely lacked arm suckers, hooks, tentacles , a hood, and an ink sac. All Cretaceous heteromorphs share an aptychus-type lower jaw with a lamellar calcitic covering. Differences in radular tooth morphology and size in heteromorphs suggest a microphagous diet. Stomach contents of heteromorphs comprise planktic crustaceans, gastropods, and crinoids, suggesting a zooplanktic diet. Forms with a U-shaped body chamber (ancylocone) are regarded as suspension feeders, whereas orthoconic forms additionally might have consumed benthic prey. Heteromorphs could achieve near-neutral buoyancy regardless of conch shape or ontog-eny. Orthoconic heteromorphs likely had a vertical orientation, whereas ancylocone heteromorphs had a near-horizontal aperture pointing upwards. Heteromorphs with a U-shaped body chamber are more stable hydrodynamically than modern Nautilus and were unable substantially to modify their orientation by active locomotion, i.e. they had no or limited access to benthic prey at adulthood. Pathologies reported for heteromorphs were likely inflicted by crustaceans, fish, marine reptiles, and other cephalopods. Pathologies on Ptychoceras corroborates an external shell and rejects the endocochleate hypothesis. Devonian, Triassic, and Jurassic heteromorphs had a preference for deep-subtidal to offshore facies but are rare in shallow-subtidal, slope, and bathyal facies. Early Cretaceous heteromorphs preferred deep-subtidal to bathyal facies. Late Cretaceous heteromorphs are common in shallow-subtidal to offshore facies. Oxygen isotope data suggest rapid growth and a demersal habitat for adult Discoscaphites and Baculites. A benthic embryonic stage, planktic hatchlings, and a habitat change after one whorl is proposed for Hoploscaphites. Carbon isotope data indicate that some Baculites lived throughout their lives at cold seeps. Adaptation to a planktic life habit potentially drove selection towards smaller hatchlings, implying high fecundity and an ecological role of the hatchlings as micro-and mesoplankton. The Chicxulub impact at the Cretaceous/Paleogene (K/Pg) boundary 66 million years ago is the likely trigger for the extinction of ammonoids. Ammonoids likely persisted after this event for 40-500 thousand years and are exclusively represented by heteromorphs. The ammonoid extinction is linked to their small hatchling sizes, planktotrophic diets, and higher metabolic rates than in nautilids, which survived the K/Pg mass extinction event.
Content may be subject to copyright.
Recent advances in heteromorph ammonoid
palaeobiology
René Hoffmann
1
*, Joshua S. Slattery
2
, Isabelle Kruta
3
, Benjamin J. Linzmeier
4
,
Robert E. Lemanis
5
, Aleksandr Mironenko
6
, Stijn Goolaerts
7
, Kenneth De Baets
8
,
David J. Peterman
9
and Christian Klug
10
1
Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, Bochum, 44801, Germany
2
School of Geosciences, University of South Florida, 4202 East Fowler Ave., NES 107, Tampa, FL, 33620, U.S.A.
3
CR2P Centre de Recherche en Paléntologie Paris, UMR 7207, Sorbonne Université-MNHN-CNRS, 4 place Jussieu, case 104, Paris, 75005,
France
4
Department of Geoscience, University of Wisconsin - Madison, Madison, WI, 53706, U.S.A.
5
B CUBE Center for Molecular Bioengineering, Technische Universität Dresden, Dresden, 01307, Germany
6
Geological Institute of RAS, Pyzhevski Lane 7, Moscow, 119017, Russia
7
OD Earth & History of Life, and Scientic Service Heritage, Royal Belgian Institute of Natural Sciences, Vautierstraat 29, Brussels, B-1000,
Belgium
8
GeoZentrum Nordbayern, Friedrich-Alexander University Erlangen-Nürnberg, Erlangen, 91054, Germany
9
Department of Earth and Environmental Sciences, Wright State University, 3640 Colonel Glenn Highway, Dayton, OH, 45435, U.S.A.
10
Paläontologisches Institut und Museum, Universität Zürich, Karl-Schmid-Strasse 4, Zürich, 8006, Switzerland
ABSTRACT
Heteromorphs are ammonoids forming a conch with detached whorls (open coiling) or non-planispiral coiling. Such
aberrant forms appeared convergently four times within this extinct group of cephalopods. Since Wiedmanns seminal
paper in this journal, the palaeobiology of heteromorphs has advanced substantially. Combining direct evidence from
their fossil record, indirect insights from phylogenetic bracketing, and physical as well as virtual models, we reach an
improved understanding of heteromorph ammonoid palaeobiology. Their anatomy, buoyancy, locomotion, predators,
diet, palaeoecology, and extinction are discussed. Based on phylogenetic bracketing with nautiloids and coleoids, hetero-
morphs like other ammonoids had 10 arms, a well-developed brain, lens eyes, a buccal mass with a radula and a smaller
upper as well as a larger lower jaw, and ammonia in their soft tissue. Heteromorphs likely lacked arm suckers, hooks, ten-
tacles, a hood, and an ink sac. All Cretaceous heteromorphs share an aptychus-type lower jaw with a lamellar calcitic
covering. Differences in radular tooth morphology and size in heteromorphs suggest a microphagous diet. Stomach con-
tents of heteromorphs comprise planktic crustaceans, gastropods, and crinoids, suggesting a zooplanktic diet. Forms with
a U-shaped body chamber (ancylocone) are regarded as suspension feeders, whereas orthoconic forms additionally might
have consumed benthic prey. Heteromorphs could achieve near-neutral buoyancy regardless of conch shape or ontog-
eny. Orthoconic heteromorphs likely had a vertical orientation, whereas ancylocone heteromorphs had a near-
horizontal aperture pointing upwards. Heteromorphs with a U-shaped body chamber are more stable hydrodynamically
than modern Nautilus and were unable substantially to modify their orientation by active locomotion, i.e. they had no or
limited access to benthic prey at adulthood. Pathologies reported for heteromorphs were likely inicted by crustaceans,
sh, marine reptiles, and other cephalopods. Pathologies on Ptychoceras corroborates an external shell and rejects the
endocochleate hypothesis. Devonian, Triassic, and Jurassic heteromorphs had a preference for deep-subtidal to offshore
facies but are rare in shallow-subtidal, slope, and bathyal facies. Early Cretaceous heteromorphs preferred deep-subtidal
to bathyal facies. Late Cretaceous heteromorphs are common in shallow-subtidal to offshore facies. Oxygen isotope data
suggest rapid growth and a demersal habitat for adult Discoscaphites and Baculites. A benthic embryonic stage, planktic
hatchlings, and a habitat change after one whorl is proposed for Hoploscaphites. Carbon isotope data indicate that some
Baculites lived throughout their lives at cold seeps. Adaptation to a planktic life habit potentially drove selection towards
*Address for correspondence (Tel: +492343223203; E-mail: rene.hoffmann@rub.de)
Biological Reviews (2021) 000000 © 2020 The Authors. Biological Reviews published by John Wiley & Sons Ltd on behalf of Cambridge Philosophical Society.
This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium,
provided the original work is properly cited.
Biol. Rev. (2021), pp. 000000. 1
doi: 10.1111/brv.12669
smaller hatchlings, implying high fecundity and an ecological role of the hatchlings as micro- and mesoplankton. The
Chicxulub impact at the Cretaceous/Paleogene (K/Pg) boundary 66 million years ago is the likely trigger for the extinc-
tion of ammonoids. Ammonoids likely persisted after this event for 40500 thousand years and are exclusively repre-
sented by heteromorphs. The ammonoid extinction is linked to their small hatchling sizes, planktotrophic diets, and
higher metabolic rates than in nautilids, which survived the K/Pg mass extinction event.
Key words: cephalopods, heteromorph ammonoids, palaeobiology, anatomy, geochemistry, facies distribution
CONTENTS
I. Introduction .........................................................................2
II. Soft body reconstruction ................................................................3
(1) Buccal mass ...................................................................... 3
(a)Jaws ......................................................................... 3
(b)Radula ........................................................................ 5
(c)Enigmatic structures found in the body chamber ............................................. 5
(2) Muscle system .................................................................... 5
(3) Inksac .......................................................................... 6
(4) Digestive tract .................................................................... 6
(5) Shell colour patterns ............................................................... 7
III. Buoyancy and locomotion ...............................................................7
(1) Physical models ................................................................... 9
(2) Virtual models .................................................................... 9
IV. Ecological interactions .................................................................11
(1) Predators ....................................................................... 11
(2) Prey ........................................................................... 13
(3) Epizoa, parasites, and commensalism ................................................. 14
V. Habitat reconstruction ................................................................15
(1) Taphonomic controls on facies distributions ............................................ 15
(2) Life habit controls on facies distributions ............................................... 16
(3) Ecological requirements based on shell geochemistry ..................................... 18
(4) Reproduction ................................................................... 20
(5) Intraspecic variability ............................................................. 20
VI. Heteromorphs through time ............................................................21
(1) Devonian heteromorphs ........................................................... 21
(2) Triassic heteromorphs ............................................................. 23
(3) Jurassic heteromorphs ............................................................. 24
(4) Cretaceous heteromorphs and ammonoid extinction ..................................... 25
VII. Conclusions .........................................................................26
VIII. Acknowledgements ...................................................................27
IX. References ..........................................................................27
X. Supporting information ................................................................35
I. INTRODUCTION
Heteromorphs are ammonoids that are dened by detached
whorls (open coiling) or non-planispiral coiling, and thus
deviate from the monomorph bauplan. The oldest mention
of the term heteromorphin relation to ammonoid shell
shape that we could identify is in Whitehouse (1926). Hetero-
morphy may occur at any growth stage and can be expressed
throughout ontogeny or be restricted to a single or a few
growth stages (cf. Wiedmann, 1969). Ammonoids with mod-
ied apertures or angular coiling at the very end of their body
chambers due to morphogenetic countdown (sensu Seila-
cher & Gunji, 1993) are not classied as heteromorphs.
Heteromorphy occurred in many evolutionary lineages,
and thus by denition, the term heteromorph ammonoid
forms a polyphyletic grouping. Heteromorphy appeared
multiple times during the history of ammonoids, beginning
with the rst ammonoids in the early Devonian to their very
end at the Cretaceous/Paleogene boundary (see Section VI).
Heteromorphy was most common in the Cretaceous, which
is the reason why many textbooks identify heteromorphs
(only) as Cretaceous Ancyloceratina, and why most of the
available data on their palaeobiology comes from Cretaceous
examples. Only Cretaceous heteromorphs show an impor-
tant modication of the suture line, namely a reduction from
ve to four lobes in the primary suture line, which was used
Biological Reviews (2021) 000000 © 2020 The Authors. Biological Reviews published by John Wiley & Sons Ltd on behalf of Cambridge Philosophical Society.
2René Hoffmann et al.
by earlier workers to dene them as a monophyletic group
(Korn et al., 2003), an outdated idea that is strictly rejected
herein. It was also the diversity of aberrantforms during
the Cretaceous that was used in the outdated hypothesis of
degeneration (typolysis) to explain the cause of their extinc-
tion (Schindewolf, 1936; Wiedmann, 1969; Korn, 2003).
Some of their complex conch shapes (conch refers to the
entire skeleton, shell refers to the material) without apparent
streamlining have been used to argue for an immobile ben-
thic or pseudoplanktic life habit for ammonoids (Arkhipkin,
2014), an interpretation that was fuelled by the rapid replace-
ment of loosely coiled (heteromorph) forms by tightly coiled
(monomorph) ammonoids during their early evolution in
the Devonian (e.g. Klug & Korn, 2004; De Baets et al.,
2012; Klug et al., 2015a). It is now widely accepted that het-
eromorphs independently and convergently evolved from
different phylogenetic lineages during the Triassic, Jurassic,
and multiple times during the Cretaceous (Cecca, 1997).
Evolutionary relationships within the heteromorphs, particu-
larly during the Cretaceous, require renement (Wright,
Callomon & Howarth, 1996; Kaplan, 2002). To what degree
heteromorphs had comparable life habits is still debated, but
is crucial for understanding their radiations, evolutionary
successes, and extinctions during each period.
Here, we review the currently known constraints of hetero-
morph palaeobiology and ecology using a wide range of
approaches to disentangle the factors driving the organisa-
tion and evolution of heteromorph conch shapes within
Ammonoidea.
II. SOFT BODY RECONSTRUCTION
Remains of soft parts are rarely preserved in ammonoids
(e.g. Closs, 1967a,b). However, two reported cases come from
heteromorphs, namely trochospiral Allocrioceras (Wippich &
Lehmann, 2004; Keupp, Saad & Schweigert, 2016b) and
orthoconic Sciponoceras (Klug, Riegraf & Lehmann, 2012;
Klug & Lehmann, 2015). In these genera, remains of the
digestive tract including the oesophagus (well preserved due
to its original chitinous cover), crop, and stomach are pre-
served as thin organic sheets (Fig. 1).
Klug et al. (2012) and Klug & Lehmann (2015) proposed
that structures surrounding the oesophagus of Sciponoceras
might represent phosphatised parts of the cephalic cartilage
with the eye capsules. Both phylogenetic bracketing with
nautiloids and coleoids (Witmer, 1995; Klug et al., 2015a,
2019; Shigeno et al., 2018) and the limited fossil evidence sug-
gest the presence of a well-developed brain in these hetero-
morphs. We speculate that the evolution of the nervous
system in cephalopods was linked with the need to control
their position in space and to handle information from sen-
sory systems like eyes and statocysts (sensory receptors for
balance and orientation).
Phylogenetically, ammonoids are positioned between nau-
tilids with their pinhole camera eye and coleoids with a lens
eye. The shared ancestry of ammonoids and coleoids
(derived from bactritids; Kröger, Vinther & Fuchs, 2011),
and their potential for a more active lifestyle similar to
coleoids, suggests that ammonoids had lens eyes. Putative
eye capsules are relatively large in Sciponoceras, suggesting
the presence of large eyes in baculitids (Klug et al., 2012;
Klug & Lehmann, 2015), which is supported by deep lateral
sinuses on their apertures providing structural space for large
eyes useful for an active lifestyle.
The number, shape, and proportions of the arms ammo-
noids possessed are subject to ongoing debate. Structures
anterior to the jaws in Sciponoceras (Klug et al., 2012; Klug &
Lehmann, 2015) might represent arm bases. Various authors
have suggested that the arms of heteromorphs were short
(Landman, Cobban & Larson, 2012a). Klug et al. (2015a)
applied phylogenetic bracketing (Witmer, 1995), and argued
that it is most parsimonious to assume that all ammonoids
including heteromorphs had 10 arms (Klug & Lehmann,
2015). The same line of reasoning suggests that ammonoid
arms lacked suckers and hooks (but see Section II.1c), and
that they had no tentacles (extendable extremities longer
than the arms), which is weakly supported by the absence of
fossil evidence of these structures. Clements et al. (2017)
found that members of the clade comprising modern
10-armed cephalopods (sepiids and squids) decompose faster
as a function of their higher ammonia content, therefore con-
necting coleoid cephalopod taphonomy with physiology.
Thus, the scarcity of soft-part preservation in ammonoids
could be linked to both the ammonia content of their soft
parts and that potentially preserved soft parts are often cov-
ered by shell remains.
Closs (1967a,b) interpreted ammonoids as possessing a
hood, but re-examination of his Carboniferous material sug-
gests a misidentication of a shell fragment situated near the
aperture as a hood (Lehmann, Klug & Wild, 2015). Using
phylogenetic bracketing and information from the embry-
onic development of modern nautilids Shigeno et al. (2008)
and Shigeno, Sasaki & Boletzky (2010) suggest that the hood
is derived from Anlagen (developmental foundation) of the
arm crown which is absent in ammonoids.
(1) Buccal mass
(a) Jaws
The buccal apparatus is known for several Ancyloceratina,
and in the Ammonitina for Spiroceras (Tanabe, Kruta &
Landman, 2015a) (see online Supporting Information,
Table S1). It follows the general pattern of ammonoid jaws:
a smaller upper jaw nested within a larger lower jaw (Fig.
1E). The upper jaw is conservative in ammonoids and has a
blunt rostrum with inner lamellae for the support of the mus-
cular tissue split into two rounded wings (Tanabe et al.,
2015a). Lower jaws are morphologically variable with differ-
ent shapes, ornamentation, and microstructure of the calcitic
covering. Most heteromorphs have an aptychus-type jaw
with a central commissure (symphysis) dividing the lower
Biological Reviews (2021) 000000 © 2020 The Authors. Biological Reviews published by John Wiley & Sons Ltd on behalf of Cambridge Philosophical Society.
Heteromorph ammonoid palaeobiology 3
Fig. 1. Soft body reconstruction of an orthocone heteromorph. (A) Empty conch with body chamber and parts of the phragmocone.
(B) Overlay image of conch and soft body with eyes and arms. (C) Reconstructed soft body with the digestive tract, buccal mass, and
ovary. (D) Lower jaw of Aegocrioceras (Hauterivian, NW Germany). Scale bar =1 cm. (E) Detailed parts of the buccal mass showing
radula (E2) and upper and lower jaws (E1, E3). (F) Six morphological heteromorph archaetypes. Abbreviations used in AC: arms,
10 arms of equal length without suckers or hooks; bc, body chamber; bm, buccal mass (see E for details); ca, caecum; cr, crop
(covered by digestive gland); dg, digestive gland; drm, dorsal retractor muscle; fu, funnel; gi, gill; in, intestine; mc, mantle cavity;
oe, oesophagus; os, ocular sinus; ov, ovary; ph, phragmocone; si, siphuncle; st, stomach.
Biological Reviews (2021) 000000 © 2020 The Authors. Biological Reviews published by John Wiley & Sons Ltd on behalf of Cambridge Philosophical Society.
4René Hoffmann et al.
jaw into two valves lacking a sharp rostral tip. In Criocerati-
dae, the symphysis is poorly expressed and due to this fact,
their jaws are interpreted by some authors as an
anaptychus-type jaw (Engeser & Keupp, 2002). In contrast
to an anaptychus interpretation, Tanabe et al. (2015a) consid-
ered the crioceratid lower jaws as the inner chitinous lamella
of the aptychus. Later, Rogov & Mironenko (2016) reported
Hauterivian praestriaptychi associated with early criocera-
tids (Criosarasinella and Crioceratites) suggesting that one of
these taxa had an aptychus-type jaw (praestriaptychus, see
Table S1).
In the aptychus-type lower jaw, the surface is covered by a
calcitic layer either with a striated surface (praestriaptychus
and striaptychus) or with rugae (rugaptychus). The outline
of the lower jaw spread open matches the shape of the whorl
cross section (Baculites,Scaphites), which led to the suggestion
that this structure had an opercular function. By contrast,
Larson & Landman (2017) and Landman & Waage (1993b)
argued that the jaws of Baculites, even when fully splayed
out, did not touch the inner surface of the conch tube.
Accordingly, they rejected an operculum function of the
lower jaw in Baculites. Parent, Westermann & Chamberlain
jr. (2014) suggested that lower jaw types like praestriaptychus
and striaptychus could fulll multiple functions: lower man-
dible, operculum, ltering device, ushing benthic prey,
pumping for jet propulsion and stabilizing against pitching
[for praestriaptychus see Parent et al. (2014) and Tanabe
et al. (2015a)].
The microstructure of the calcitic covering of aptychi varies
in ammonoids between tubular and lamellar [Farinacci et al.
(1976); see Kruta et al. (2009) for heteromorphs]. The calcitic
covering in all Cretaceous heteromorph jaws is lamellar. In
Baculites, the calcitic covering can reach 1 mm in thickness
and two main layers have been described (R1 and R2) consist-
ing of overlapping thin (3 μm thickness) calcitic increments
(Kruta et al., 2009). The aptychus is 0.5 mm thick in Hoplosca-
phites and 0.03 mm in Polyptychoceras without ornamentation.
(b) Radula
The use of tomographic techniques on three in situ Baculites
radulae allowed description of their jaw apparatus (Kruta
et al., 2011). The radular ribbon lies between the upper and
lower jaws. Each tooth row of the radula is composed of nine
elements (one central tooth, two pairs of lateral teeth, one
pair of marginal teeth, and one pair of marginal plates)
(Fig. 1E). The central rachidian tooth and the two pairs of lat-
eral teeth are delicately multicuspidate, while the marginal
tooth is long and sabre-like. The same radula composition
and morphology was revealed for a baculitid (?Sciponoceras)
from the Late Cretaceous of Germany (Klug et al., 2012).
Multicuspidate radular teeth found in situ in Rhaeboceras have
been described by Kruta et al. (2013) and Kruta, Landman &
Tanabe (2015). The only other report (see Table S1) of a rad-
ula from heteromorphs is from a Spiroceras (Tanabe et al.,
2015a) but the number of radular teeth per row and their
morphology remains unknown.
(c) Enigmatic structures found in the body chamber
Cuspidate structures associated with jaws have been found in
the body chamber of several species of well-preserved hetero-
morphs and re-coiled scaphitids. Landman & Waage (1993b)
described bicuspidate structures in two Maastrichtian species
from South Dakota: Hoploscaphites spedeni and H. nicolletii.
Kennedy et al. (2002b) documented similar cuspidate struc-
tures in Rhaeboceras halli, a re-coiled scaphitid from the Cam-
panian of Montana, and interpreted them as radular teeth.
Kruta et al. (2013) rejected this interpretation after nding
actual radular teeth in three specimens of R. halli. These in situ
teeth match the size and morphology of radular teeth known
from other aptychophoran ammonoids (Kruta et al., 2015;
Keupp et al., 2016a). Landman et al. (2013) documented addi-
tional structures associated with H. gilberti from the Campa-
nian of South Dakota. Ongoing studies are characterising
the diverse morphology of these enigmatic structures in
R. halli (Kruta et al., 2019) and their function in scaphitids,
e.g. hooks on the hectocotylised arm.
(2) Muscle system
Although there is no unequivocal report of fossilised muscles
from ammonoids we can make inferences about their muscle
system (Doguzhaeva & Mapes, 2015). Based on dimensions
of attachment scars, the cephalic retractor muscles inserted
at the largest postero-lateral attachment sites. However, Mir-
onenko (2015b) argued that these muscles were attached dor-
sally with the nuchal retractors in Jurassic ammonoids like
Kachpurites. In his opinion, the hyponome muscles inserted
laterally in the annular elevation or the lateral sinus. Miro-
nenko (2015b) proposed that the collar was attached laterally.
Mironenkos (2015b) interpretation of their function and
homology with muscles in nautilids and coleoids raises sev-
eral questions, which cannot yet be fully answered given the
paucity of available anatomical information.
(1) Does the arrangement of the main muscles in ammo-
noids correspond to that of modern Nautilus? Jacobs &
Landman (1993, p. 101) stated that Nautilus is in some
respects a poor model for the function and behaviour
of ammonoids, but is the only living ectocochleate
cephalopod for comparison. Nevertheless, in the
absence of coleoids with chambered conchs and a long
body chamber, Nautilus represents one of the most
important living relatives used to reconstruct ammo-
noid palaeobiology. Jacobs & Landman (1993) support
Mironenkos (2015b) interpretation of a dorsal inser-
tion of retractor muscles. The muscle attachment pat-
terns of Audouliceras renauxianum from the Russian
Cretaceous gured in Doguzhaeva & Mapes (2015,
Figs. 14.114.4) may support this hypothesis, because
there are large dorsoventral (umbilical) tongue-like
marks. In this ancylocone, the adult body chamber
forms a characteristic hook. To enhance efciency of
the retractor muscles, the distance to the head must
be minimised. Due to the curve in the adult body
Biological Reviews (2021) 000000 © 2020 The Authors. Biological Reviews published by John Wiley & Sons Ltd on behalf of Cambridge Philosophical Society.
Heteromorph ammonoid palaeobiology 5
chamber, a posterior insertion of straight cephalic
retractors is impossible. By contrast, an anterior inser-
tion of retractor muscles would have reduced the
length of these muscles, thereby increasing their ef-
ciency. Nevertheless, this homologisation requires fur-
ther evidence. Ultimately, we will have to await
specimens preserving remains of the musculature.
(2) Are coleoid muscles homologous with ammonoid mus-
cles? Homology of the mantle in coleoids (cf. Jacobs &
Landman, 1993; Fuchs et al., 2016) with that of ammo-
noids is likely. However, it is also probable that the
ammonoid mantle could not serve the same purposes
as in coleoids because of the external conch. The man-
tle secretes the mineralised hardparts (ectocochleate or
endocochleate) in most cephalopods, except for Argo-
nauta. Klug et al. (2008) proposed that the mantle mus-
culature in ammonoids enabled the animal to move its
soft parts within the body chamber. Jacobs & Landman
(1993) suggested that the mantle played an active role
in swimming. This assumption would support a mod-
erately thick mantle with the potential to be fossilised,
which has yet to be reported. The homologisation of
other muscles would require soft tissue preservation.
(3) Did ammonoids have a collar like coleoids? Miro-
nenko (2015b) found a lateral soft tissue attachment
site close to the aperture and suggested that the collar
and funnel-locking apparatus attached at this site.
The soft tissues were presumably attached to the body
chamber wall anteriorly and posteriorly. The presence
of a nuchal and funnel cartilage in Nautilus and Sepia
implies that these were also present in ammonoids. If
we accept the intermediate position of ammonoids
between nautilids and coleoids with respect to locomo-
tory abilities, we can assume that the funnel was some-
what stiff and could be locked rmly near the aperture
to maximise the thrust of the hyponomic water jet.
Although fossil remains of the nuchal cartilage have
been reported from coleoids (Klug et al., 2016), they
have not been found in ammonoids.
We conclude that ammonoids likely had paired cephalic
and hyponome retractor muscles, a ventral muscle, and man-
tle musculature. Since ammonoid muscles are not preserved
well enough (only a few doubtful remains), homologisation
of ammonoid muscles and their attachment sites with those
of coleoids and nautilids remains uncertain. Conch morphol-
ogy of heteromorphs and the likely arrangements of muscles
in their body chambers suggest that their locomotory abilities
were limited compared to most modern coleoids because of
the indirect action of the main muscles. Efciency of the mus-
cle system and the locomotory apparatus probably improved
towards the end of their lives, which is achieved through the
commonly widened body chamber and the more horizontal
orientation of the adult aperture. This ontogenetic change
in orientation allowed the approximate horizontal alignment
of the jet with the centre of mass (e.g. Klug, 2001; Klug &
Korn, 2004).
(3) Ink sac
There are four categories of colour or light display in ani-
mals: (i) pigments, (ii) bioluminescence, (iii) structural colours,
and (iv) liquid crystals. While the ink of modern coleoids is
made of the pigment melanin, coleoids also evolved biolumi-
nescence such as in the photophore of Spirula spirula. Some
cephalopod light organs are also situated within the ink sac,
for example in Sepiola atlantica (Herring et al., 1981). Struc-
tural colour is present as mirrors in photophores composed
of collagen bres forming a multilayer reector e.g. in Seleno-
teuthis,Abralia,orEnoploteuthis (Parker, 2000). Reports on fos-
silised colour mostly refer to the presence of pigments in
shells or the ink sac (e.g. Roy et al., 2020).
Melanin macromolecules are classied into eumelanins
(black) that contain nitrogen and pheomelanins (broad col-
our range: yellow-red-brownish) containing sulfur (Schäfer,
2013). Melanins are stored in melanosomes, specic cell
organelles of pigment cells that share the size and shape of
bacteria (Lindgren et al., 2012). Identication of melanin
based on structural information can be misleading. In addi-
tion, the presence of melanin in soft tissues such as visual sys-
tems (Schäfer, 2013), liver (Scalia et al., 1990), or others
(Wakamatsu & Ito, 2002) complicates the recognition of ink
in fossil cephalopods.
Reports of ink sacs in ammonoids by Lehmann (1967) for
Eleganticeras, Mathur (1996) for Eopsiloceras, and Wetzel (1969)
for Bochianites have been discarded by subsequent researchers
(Wippich & Lehmann, 2004; Klug & Lehmann, 2015). Doguz-
haeva et al. (2004) interpreted bituminous matter in attened
ammonoid body chambers of Austrachyceras as ink sacs. This
interpretation is rejected here because modern ectocochleate
cephalopods lack an ink sac and melanin occurs in other
ammonoid structures such as the black layer (Klug et al.,
2004). Unequivocal evidence for fossil ink and ink sacs are com-
moninfossilcoleoidsfromtheCarnianAustrianRheingraben
Shale, the German Posidonia Shale (Toarcian), German plat-
tenkalks (KimmeridgianTithonian), and Lebanese platten-
kalks (Cenomanian) as summarised by Schäfer (2013), but are
absent in co-occurring ammonoids. Raman spectroscopy
revealed the presence of the typical melanin spectra, revealing
the effect of the carbonate matrix or the presence of iron on
these spectra (Schäfer, 2013). This method is recommended
to test for the presence of ink and related structures in hetero-
morphs (see Section II.5).
(4) Digestive tract
Fossil remains of the digestive tract (except the buccal mass;
Section II.1; Fig. 1B, C) have been documented from the het-
eromorphs Allocrioceras (Wippich & Lehmann, 2004) and Sci-
ponoceras (Klug et al., 2012; Klug & Lehmann, 2015). In both
cases, attened specimens with dissolved conchs preserve
organic remains of crop and stomach. In Allocrioceras
(Wippich & Lehmann, 2004) from the Cenomanian of Leba-
non, the remains of the digestive tract lie far behind the buc-
cal mass, suggesting that it is the stomach and not the crop. In
Biological Reviews (2021) 000000 © 2020 The Authors. Biological Reviews published by John Wiley & Sons Ltd on behalf of Cambridge Philosophical Society.
6René Hoffmann et al.
the case of Sciponoceras from the CenomanianTuronian
boundary in Germany, carbonised impressions show details
of the digestive tract (Klug et al., 2012; Klug & Lehmann,
2015). It begins with the oesophagus, which was identied
based on its elongate shape and its spatial association with
the buccal mass. Posteriorly, the digestive tract displays a
swelling, which probably represents the elongate crop, which
is known to be large in cephalopods. The elongate crop in Sci-
ponoceras is likely linked with the long body chamber in bacu-
litids. A third, further posteriorly situated elongate structure
found in Sciponoceras was interpreted as the stomach by Klug
et al. (2012).
(5) Shell colour patterns
Shell colour patterns in molluscs can be the result of pigments
or of structural colour (see Section II.3). Identication of true
colour patterns including bands and spots is complicated by
the existence of false colour patterns, which often run parallel
to growth lines and shell thickening. Klug et al. (2012)
reported on false colour patterns, which are related to growth
halts (megastriae) accompanied by faint ribs and the secre-
tion of melanin along transverse stripes (parallel to former
apertures). Muscle attachment structures can also result in
dark patches mimicking colour patterns regarded herein as
pseudo colour patterns.
The oldest true colour patterns in ammonoids were
described from a monomorph from the Early Triassic Crit-
tenden Springs Lagerstätte (Mapes & Sneck, 1987), although
there are reports of colour patterns for Palaeozoic nautiloids
(Manda & Turek, 2009a,b, 2015) and one Devonian ammo-
noid (Ebbighausen et al., 2007). In mollusc shells, polyenes
like carotenoids represent most shell pigments as outlined
by Hedegaard, Bardeau & Chateigner (2006) and Barnard &
de Waal (2006) using in situ Raman spectroscopy. Mapes &
Larson (2015) reported true colour patterns from Hoplosca-
phites nicolletii and H. reesidei. In both species, longitudinal iri-
descent bands are preserved in nacre. This colour pattern is
a structural colour with multilayer reectors, where the
nanometer thick nacre tablets fulll the Bragg condition for
diffraction. Bragg diffraction by a crystal lattice occurs when
radiation (e.g. X-rays or light) with a wavelength comparable
to atomic spacing is scattered from the lattice plane undergo-
ing constructive interference (Bragg & Bragg, 1913). These
structural colours are comparable to the colours of buttery
wings, gold bugs, polychaetes, and sh (Parker, 2000; Par-
ker & Martini, 2006). Based on transmission electron micros-
copy analyses, Snow et al. (2004) described a new type of
structural colour of nacre, the edge-band structure, which
produces interference colours. These are characteristic for
different widths of the edge-band structure ranging from sil-
ver tones (74 nm) to creams (80 nm) to yellow and gold (90
nm) in bivalve pearls. The same phenomenon could be
responsible for structural colours in ammonoids (Keupp,
2005; Mapes & Larson, 2015). Keupp (2005) suggested that
spiral bands resulted from original pigment-based colour
patterns in the outer prismatic layer. It is noteworthy that
the polyene pigments in modern Nautilus are restricted to
the outer prismatic layer (Fig. 2). These colour patterns can
be traced in the uppermost part of the underlying nacreous
layer due to minute differences in ultrastructure. This is
important because most ammonoids lack a preserved outer
prismatic layer. For example, spot patterns in Jurassic amalt-
heid ammonoids (Schindewolf, 1928, 1931) are visible after
the dark (pyrite) spots are lost. Mironenko (2015a) reported
on remnants of radial pigment-based colour patterns coin-
ciding with temporary apertures, representing an example
of false colour patterns.
Externally shelled (ectocochleate) cephalopods with longi-
tudinal bands fall into the plankticvertical migrant life habit.
By contrast, spiral structural colour patterns, like in Quensted-
toceras, have been suggested to indicate a potential demersal
life habit (Keupp, 2005). Relationships between conch form,
biofacies, and colour patterns have been discussed for broad
longitudinal colour patterns on orthocone Palaeozoic nauti-
loids (Manda & Turek, 2009b). Turek & Manda (2011)
described zig-zag or wave-like transverse colour patterns of
the cyrtoconic barrandeocerids Peismoceras and Phragmoceras
as well as the oncocerids Octameroceras and Pentameroceras.
Manda & Turek (2009a)gured a similar zig-zag or wave-
like transverse colour pattern for the cyrtoconic Silurian
oncocerid Euryrizocerina, which was likely demersal. A marked
polymorphism in colour patterns has been described for
modern Nautilus (Ward et al., 1977), Quenstedtoceras (Keupp,
2005), and the Silurian nautiloid Phragmoceras (Turek &
Manda, 2011) suggesting a camouage function. In this
regard, the suggestion by Balinski (2010) that marine inverte-
brates that exhibit colour patterns usually live no deeper than
200 m (photic zone) is convincing.
III. BUOYANCY AND LOCOMOTION
The bizarre morphology of many heteromorphs invites specu-
lation on the function of these convoluted conchs, which are
often considered to be poor swimmers that evolved from more
streamlined monomorphs (Jacobs, Landman & Chamberlain,
1994; Monks & Young, 1998; Mikhailova & Baraboschkin,
2009). Functional morphological work has focused on buoy-
ancy and hydrostatics to determine what life habits, typically
adult, heteromorph conch shapes might have permitted. It is
widely accepted that most ectocochleate cephalopods were
nearly neutrally buoyant during life regardless of conch shape
or ontogeny. This was controlled by adding uid to a positively
buoyant conch or counteracting slightly negative buoyancy
through swimming (Denton & Gilpin-Brown, 1966; Klinger,
1981; Lukeneder, 2012; Hoffmann et al., 2015; Lemanis
et al., 2015; Tajika et al., 2015; Peterman, Barton &
Yacobucci, 2019a; Peterman et al., 2020a,b). Thus, neutral
buoyancy is usually assumed for hydrostatic analyses, although
some authors have proposed a mobile benthic to nektobenthic
life habit for some heteromorphs (e.g. Wiedmann, 1973b;
Biological Reviews (2021) 000000 © 2020 The Authors. Biological Reviews published by John Wiley & Sons Ltd on behalf of Cambridge Philosophical Society.
Heteromorph ammonoid palaeobiology 7
Fig. 2. Raman image showing the distribution of aragonite (A) and polyenes (B) in the shell of modern Nautilus pompilius. Shell
colouration, i.e. distribution of polyenes within the shell, is limited to the outer prismatic layer.
Biological Reviews (2021) 000000 © 2020 The Authors. Biological Reviews published by John Wiley & Sons Ltd on behalf of Cambridge Philosophical Society.
8René Hoffmann et al.
Kakabadze & Sharikadze, 1993). These analyses can utilise
both physical and virtual models.
(1) Physical models
Physical models include the construction of a physical replica
of the conch that is immersed in uid. Reyment (1973) was
among the rst to use physical models to examine static
conch orientation, although these experiments were designed
to understand post-mortem drift rather than life orientation,
hence soft tissue was not considered. To determine life orien-
tation, differentiation between phragmocone and body
chamber densities must be replicated to reconstruct the posi-
tions of the centres of volume (buoyancy) and mass (gravity).
Ward (1976) used microcrystalline sculpting wax to create
models of the heteromorphs Glyptoxoceras subcompressum,Didy-
moceras elongatum,Ryugasella ryugasensis,Pseudoxybeloceras nanai-
moense,Baculites inornatus, and B. anceps pacicus. Density
differences were achieved by adding carborundum (silicon
carbide), metal powder, and lead or solder strips to the
wax. G. subcompressum likely had the most signicant changes
in aperture orientation in early ontogeny during a switch
from orthoconic to torticonic to gyroconic coiling with less
variation in aperture orientation in the gyroconic stage.
D. elongatum showed a stable aperture orientation during the
growth of the torticonic conch with the nal growth stage
showing a slight upwards turn of the aperture. The orthoco-
nic B. inornatus,B. anceps pacicus, and ancylocone R. ryugasensis
had a vertical conch orientation with the aperture directed
downwards. P. nanaimoense with its ancylocone conch showed
the greatest variation of orientation through ontogeny with
the aperture changing by up to 180, similar to the hamiti-
cone Dissimilites that was investigated virtually (Lukeneder,
2012). A vertical orientation of orthoconic heteromorphs is
a common result in both physical and virtual models
(e.g. Trueman, 1941; Westermann, 1977; Peterman et al.,
2019a,b). A ooded apex of the phragmocone, however,
might have allowed the animals to achieve a more horizontal
orientation temporarily. This hypothesis was tested by Wes-
termann (2013) using models of Baculites composed of plexi-
glass and styrofoam with iron weights used to simulate
different weight distributions. Westermann (2013) conrmed
the possibility of such conchs achieving a roughly horizontal
orientation and proposed a potential ontogenetic shift from a
vertical to a subvertical conch orientation following the
growth of conch curvature and re-ooding of parts of the
phragmocone. Peterman et al. (2019b) simulated an apically
ooded Baculites that was not able to deviate from a vertical
orientation while remaining neutrally buoyant. Experiments
on physical models with the same hydrostatic properties as
their virtual counterparts support a stable vertical posture.
The models represent density-corrected, neutrally buoyant
three-dimensional prints of reconstructed cephalopod
conchs (Peterman et al., 2019a,b, 2020a). These weighted
models (Fig. 3) allow the approximate analysis of the restor-
ing moment and hydrodynamic drag during rotational and
translational movement. Changes in conch orientation
during evolution from orthocones to early ammonoids were
investigated by balancing sculpted models of the whole conch
and the isolated body chamber to determine the relative posi-
tions of the centres of mass and buoyancy (Klug & Korn,
2004). They found an evolutionary shift from the aperture
horizontally pointing downwards (Bactritida) via oblique
apertures (early heteromorphs) to a near horizontal aperture
facing upwards (derived monomorphs), which implies an
increase in swimming velocity and manoeuvrability.
(2) Virtual models
Virtual models include mathematical models and simulated
conchs, either generated ex nihilo or from tomographic data
(Hoffmann et al., 2014; Naglik et al., 2015; Lemanis,
Zachow & Hoffmann, 2016; Naglik, Rikhtegar & Klug,
2016; Hebdon, Ritterbush & Choi, 2020). Trueman (1941)
rst attempted to calculate the buoyancy and hydrostatic ori-
entation of several heteromorph conchs, taking the body
chamber length versus total conch length into account. Plani-
spiral to gyrocone Crioceras ssicostatum,C. duvali,andC. mulsanti
were calculated with a sub-vertical aperture orientation,
whereas the planispiral Macroscaphites yvani,Scaphites aequalis,
and Oecoptychius sp. were calculated with a near-horizontal
aperture that pointed upwards. Trueman (1941) and later
authors (e.g. Klinger, 1981; Westermann, 1996; Peterman
et al., 2020a) noted that the uncoiling of the conch has the
tendency to increase the distance between the centre of
buoyancy and the centre of mass, thereby increasing the
conchs hydrodynamic stability (its tendency to resist changes
in orientation). Tanabe (1975) calculated the buoyancy for
Otoscaphites by volume approximations based on coiled coni-
cal geometries and the equations of Raup (1973). Otoscaphites
had a negative buoyancy that was interpreted as a shift
through the groups evolutionary history from a benthic to
a nektoplanktic habitat. Ward & Westermann (1977) calcu-
lated the buoyancy of the vermicone Nipponites occidentalis
based on conch length, radius, cross-sectional area, and sev-
eral corrective factors to account for ornamentation, septa,
and the siphuncle. They interpreted N. occidentalis as being
planktic due to its near-neutral buoyancy, lack of hydrody-
namic streamlining or perceived adaptations to a benthic life
habit (shell thickening, reduction in sutural complexity). The
assumption of neutral buoyancy has a strong impact on
reconstructed orientations. This impact was shown for differ-
ent heteromorphs with both neutral and negative buoyancy
(Okamoto, 1988b; Okamoto & Shibata, 1997; Higashiura &
Okamoto, 2012). On the basis of simulated conchs, Oka-
moto argued for a nektobenthic life habit, in some cases with
the conch occasionally resting on the sea oor like the torti-
cone Eubostrychoceras muramotoi, the ancylocone Polyptychoceras
pseudogaultinum, and torticone nostoceratids (Okamoto,
1988b; Okamoto & Shibata, 1997; Higashiura & Okamoto,
2012). But he agreed with Ward & Westermann (1977) that
Nipponites had a near-neutral buoyancy and a planktic life
habit (Okamoto, 1988a).
Biological Reviews (2021) 000000 © 2020 The Authors. Biological Reviews published by John Wiley & Sons Ltd on behalf of Cambridge Philosophical Society.
Heteromorph ammonoid palaeobiology 9
Olivero & Zinsmeister (1989) calculated a positive buoy-
ancy and near-horizontal upwards aperture orientation for
the heteromorph Diplomoceras maximum.Diplomoceras is a hami-
ticone (paperclip-like ancylocone subtype) heteromorph sim-
ilar in morphology to the proposedly negatively buoyant
Polyptychoceras (Okamoto & Shibata, 1997). The role of the
U-shaped body chamber of some heteromorphs was tested
by Kaplan (2002) who abstracted the complex conch shapes
into simpler geometric forms (e.g. cones, cylinders, tori) to
calculate volumes, surface areas, and centres of mass and vol-
ume. Kaplan (2002) also took into account the effect of par-
tial phragmocone relling and alterations of the centre of
mass of the soft body due to movement within the body
chamber (Monks & Young, 1998). Kaplan (2002) found that
only heteroconic (combination of torticone and ancylocone)
conchs could consistently attain an orientation that would
allow them to feed on benthos; scaphiticones and praviti-
cones (a combination of planispiral and ancylocone) showed
limited access to benthos through manipulation of the ll
fraction of the phragmocone, while ancylocones and hamiti-
cones showed no easy access to benthic prey regardless of
orientation modication. This analysis challenged the idea
that the evolution of the U-shaped body chamber was an
adaptation for a benthic life habit. Landman et al. (2012a)
agreed with this interpretation by arguing for a more passive,
lter-feeding strategy for Scaphites.
Heteromorphs with a U-shaped body chamber (nosto-
cones, scaphiticones, ancylocones; Fig. 4) are more stable
than modern Nautilus and are unlikely to be able to substan-
tially modify their orientation by active swimming locomo-
tion by jet propulsion (Peterman et al., 2020a,b). The U-
shaped body chamber distributes organismal mass away
from the centre of buoyancy, while maintaining an upward-
facing posture (as suggested by Trueman, 1941; Klinger,
1981; Westermann, 1996). Horizontal alignment of the
hyponome with the centres of rotation, such as in ancylo-
cones, suggests that more energy would be transmitted to
translational movement with minimal rocking, i.e. these mor-
photypes are well suited for backwards horizontal swimming
at maturity. Hydrodynamic efciency was investigated for
various scaphiticones (Peterman et al., 2020a) and an ancylo-
cone (Peterman et al., 2020d). The uncoiled shape increases
Fig. 3. Neutrally buoyant physical experiments on scaphitid heteromorph ammonoids. (A) Virtual model with complex internal
geometry. (B) Modied virtual model for 3D printing. Bismuth counterweight corrects for differences in mass and stability between
A and B. (C) 3D printed model showing counterweight and a control valve in case of ooding. (D) Neutrally buoyant model with
proper hydrostastics and tracking points for hydrodynamic analyses. From Peterman et al. (2020a).
Biological Reviews (2021) 000000 © 2020 The Authors. Biological Reviews published by John Wiley & Sons Ltd on behalf of Cambridge Philosophical Society.
10 René Hoffmann et al.
drag, but with Nautilus-like thrust values they were able to
move horizontally backwards at velocities similar to Nautilus
(when normalised by mass). Coleoid-like thrusts would have
allowed them to surpass the velocities of similarly sized Nauti-
lus. These calculations depend on the largely unknown
ammonoid soft body morphology (see Section II). Thus, it
remains unclear whether they are realistic. Aside from these
open questions, the results of these studies contrast with the
interpretation of such heteromorphs as being conned to
benthic habitats and the interpretation of limited horizontal
mobility (Westermann, 1996).
The adult change in coiling (U-shaped body chamber)
brought the hyponome closer to the level of the centre of
mass. This implies improved swimming capabilities. Klug
(2001) and Klug & Korn (2004) found this phenomenon in
several ammonoid clades and suggested that improved swim-
ming capabilities represent selective advantages during
reproduction particularly for the choice of mating partners
and spawning grounds. Sexual selection is a strong evolution-
ary agent and might be the main driver behind the conver-
gent evolution of more horizontally oriented apertures in
adult conch forms in many ammonoid clades, including
numerous heteromorphs.
IV. ECOLOGICAL INTERACTIONS
(1) Predators
Abnormal shell growth in modern and fossil molluscs, includ-
ing ammonoids, is often the result of sublethal injuries, which
may inform on predatorprey relationships (Keupp, 2012;
Hoffmann & Keupp, 2015). Like modern cephalopods,
ammonoids were an important part of marine food webs
and served as prey for vertebrates and invertebrates. For
food-web reconstructions, digestichnia (Vallon, 2012),
including stomach contents, cololites, coprolites, regurgita-
lites, and palaeopathologies (Keupp, 2012) have been used.
Since digestichnia including heteromorph remains were
reviewed by Hoffmann et al. (2020) we herein focus on
palaeopathologies. Various heteromorphs bear large (often
hollow) spines which might have had a protective function
against predators or encrusters (Klinger, 1981; Cecca,
1997) but other authors have speculated that these spines
were too fragile and might have had other functions such as
stabilizing the shell in the water column (Klinger, 1981; Ifrim,
Bengtson & Schweigert, 2018).
Specic reactions of the prey often result in a characteristic
abnormal growth of the repaired shell (symptoms). Assump-
tions about the ammonoid life habit including habitat depth
and proximity to the seaoor are possible (Hoffmann &
Keupp, 2015). However, sublethal injuries in cephalopods
can be inicted not only by predators, but also by the same
species such as in Nautilus during mating (Ward, 1987) or by
dangerous prey such as crustaceans [e.g. see Ward (1981)
for Nautilus and Landman & Waage (1986) for scaphitids].
Pathologies have been primarily reported for Jurassic and
Cretaceous heteromorphs (Table S2) but were also known
from Devonian heteromorphs, which suggest damage caused
by other cephalopods (Klug, 2007). The oldest known
injured Mesozoic heteromorph is the Late Bajocian Spiroceras
(Bayer, 1970: plate 8, Fig. 9; Dietl, 1978: Fig. 6f ). Here, we
document palaeopathologies for the Hauterivian Aegocrioceras
raricostatum (Fig. 5B, G), Aptian Proaustraliceras tuberculatum
(Fig. 5D), Albian Pictetia astieriana (Fig. 5A), Aptian Ptychoceras
minimum (Fig. 5C, E), and Ptychoceras renngarteni (Fig. 5F).
Among Cretaceous heteromorphs, palaeopathologies are
common and have been mostly reported for scaphitids
(Landman & Waage, 1986; Hengsbach, 1996; Larson,
2002; Landman et al., 2010b, 2012a, 2019; Keupp, 2012;
Fig. 4. Virtual hydrostatic models of heteromorphs with recurved body chambers at maturity. (A) Didymoceras nebrascense (Peterman
et al., 2020b). (B) Audouliceras renauxianum (Peterman et al., 2020d). (C) Hoploscaphites crassus microconch (Peterman et al., 2020a). Tip
of upper yellow cone, centre of buoyancy; tip of lower yellow cone, centre of mass. Scale bars =3 cm.
Biological Reviews (2021) 000000 © 2020 The Authors. Biological Reviews published by John Wiley & Sons Ltd on behalf of Cambridge Philosophical Society.
Heteromorph ammonoid palaeobiology 11
Fig. 5. Palaeopathologies of heteromorphs. (A) Pictetia asteriana (Early Albian, Madagascar) with a band-slit pathology potentially
caused by a benthic crustacean suggesting a demersal life habit of the ammonoid. Private collection of Wolfgang Grulke (Dorset,
UK). (B) Aegocrioceras raricostatum (Hauterivian, NW-Germany) with a bulbous shell breakage. (C) Ptychoceras minimum (Late Aptian,
Krasnodar region, Russia). Geological institute RAS, No. GIN MPC 5/2. Arrow indicates a healed sublethal injury on the second
shaft of the conch. Post-traumatic ination indicates damage of the shell-secreting mantle edge. (D) Proaustraliceras tuberculatum
(Early Aptian, Ulyanovsk region, Russia). Private collection of Dmitry Vinogradov (Moscow), specimen No. 438. Arrow indicates
a healed sublethal injury on the dorsal side of the conch likely produced by durophagous sh. (E) Ptychoceras minimum (Late Aptian,
Krasnodar region, Russia). Geological institute RAS, No. GIN MPC 5/5. Feather-like wrinkles on the dorsal surface of the second
shaft that start at the point of truncation of the rst shaft. (F) Ptychoceras renngarteni (Late Aptian, Krasnodar region, Russia).
Geological institute RAS, No. GIN MPC 5/1. Arrows mark healed sublethal injuries on the second shaft of the conch.
(G) Aegocrioceras sp. body chamber (Hauterivian, NW-Germany) with a band-slit.
Biological Reviews (2021) 000000 © 2020 The Authors. Biological Reviews published by John Wiley & Sons Ltd on behalf of Cambridge Philosophical Society.
12 René Hoffmann et al.
Hoffmann & Keupp, 2015). Typically, injuries affected the
margin of temporary apertures and usually occur in early
ontogenetic stages (coiled conch part). Severe injuries of the
shell-secreting mantle epithelium sometimes resulted in the
disappearance of ventral tubercles (Hoffmann & Keupp,
2015: Fig. 21.8d). Crustaceans, sh, marine reptiles, coleoids,
ammonoids, and nautiloids were discussed as potential pro-
ducers of such damage (Landman & Waage, 1986;
Hengsbach, 1996; Larson, 2002; Landman et al., 2010b;
Keupp, 2012; Hoffmann & Keupp, 2015).
Orthoconic baculitids display palaeopathologies on their
apertural edges of different growth stages (Klinger &
Kennedy, 2001; Henderson, Kennedy & Cobban, 2002;
Kennedy, Cobban & Klinger, 2002a). In some cases, the
complete aperture was fragmented, presumably by pycno-
dontid sh (Kennedy et al., 2002a: plate 8, Fig. 9) or the conch
had long and narrow V-shaped injuries on both anks
(Kennedy et al., 2002a: plate 8, Figs. 24, 68). For the latter,
Kennedy et al. (2002a) suggested a coleoid attack, but Keupp
(2012) argued for a benthic crustacean as the predator.
Palaeopathologies may help reconstructing the soft body
organisation of the Aptian Ptychoceras. Doguzhaeva & Mutvei
(1989, 1993, 2015) argued that Ptychoceras had an internal
shell. Kakabadze & Sharikadze (1991, 1993) and Keupp
(2012) rejected that interpretation based on similarities of
Ptychoceras palaeopathologies with injuries found in other
monomorph and heteromorph ammonoids. Injured and
repaired temporal apertures (forma aegra substructa)ofPtycho-
ceras caused a strong distortion of the subsequent shaft, indi-
cating severe damage of the mantle edge (Fig. 5C, F). A
healed ventral injury located behind the terminal aperture
(forma aegra fenestra sensu Keupp, 2006) was found in another
Ptychoceras. In all cases, the healed shell is attached to the
inner surface of the preserved pre-traumatic conch wall like
in other ammonoids and modern Nautilus with repaired inju-
ries. No additional layers were secreted on top of the outer
prismatic layer, as expected for an internal shell.
Truncation of shell is restricted to the rst shaft in Ptycho-
ceras (Doguzhaeva & Mutvei, 1989, 1993, 2015). Truncation
also occurs in Palaeozoic nautiloids (e.g. Turek & Manda,
2012) and coleoids (Doguzhaeva, Mapes & Mutvei, 2002).
Doguzhaeva & Mutvei (1989) suggested that the truncation
in Ptychoceras occurred due to muscular action of the ammo-
noid body, during the formation of the second shaft. Kaka-
badze & Sharikadze (1993) assumed that the truncation
could have occurred randomly during the formation of the
rst to the beginning of the second shaft. Reported fragments
preserved in the partially broken phragmocone chambers
near the point of truncation (Doguzhaeva & Mutvei, 1989,
pl. 9, 10) support the hypothesis of Doguzhaeva & Mutvei
(1989). The feather-like structure (Fig. 5E) implies the pres-
ence of the shell-secreting mantle edge and periostracum in
Ptychoceras, which disproves the endocochleate hypothesis
for this ammonoid.
Palaeopathologies of heteromorphs include various shell
abnormalities and deformed suture lines. Pseudoinversion
of suture lines refers to pointed saddles and rounded lobes.
Pseudoinversion has been reported for the Late Cretaceous
heteromorphs Glyptoxoceras (Westermann, 1975; Ward &
Westermann, 1976) and Baculites (Henderson et al., 2002).
Rogov (2018) found this in several Jurassic and Cretaceous
monomorphs. Another internal abnormality of the ammo-
noid conch is sutural asymmetry relative to the venter, while
the conch wall is unaffected [forma aegra juxtalobata
(Landman & Waage, 1986; Keupp, 2012)]. In this case, the
siphuncular tube moved out of its ventral central position.
This type of palaeopathology occurs in both heteromorphs
and monomorphs (Keupp, 2012). However, the question of
what causes these pathologies remains unanswered. Various
authors speculated that the asymmetries were caused either
by the asymmetric development of reproductive glands due
to parasites or due to epithelial diseases(Keupp, 2012; De
Baets et al., 2015a). Mortons syndromealso affected conch
symmetry, where the asymmetry is unrelated to external inju-
ries and occurs in 36% of scaphitids from South Dakota
(Landman & Waage, 1986). This phenomenon is widespread
in keeled monomorphs, such as Jurassic Graphoceras and Pleur-
oceras (Morton, 1983; Keupp, 2012). As in many other cases
of abnormalities with unclear aetiology, this asymmetry has
been explained by parasitic infestation (Morton, 1983; De
Baets, Keupp & Klug, 2015b), although traces of parasites
have not been documented in asymmetric conchs.
(2) Prey
Reports of fossil cephalopod stomach contents are rare (Table
S3). Planktotrophy was suggested for ammonoids that have
aptychus-type, calcied lower jaws (Kruta et al., 2011, 2015;
Tanabe, 2011; Tanabe et al., 2015a;Keuppet al., 2016a). This
is supported by nds of stomach contents associated with a rad-
ula in Baculites (Kruta et al., 2011) and Allocrioceras (Wippich &
Lehmann, 2004), a pelagic, aperture-upwards drifter that
probably fed on echinoderms (comatulid crinoids or ophiu-
roids; Fig. 6). Diet remains from Jurassic and Triassic mono-
morphs comprise foraminifera, ostracoda, decapod
crustacean, bivalves, aptychi, and crinoid hardparts (e.g. Sacco-
coma)inNeochetoceras and corroborate planktotrophy in ammo-
noids (Wippich & Lehmann, 2004; Ritterbush et al., 2014;
Klug & Lehmann, 2015). Interestingly, the modern coleoid
Japetella shares radular tooth morphology with some Jurassic
ammonoids (Kruta et al., 2011; Keupp et al., 2016a), which
have been regarded as microphagous planktotrophs.
JurassicCretaceous heteromorphs are widely accepted as
microphagous zoo-planktotrophic suspension feeders (Nesis,
1986, 2005; Kruta et al., 2011; Tajika, Nützel & Klug,
2018). Many heteromorphs reached large sizes (e.g. Baculites
grandis with a conch length of 250 cm; N. Larson, personal
communication) and are considered passive drifters with
low manoeuvrability (Westermann, 1996; Mikhailova &
Baraboschkin, 2009). Without direct evidence, Nesis (1986,
2005) suggested the presence of membranes between the
arms or a special mucus web to collect planktic prey out of
the water column. Heteromorphs likely lived in the water col-
umn (Cecca, 1997; Guex, 2006), i.e. with direct access to
Biological Reviews (2021) 000000 © 2020 The Authors. Biological Reviews published by John Wiley & Sons Ltd on behalf of Cambridge Philosophical Society.
Heteromorph ammonoid palaeobiology 13
meso- and microplankton. Based on the reconstructed swim-
ming position for heteromorphs with U-shaped body cham-
bers in which the aperture pointed upwards (Kaplan,
2002), it is assumed that their diet was composed of micro-
scopic particles or organisms (Kruta et al., 2011; Keupp,
2012; Ritterbush et al., 2014; Klug & Lehmann, 2015; Keupp
et al., 2016b). Remarkably, the jaw morphology of modern
coleoids does not entirely relate to diet (Clarke & Maddock,
1988). Jaw morphology only reects the maximum size of
prey and dictates the area for the insertion of mandibular
muscles and hence its mass and biting force (Kear, 1994).
All Cretaceous heteromorphs, except the crioceratids, share
an aptychus-type lower jaw (Section II.1.a). The aptychus is
potentially not involved in determining prey size but prey
type. Accordingly, both the Crioceratidae with an anapty-
chus and other aptychus-bearing Cretaceous heteromorphs
may differ in their food preferences.
All modern cephalopods are carnivores ranging from
active predators to more passive forms like Vampyroteuthis
(Norman, 2000; Golikov et al., 2019). Hanlon & Messenger
(2018) documented cannibalism for every known species of
modern coleoids except Sepioteuthis sepoidea. The occurrence
of cannibalism appears to be related to poor food supply
(Ennis & Collins, 1979). Research on stomach contents in
modern cephalopods is difcult because the jaws reduce the
prey to unidentiable pieces. Hardparts needed for the iden-
tication of prey organisms are often regurgitated, leading to
an underestimation of soft-bodied prey in estimations of diet
(Rodhouse & Nigmatullin, 1996). This is because the oesoph-
agus diameter is limited because it passes through the brain.
This likely also applied to most extinct neocephalopod
groups such as ammonoids (Engeser, 1996; Klug et al.,
2016, 2019). Modern cephalopod stomachs often contain
skeletal parts of sh, crustaceans, and cephalopods (even
from the same species). A shift of food resources during
ontogeny is evident. The juveniles of most modern cephalo-
pods feed on crustaceans and shift towards larger sh and
other cephalopods later in ontogeny (Rodhouse &
Nigmatullin, 1996). Only a few coleoids seem to be special-
ists; the majority appear to be non-selective in their choice
of prey [e.g. Mather (1993) for observations on octopuses].
Rodhouse & Nigmatullin (1996) observed that squid and cut-
tlesh preying on pelagic crustaceans ingest the exoskeleton,
whereas octopuses paralyse their prey with cephalotoxin,
digest only the esh and reject the exoskeleton. This is rele-
vant for the reconstruction of predatorprey relationships
in heteromorphs. This difference in behaviour might be
related to the need to form a mineralised internal skeleton
or other unknown factors. Larson (2002) speculated on can-
nibalism in scaphitids. Based on their high demand for cal-
cium, Larson (2002) postulated that ammonoids fed on
crustaceans, echinoids, molluscs, and possibly sh.
(3) Epizoa, parasites, and commensalism
Syn vivo encrustations, i.e. epizoa (in contrast to post-mortem
encrustation of dead shells epicole; see Davis, Mapes &
Fig. 6. Stomach content of Allocrioceras (specimen shown in
Wippich & Lehmann, 2004). (A) Image taken under daylight.
(B) Same specimen imaged under ultraviolet light showing
preserved soft parts such as the siphuncle and remnants of the
stomach. (C) Close up of assumed but unidentiable stomach
remains. (D) Stomach remains of planktic comatolids
(crinoids). Scale bars: A and B =5 mm, C and D =0.5 mm.
Biological Reviews (2021) 000000 © 2020 The Authors. Biological Reviews published by John Wiley & Sons Ltd on behalf of Cambridge Philosophical Society.
14 René Hoffmann et al.
Klofak, 1999), provide a unique source for palaeoecological
reconstructions (Lukeneder, 2008). Some cases have been
attributed to parasitism, while others might indicate syn vivo
epizoism, which ranges from associations that might have
been detrimental to both or benecial for one or both
organisms.
Four criteria have been applied to identify cases of syn vivo
associations of epizoans with ammonoid conchs (see Keupp,
2012). (i) Associations found on both anks, but not at the
apertural region mostly refer to ectoparasites or epizoans
[see exceptions in Mironenko (2016) and Landman, Slat-
tery & Harries (2016b)]; in the case of disease or endoparasit-
ism, no external injuries are visible, but pathologies develop
within soft tissues. (ii) The growth of associated organisms
stops precisely at a whorl or is otherwise outpaced by the
conch growth of the cephalopod. (iii) Associated organisms
show a growth direction consistent with shape and life posi-
tion of the cephalopod and may follow changes in growth
direction of the host. (iv) The cephalopod reacts to its epizo-
ans by altering its conch form such as non-planispiral coiling,
shortening of body chamber (Klug et al., 2004), and other
pathologies.
Cases of conch deformation or deviation from normal pla-
nispiral coiling caused by encrusters, which are still pre-
served, provide incontrovertible evidence for syn vivo
encrustation in ammonoids (Ramming et al., 2018). Syn vivo
encrustations may result in monomorphs resembling hetero-
morphs (Checa, Okamoto & Keupp, 2002). Patterns related
to criteria 1 and 3 are the only indicators useful to infer syn
vivo encrustation when host growth has stopped, but could
potentially develop in post-mortem sclerobiont attachment dur-
ing necroplanktic drift (Stilkerich, Smrecak & De Baets,
2017). However, marked post-mortem drift is unlikely in the
case of heavy encrustation and smaller conchs (Yacobucci,
2018; Wani et al. 2005). Furthermore, the conch might
change orientation during post-mortem drift and due to added
weight by epicoles. The orientation and position of encrus-
ters during life could also support the idea that heteromorphs
were nektoplanktic or at least neither demersal nor benthic.
Encrustation by cirripedes (Hauschke, Schöllmann &
Keupp, 2011), brachiopods (Landman et al., 2016b), and
bivalves (Misaki et al., 2014) have been reported on Creta-
ceous heteromorphs. Misaki et al. (2014) found anomiid
bivalves on both sides of Pravitoceras conchs, suggesting these
are epizoa. These heteromorphs come from mudstones with-
out signs of strong currents or wave-induced transport. Mis-
aki et al. (2014) suggested that Pravitoceras was neutrally
buoyant and could move vertically, thus contradicting the
hypothesis of a benthic life habit (Matsumoto et al., 1981b).
Syn vivo encrustation of a Palaeozoic Ivoites specimen by
hederelloids (?phoronids) caused trochospiral instead of
gyrocone coiling (Stilkerich et al., 2017). The pathological
specimen survived into adulthood despite multiple encrusta-
tions (Stilkerich et al., 2017). If cephalopods reached adult-
hood showing such deformations, these may not have
altered ammonoid health substantially, irrespective of
whether such trochospiral coiling was a normal variant or
pathological. Regular covariation patterns might relate to
structural constraints and may provide information about
growth. Highly irregular patterns within the same specimen
might indicate a lack of morphogenetic constraints.
Associations between heteromorphs and algae have been
suggested repeatedly. Arkhipkin (2014) suggested that Spiro-
ceras was associated with macroalgae and that adult life stages
of Cretaceous heteromorphs (e.g. scaphitids) clung to macro-
algae with their umbilical opening [see also Vašícˇek & Wied-
mann (1994) for Leptoceratoidinae such as Karsteniceras].
However, direct evidence for such associations has not been
found yet, rendering this idea speculative (Landman et al.,
2016a). The fossil record of non-calcied macroalgae is
sparse; potential biomarkers like sterols may help to identify
their presence.
Hatchlings of the trochospiral Mariella or orthoconic Scipo-
noceras have been interpreted as semi-sessile based on associ-
ations with algal mats (Stinnesbeck, Frey & Zell, 2016).
However, this is likely a taphonomic artefact and solid evi-
dence for this life habit is needed because such life habit is
unknown in modern cephalopods.
V. HABITAT RECONSTRUCTION
(1) Taphonomic controls on facies distributions
The relationship between heteromorphs and facies has been
debated due to post-mortem drift. This issue stems from exter-
nally shelled cephalopods having phragmocones, which can
remain gas-lled after death allowing the conch to oat and
drift varying distances away from the living animalshabitat
[Fig. 7; Reyment (1958, 1973, 2008); for alternative perspec-
tives see Wani & Gupta (2015) and Yacobucci (2018)]. How-
ever, numerous studies have argued that post-mortem drift was
rare among ammonoids, and that their fossils closely reect
their life distributions (e.g. Kennedy & Cobban, 1976;
Tanabe, 1979; Chamberlain, Ward & Weaver, 1981; Wani
et al., 2005; Wani & Gupta, 2015; Yacobucci, 2018). These
interpretations are based on the low frequency of encrusters,
degree of shell breakage, frequency of preserved jaws in body
chambers, sizefrequency distribution of conchs, low drift
potential of numerous conch morphotypes, and geographic
distribution patterns. Kennedy & Cobban (1976) proposed
that facies independence and broad geographic ranges as
well as distinct facies associations and limited ranges should
reect high and low post-mortem drift, respectively. They state
that the large numbers of unbroken, well-preserved ammo-
noids at many localities represent autochthonous assem-
blages, which cannot be explained by post-mortem drift. This
is supported by taphonomic and experimental studies of
modern nautilid conchs, which revealed that drifted conchs
show signs of fragmentation and encrustation (Wani, 2004;
Reyment, 2008). Yacobucci (2018) argues that most ammo-
noid conchs, except for highly inated monomorphs, were
unlikely to undergo long-distance post-mortem drift. This study
Biological Reviews (2021) 000000 © 2020 The Authors. Biological Reviews published by John Wiley & Sons Ltd on behalf of Cambridge Philosophical Society.
Heteromorph ammonoid palaeobiology 15
also shows that most Cretaceous ammonoids, even those with
conch forms conducive to post-mortem drift, rarely had sub-
stantial biogeographic ranges (i.e. basinwide, intercontinen-
tal), which would be expected if they were oating over
great distances after death.
Post-mortem drift in heteromorphs has not been considered
to the same extent as for monomorphs; the latter being mor-
phologically more like nautilids, which are assumed to be
reasonable analogs. However, the few available studies sug-
gest that the impact of post-mortem drift on heteromorph spa-
tial distributions is minimal (Tanabe, Obata & Futakami,
1978; Tanabe, 1979; Kawabe, 2003; Reboulet, Giraud,
& Proux, 2005; Landman et al., 2010b, 2019; Landman &
Klofak, 2012; Misaki et al., 2014; Slattery, Harries &
Sandness, 2018). Stable isotopic analysis of heteromorphs
and associated benthic faunas provides evidence for limited
post-mortem drift. These studies reveal that the δ
18
O and
δ
13
C composition of scaphitids, baculitids, and nostoceratids
more closely match the isotopic composition of associated
benthic faunas than known planktic organisms. This has been
interpreted as an indication that they were living near where
they were collected (Tsujita & Westermann, 1998; He,
Kyser & Caldwell, 2005; Landman & Klofak, 2012; Land-
man et al., 2012b; Sessa et al., 2015).
Berry (2018a,b) and Landman et al. (2019) are among the
few studies to suggest that some heteromorph conchs could
undergo long-term and/or long-distance post-mortem drift.
Berry (2018a,b) utilised changes in both bryozoan zooid size
and the direction of growth in colonies encrusting the inter-
nal body chambers of Baculites and Scaphites to suggest that
these heteromorphs experienced long-term post-mortem drift.
Landman et al. (2019) argued that the occurrences of North
American Hoploscaphites in Europe are best explained by
long-distance post-mortem drift due to their rarity. However,
Machalski et al. (2007) suggested that North American
Hoploscaphites and Discoscaphites in Europe are more likely
related to immigrant populations rather than post-mortem drift
due to the greater potential for small cephalopod shells to be
buried close to their habitats.
(2) Life habit controls on facies distributions
The association of water-column-dwelling ammonoids with a
benthic bio- and lithofacies or a lack thereof is likely due to
their preferred habitats and positions in the water column
(Ziegler, 1967; Kennedy & Cobban, 1976; Batt, 1986,
1989; Westermann, 1996; Lukeneder, 2015). They would
have tracked these environmental preferences both spatially
and temporally, which in turn would be expressed in the
stratigraphic record as changes in faunal occurrences and/or
ranges. Environmental changes in the upper water column
would be expressed in the stratigraphic record as changes in
occurrences and abundances of nektic and planktic organ-
isms (e.g. ammonoids, sh, planktic foraminifera, nanno-
plankton), while potentially showing no concomitant
pattern among benthic organisms, or lithofacies [Soule &
Kleppel (1988) and references therein]. Nektic (i.e. actively
swimming in the water column), nektoplanktic (i.e. passively
oating in the water column), and vertically migrating
(i.e. moving vertically through the water column) ammonoids
Fig. 7. Potential taphonomic pathways of modern Nautilus prior to burial (modied from Maeda & Seilacher, 1996; Mapes
et al., 2010a,b). Nautilus habitat ranges from 20 to 600 m depth; however, most live within a few meters of the seabed. The Depth
Limit to Surfaceis the theoretical limit where an empty conch cannot ascend to the sea surface because hydrostatic pressure
forces water into the phragmacone and causes the conch to become negatively buoyant and sink.
Biological Reviews (2021) 000000 © 2020 The Authors. Biological Reviews published by John Wiley & Sons Ltd on behalf of Cambridge Philosophical Society.
16 René Hoffmann et al.
likely tracked their preferred ecological conditions and food
sources through the water column. This would have
decoupled them from benthic faunas, environments, and
facies, unless their prey was tied to a benthic environment
(Westermann, 1996). By contrast, nektobenthic (i.e. living
near the sea bottom) ammonoids would have been inuenced
by the same conditions (e.g. oxygen, turbidity) that controlled
the distribution of benthic organisms, which explains the
association of many ammonoids with specic benthic bio-
and/or lithofacies (e.g. Tsujita & Westermann, 1998; Land-
man et al., 2012b; Slattery et al., 2018). Documenting changes
in heteromorph occurrences and abundances across benthic
bio- and lithofacies can reveal important details about their
preferred habitats, specically when examined within a
sequence stratigraphic framework. Kennedy & Cobban
(1976) showed that occurrences of Cretaceous ammonoids,
including heteromorphs, rarely exhibit discernable lithofa-
cies patterns. However, they also noted that there are obvious
changes in abundances among taxa across lithofacies. Varia-
tion in heteromorph abundances has also been documented
across benthic biofacies, which are attributed to changes in
marine environmental conditions (e.g. oxygen) that are inde-
pendent of lithofacies (e.g. Lukeneder, 2003, 2004, 2005;
Slattery et al., 2018).
Heteromorphs are known from an array of facies spanning
tidally inuenced littoral to oceanic environments (Fig. 8;
Westermann, 1996). Most heteromorphs occur in shallow-
subtidal to offshore facies deposited in neritic (i.e. open-
ocean-facing seas) and epeiric seas (i.e. seas covering
continental interiors). They are common in continental slope
facies, but rare in bathyal, foreshore, and littoral facies
(Westermann, 1996). Most heteromorphs preferred epipe-
lagic depths (0200 m), and for certain taxa, upper mesope-
lagic depths (200600 m) due to depth limitations imposed
by the strengths of their shells (Hewitt, 1996). Taxa found
in oceanic facies were likely living within a few meters of
the continental slope and/or in the water column well above
the slope and bathyal seaoor at epi- and/or upper mesope-
lagic depths (Hewitt, 1996; Westermann, 1996). The few
examples of heteromorphs in foreshore and littoral facies
(e.g. Hoganson & Murphy, 2002) are likely due to post-mortem
transport via drift or storms.
Devonian, Triassic, and Jurassic heteromorphs show a
preference for deep-subtidal to offshore facies but are rare
in shallow-subtidal, slope, and bathyal facies (Fig. 8A;
Table S4; Dietl, 1978; Laws, 1982; Chlupácˇ& Turek,
1983; www.paleodb.org). The rarity of Pre-Cretaceous het-
eromorphs in shallow-subtidal, slope, and bathyal facies sug-
gests that these ammonoids did not inhabit these
environments but were transported into these settings via
storms or post-mortem drift from their epipelagic, deep-
subtidal to offshore habitats.
Early Cretaceous heteromorphs preferred deep-subtidal to
bathyal facies (Fig. 8B; Table S4). Barragan, Gonzalez-
Arreola & Villaseñor (2004) documented distributions of
BarremianammonitesinMexico,whichwerecontrolledby
water-column oxygenation and the preferred water depth of
each species. They suggest that ancylocone heteromorphs were
nektic or nektoplanktic based on their abundance in anoxic
facies, which indicates a preference for offshore settings
(Barragan et al., 2004). Lukeneder (2003, 2004, 2005) suggested
that the Barremian ancyloceratid Karsteniceras preferred low-
oxygen conditions based on its mass occurrences in dysoxic
facies. Marcinowski & Wiedmann (1990) noted that Albian het-
eromorphs in Poland are restricted to clay- and marlstones
rather than sandstones, which suggests that these ammonites
preferred offshore facies rather than subtidal facies. Reboulet
et al. (2005) studied Albian ammonite occurrences and abun-
dances across Oceanic Anoxic Event 1d in the Vocontian Basin
(France) and found that most heteromorphs are associated with
offshore neritic to oceanic facies. For example, the baculitid
Lechites is most abundant in continental-slope facies. The torti-
cones Turrilitoides and Mariella as well as the hamiticones Anisoceras
and Hamites are associated with offshore neritic and oceanic
facies. Tajika et al. (2017) showed that Cretaceous cephalopod
associations of Switzerland, including heteromorphs
(e.g. Emericiceras,Mariella), are dominant in deep-subtidal to off-
shore facies and rare to absent in shallower, carbonate facies.
Several studies suggest that bochianitids are deeper-water indi-
cators due to their abundances in limestones and claystones
depositedincontinentalslopefacies(>200 m) and absence in
neriticaswellasepeiricfacies (Company, 1987; Reboulet &
Atrops, 1997; Baraboschkin & Enson, 2003; Reboulet et al.,
2005; Arkadiev, 2008; Lukeneder, 2015).
Late Cretaceous heteromorphs have a preference for
shallow-subtidal to offshore facies, but are rare in slope and
bathyal facies (Fig. 8C; Table S4). Baculitids are broadly dis-
tributed with juveniles being common in well-oxygenated,
shallow- to deep-subtidal facies and adults being common
in well-oxygenated, deep-subtidal to anoxic, offshore facies
(Fig. 9; Tsujita & Westermann, 1998; Kawabe, 2003; Slat-
tery et al., 2018). The earliest juvenile Baculites have been
recorded in shallow-subtidal to offshore facies, which indi-
cates that they might have been more facies independent
and/or more subject to drift as compared to more mature
individuals (Landman & Klofak, 2012; Slattery et al., 2018;
N.H. Landman, personal communication; Rowe et al.,
2020). The giant baculitid Pseudobaculites is common in subti-
dal facies but rare in offshore facies (W.A. Cobban, personal
communication). Turrilitids, scaphitids, diplomoceratids,
and nostoceratids are typically associated with well-oxygen-
ated, subtidal to offshore facies due to their nektobenthic life
habit (Fig. 9; Scott, 1940; Kennedy & Cobban, 1976;
Tanabe et al., 1978; Tanabe, 1979; Matsumoto et al.,
1981a; Batt, 1986, 1989; Tsujita & Westermann, 1998;
Kawabe, 2003; Landman et al., 2012b; Slattery et al., 2012,
2018; Olivero & Raf, 2018; Rowe et al., 2020; J. S. Slattery,
personal observations). The scaphitid Worthoceras likely
inhabited the middle to upper water column based on their
broad distribution in well-oxygenated, subtidal to anoxic, off-
shore facies (Scott, 1940; Batt, 1986, 1989). Several nostocer-
atids in North America are common in shallow-subtidal
facies but rare to absent in deep-subtidal to offshore facies
(Table S4; Fig. 9; Kennedy & Cobban, 1976, 1994;
Biological Reviews (2021) 000000 © 2020 The Authors. Biological Reviews published by John Wiley & Sons Ltd on behalf of Cambridge Philosophical Society.
Heteromorph ammonoid palaeobiology 17
Cobban, 1993; Cobban et al., 1993; Kennedy, Cobban & Scott,
2000a,b). Hamitids and anisoceratids are commonly restricted
to well-oxygenated, subtidal facies, however, the hamitid Stomo-
hamites and anisoceratid Allocrioceras have broad distributions
ranging from well-oxygenated, subtidal to anoxic, offshore
facies (Kennedy & Cobban, 1976; Batt, 1986, 1989). The
increase in heteromorph diversity in shallow-subtidal settings
duringtheLateCretaceouslikelyreects an evolutionary shift
among Ancyloceratina to shallower-water settings.
(3) Ecological requirements based on shell
geochemistry
Stable isotope proxy systems provide information on habitat
depth, depth change through ontogeny, and growth rates of
individual monomorphs and heteromorphs (e.g. Moriya
et al., 2003; Lukeneder et al., 2010; Fig. 10). Most analyses
to date have been focused on baculitids and scaphitids from
the Late Cretaceous (He et al., 2005; Landman et al.,
Fig. 8. Heteromorph distributions across marine environments and depth zones during the (A) Devonian, Triassic, and Jurassic,
(B) Early Cretaceous, and (C) Late Cretaceous. Based on data from Table S4.
Biological Reviews (2021) 000000 © 2020 The Authors. Biological Reviews published by John Wiley & Sons Ltd on behalf of Cambridge Philosophical Society.
18 René Hoffmann et al.
2012b). Variation in the oxygen isotope ratio (δ
18
O) reects
changes in temperature or the oxygen isotope ratio of sea
water, which often covaries with salinity (Mátyás et al., 1996).
Combined temperature and salinity gradients create a vari-
able prole in δ
18
O between surface water and bottom water,
which is recorded in shell carbonate precipitated in equilib-
rium with those waters. Seasonal variation in these parameters
can impart sinusoidal variation and therefore a chronometer
in the shells and aptychi of heteromorphs (Fatherree, Harries &
Quinn, 1998; Kruta, Landman & Cochran, 2014; Ellis &
Tobin, 2019), although swimming behaviour and growth rate
can complicate these patterns in ammonoids (Linzmeier,
2019). Cephalopods are thought to precipitate shell aragonite
in oxygen isotope equilibrium with ambient seawater stable
oxygen isotope composition (δ
18
O
SW
)andtemperature
(Landman et al., 1994). Stable carbon isotope ratios (δ
13
C)
record a mixture of carbon dominated by dissolved organic
carbon from water and minor respired carbon
(McConnaughey & Gillikin, 2008). Therefore, the habitat
depth of modern cephalopods can be directly inferred from
measured carbonate oxygen isotope ratios of their aragonitic
shell or calcitic aptychi (Auclair et al., 2004; Lukeneder et al.,
2008; Kruta et al., 2014; Lukeneder, 2015; Linzmeier et al.,
2016). To interpret habitat depth or growth rate from δ
18
O
and δ
13
C of fossils, additional isotopic information derived
from co-occurring benthic and planktic organisms is required.
The habitats of Late Cretaceous heteromorphs have been
studied using stable isotope analyses. Discoscaphites and Bacu-
lites from Mississippi show δ
18
O values that are more similar
to both benthic molluscs and foraminifera than to planktic
foraminifera (Sessa et al., 2015). Individuals have consistent
δ
18
O through the gerontic whorl, supporting an interpreta-
tion of rapid growth and consistently benthic habitat
(Ferguson et al., 2019). Analysis of the earliest whorls of
Hoploscaphites from South Dakota suggest a benthic embry-
onic development, planktic hatchlings, and a habitat change
after about one whorl (Linzmeier et al., 2018). These data
may also suggest seasonally protracted spawning like some
modern cephalopods (Rocha, Guerra & González, 2001).
Other work spanning multiple Campanian ammonoid zones
from the Western Interior Seaway suggests Didymoceras was
living in warmer or more brackish water than co-ocurring
Scaphites and Baculites (He et al., 2005).
The interpretation of δ
13
C in heteromorphs, like other
marine molluscs, reects the δ
13
C of dissolved inorganic car-
bon (DIC) (McConnaughey & Gillikin, 2008). In the modern
ocean, the δ
13
C varies with changes in the ratio of respiration
to photosynthesis and proximity to sources of unusual δ
13
C
from DIC (e.g. methane seeps). Analyses of Baculites from
the Western Interior Seaway (WIS) suggest that some hetero-
morphs lived on or close to cold seeps throughout their lives
and exploited the diverse prey available at the seeps
Fig. 9. Shoreface to offshore ammonoid facies and habitat distributions in the late Campanian Baculites compressus Biochron (Late
Cretaceous) in the Western Interior Seaway. Based on data from: Kennedy & Cobban (1976); Cobban, Kennedy, & Scott (1993);
Cobban et al. (1993); Larson et al. (1997); Landman & Klofak (2012); Landman et al. (2012a); and Meehan & Landman (2016).
Biological Reviews (2021) 000000 © 2020 The Authors. Biological Reviews published by John Wiley & Sons Ltd on behalf of Cambridge Philosophical Society.
Heteromorph ammonoid palaeobiology 19
(Landman et al., 2018). Hoploscaphites and Didymoceras also
appear to use methane seep habitats both as adults
(Landman et al., 2012b) and juveniles (Rowe et al., 2020).
The growth rates of Baculites have been estimated using sta-
ble isotopic methods. Their orthoconic conchs are relatively
easy to sample serially. By interpreting the pronounced sinu-
soidal pattern in δ
18
O, Fatherree et al. (1998) inferred a growth
rate of 33 cm/year for a subadult Baculites compressus. Sampling
of other Baculites suggest similarly high growth rates if inter-
preting sinusoidal variability in either δ
18
Oorδ
13
C, although
there are caveats about geographic differences in both isotope
proxies that could reect migration (Ellis & Tobin, 2019).
Future investigation of heteromorph living depths and growth
rates using stable isotope methods will yield important infor-
mation that cannot be gathered from other methods.
(4) Reproduction
Early ammonoids (Anetoceratinae) have slightly curved
embryonic conchs, which are among the largest of all ammo-
noids (De Baets et al., 2013a; De Baets, Landman & Tanabe,
2015c). They range from about 6 mm in Metabactrites to
5mminIvoites to 3.7 mm in Erbenoceras. This, together with
their small body chamber volume, implies low fecundity
(35500 eggs). De Baets et al. (2012) argued that increased
coiling and a reduction in embryonic conch size with an
increase in adult size affected fecundity. To date, no embry-
onic conchs of Triassic heteromorphs have been reported
(De Baets et al., 2015c; Laptikhovsky, Nikolaeva & Rogov,
2018), but adults are generally small suggesting low fecundity.
The size of Spiroceras hatchlings is 0.8 mm (Landman,
Tanabe & Shigeta, 1996) and falls within the 95% condence
interval for all Jurassic ammonoids (De Baets et al., 2015c).
Heteromorphs have the smallest embryonic conchs of all Cre-
taceous ammonoids with median measurements of 0.8 mm,
which are smaller than in Ammonitina (0.9 mm), Phyllocera-
tina (0.9 mm), and Lytoceratina (1.2 mm; De Baets et al.,
2015c). It is tempting to attribute this to constraints imposed
by adult size and mobility, which probably drove selection
towards an increasing number of smaller hatchlings that are
better adapted to a planktic life habit (e.g. Laptikhovsky
et al., 2018; Tajika et al., 2018, 2020). However, additional
data comparing adult and embryonic sizes and distributions
will be necessary to corroborate this hypothesis.
(5) Intraspecic variability
Herein, we use the six archetype morphologies (orthocone,
gyrocone, cyrtocone, ancylocone, torticone, and vermicone)
rather than the 44 subtypes distinguished by Kakabadze
Fig. 10. Habitat depth reconstruction based on oxygen isotope (δ
18
O) values collected from well-preserved, coeval monomorphs,
heteromorphs, benthic faunas, and planktic faunas. Modied after (A) Ferguson et al. (2019), (B) He et al. (2005), (C) Landman
et al. (2012b), (D) Linzmeier et al. (2018), (E) Moriya et al. (2003), and (F) Sessa et al. (2015), VPDB, Vienna Pee Dee belemnite.
Biological Reviews (2021) 000000 © 2020 The Authors. Biological Reviews published by John Wiley & Sons Ltd on behalf of Cambridge Philosophical Society.
20 René Hoffmann et al.
(2015); Figs. 1F, 11; see also Section III). Phenotypic varia-
tion in conch shape, ornamentation, size, and suture line is
common in ammonoids (De Baets et al., 2015a). Intraspecic
variation rarely has been considered in ammonoid taxon-
omy, even less so in heteromorphs (Ropolo, 1995;
Kakabadze, 2004, 2015; Witts et al., 2020), where it might
partially relate to difculties in quantitatively analysing and
comparing shape variation in orthoconic, gyroconic, or torti-
conic forms. As in monomorphs, intraspecic variation can
be continuous between two or more extremes (e.g. De Baets,
Klug & Monnet, 2013b) or discontinuous between two or
more morphs (e.g. Bert, 2013). More general principles like
an association between strength of ornamentation and whorl
cross section (Buckmans law of covariation) illustrate com-
mon ancestry and growth principles among ammonoids
and molluscs as a whole (Monnet, De Baets & Yacobucci,
2015b). Various authors have argued that intraspecic varia-
tion might be higher in heteromorphs than in their mono-
morph relatives (e.g. Dietl, 1978; Kakabadze, 2004; De
Baets et al., 2013b). This might make sense as coiling in het-
eromorphs is less constrained by the previous whorl and
might even deviate from the planispiral plane as in gastro-
pods. In the most extreme examples like Spiroceras, orthoco-
nic, crioconic, and trochospiral coiling occurs within one
species (e.g. Dietl, 1978). The large degree of variation and
differences between different sites have been used to argue
that Spiroceras was pseudoplanktic (Westermann, 1996). Var-
ious environmental factors could explain the degree of phe-
notypic plasticity, and the association of the mode and
degree of variation in relationship to environmental factors,
partially reected in the litho- and biofacies, still need to be
analysed quantitatively. Differences in coiling and ornamen-
tation occur in gastropods, even within the same species, and
are related to differences in environmental factors (Urdy,
2015). Novel methods will allow us to study intraspecic var-
iation quantitatively in orthoconic, gyroconic, and vermi-
form shapes (Okamoto, 1988ab,c, 1996; Tsujino, Naruse &
Maeda, 2003; De Baets et al., 2013a; Urdy, 2015; Ward
et al., 2015; Hoffmann et al., 2019). Even taxa that seemingly
grow chaotically like Nipponites have been demonstrated to
have a regular growth pattern consistent with a free-living,
pelagic life habit (Okamoto, 1988c; Peterman, Mikami &
Inoue, 2020c). In many cases, more traditionally dened spe-
cies have been synonymised, leading to more comprehensive
analyses of drivers of the mode and degree of intraspecic
variation in different taxa (Ward et al., 2015; De Baets et al.,
2015b; Hoffmann et al., 2019).
Ammonoid conch characters vary ecophenotypically with
environmental factors such as water depth and energy
(Landman, n.d.; Landman & Waage, 1993a; Jacobs et al.,
1994; Ikeda & Wani, 2012; Yahada & Wani, 2013;
Lukeneder, 2015; Klein & Landman, 2019). Among hetero-
morphs, only scaphitid conch characters have been quantita-
tively shown to vary with environment, which is likely linked
to their nektobenthic life habit (Landman et al., 2012a). Other
clades, such as baculitids, appear to show variation in conch
characters with facies but this remains to be explored in detail
(N.H. Landman, personal communication). Jacobs et al.
(1994) documented the degree of whorl compression within
Turonian Scaphites whiteldi in near- and offshore facies in
the WIS. They show that whorls are more compressed in
nearshore facies, and that compressed whorls were hydrody-
namically more efcient in these higher-energy environ-
ments. Landman (n.d.) showed that Hoploscaphites became
more compressed and nodose in response to a late Campa-
nian to early Maastrichtian shift from lower-energy offshore
to higher-energy nearshore environments in the WIS. Land-
man & Waage (1993a) showed how several different conch
characters vary within late Maastrichtian Hoploscaphites and
Discoscaphites in near- and offshore facies in the WIS. Hoplo-
scaphites in nearshore facies are less umbilicate, more nodose,
and have more compressed whorl sections compared to spec-
imens in offshore facies (Landman & Waage, 1993a). Discos-
caphites are also more nodose in nearshore than in offshore
facies (Landman & Waage, 1993a). These facies-linked pat-
terns indicate that intraspecic variation in conch characters
reect local adaptations of populations to the differing envi-
ronments associated with the nektobenthic life habit across
a spectrum from shallower to deeper habitats. Jacobs et al.
(1994) hypothesised that iterative evolutionary changes in
ammonoid lineages might be driven by selection for conch
characters specically adapted to environments that uctuate
through time with sea level.
VI. HETEROMORPHS THROUGH TIME
(1) Devonian heteromorphs
Ammonoids originated from orthoconic orthoceratids via
cyrtocone bactritoid ancestors during the Early Devonian
(Kröger & Mapes, 2007; Kröger et al., 2011; De Baets et al.,
2013a; Klug et al., 2015a). This is based on a stratigraphically
consistent record of evolutionary transitions in both embry-
onic and later ontogenetic stages within these lineages
(De Baets et al., 2013a; Klug et al., 2015a). Ammonoids com-
pleting at least one whorl evolved only once (Anetoceratinae
ranging from loosely coiled Metabactrites to coiled Erbenoceras
and closely related forms). However, multiple bactritoid line-
ages evolved loosely coiled representatives including a line-
age leading to Kokenia and a lineage including Cyrtobactrites
and Pseudobactrites with a mix of morphological characters.
Irrespective of these iterative coiling trends in bactritoids
around the origin of ammonoids, heteromorph ammonoids
have a short (about 6 million years) appearance during the
Palaeozoic (Klug et al., 2015a). Early Devonian ammonoids
show a rapid trend towards increased coiling in embryonic
and post-embryonic development (Klug & Korn, 2004; De
Baets et al., 2012). This rapid coiling trend has been linked
to increased swimming velocity and manoeuvrability in the
face of increased predation pressure (Klug & Korn, 2004;
Klug, 2007), although the coiling of the early whorls and
Biological Reviews (2021) 000000 © 2020 The Authors. Biological Reviews published by John Wiley & Sons Ltd on behalf of Cambridge Philosophical Society.
Heteromorph ammonoid palaeobiology 21
increased body chamber volume also inuenced fecundity
and reproductive strategies (De Baets et al., 2012; Klug
et al., 2015a). Constructional constraints might also have
played a role (Monnet, De Baets & Klug, 2011). Both biotic
and abiotic factors impacted this trend as there are both con-
vergent coiling trends within lineages and the preferential
Fig. 11. Stratigraphic distribution of the six arche-morphotypes of heteromorphic conchs from the Devonian, Triassic, Jurassic, and
Cretaceous. Also shown are secondarily monomorphic forms derived from heteromorphs. After Wright et al. (1996).
Biological Reviews (2021) 000000 © 2020 The Authors. Biological Reviews published by John Wiley & Sons Ltd on behalf of Cambridge Philosophical Society.
22 René Hoffmann et al.
extinction of loosely coiled ammonoids during Devonian
extinction events (House, 1996; De Baets et al., 2012).
Mesozoic heteromorphs repeatedly underwent convergent
uncoiling trends, which is the inverse to that observed in post-
embryonic Devonian ammonoids (Fig. 11). These forms were
not an evolutionary end stage as some show secondary trends
back to planispiral forms such as scaphitids or douvilleicera-
tids (Wiedmann, 1969; Korn, 2003). Embryonic conchs of
all Mesozoic heteromorphs are planispirally coiled unlike
those of most Early Devonian ammonoids. Torticone conchs
did not exist among Palaeozoic ammonoids. The only excep-
tion is a slightly trochospirally coiled Ivoites (De Baets et al.,
2013a), which is interpreted as a pathological phenomenon
related to in vivo encrustation (Stilkerich et al., 2017).
Early Devonian gyroconic (subtype: crioconic) hetero-
morphs (Anetoceratinae) are palaeogeographically widely
distributed from about 30N in the Kolyma basin to 45S
in the Anti-Atlas, i.e. a similar range to normally coiled
ammonoids. Other ammonoids and loosely coiled bactritoids
have a more endemic distribution, which likely indicates
poorer dispersal abilities and K-strategy selective reproduc-
tion (De Baets et al., 2012). These differences could also relate
to collection or preservation artefacts. Fragments of loosely
coiled and attened ammonoids are not considered here as
they are difcult to identify or to compare with better pre-
served specimens. The oldest heteromorphs occur 404 mil-
lion years ago (Ma) during the early Emsian, and these
lineages disappear 398 Ma at the beginning of the late
Emsian (Klug et al., 2015a). Advolute ammonoids with a
closed umbilical appear 396 Ma, while advolute forms with
an open umbilicus disappear 390 Ma in the early Eifelian.
Kokenia, another loosely coiled form, is known from the late
Eifelian but is usually considered to be a bactritoid due to
the age gap as well as differences in morphology. Loosely
coiled Palaeozoic ammonoids are therefore maximally dis-
tributed for about 16 million years (Myr) and do not reap-
pear for over 170 Myr until the Late Triassic [late Norian,
at the beginning of the Sevatian following Jenks et al. (2015)].
In summary, loose coiling was successful for a short period
in the Palaeozoic (Fig. 11) probably because it provided lim-
ited swimming capabilities. Early heteromorphs represent
evolutionarily transitional forms to the fully coiled mono-
morphs. Later, specic environmental or ecological condi-
tions allowed the convergence and longer evolutionary
success of heteromorphs in the Mesozoic. However, this
was only a post-embryonic phenomenon, highlighting the
evolutionary importance of, and constraints related to, a
coiled embryonic conch.
(2) Triassic heteromorphs
Triassic heteromorphs rarely attain sizes larger than 30 mm,
possess a coarse ornamentation comprised of simple straight
ribs, and have low diversity (about 30 species) and disparity
(morphologically stable: semi-evolute, orthocone, cyrtocone,
torticone, or monomorph trochospiral; Fig. 11) compared to
Jurassic and Cretaceous heteromorphs (Monnet, Brayard &
Brosse, 2015a). Wiedmann (1973a) considered the Triassic
four-lobed heteromorphs (Choristoceratidae) as a monophy-
letic family of ceratitic ammonoids (Wiedmann, 1969;
Shevyrev, 2005). Choristoceratidae contains the subfamilies
Rhabdoceratinae including Rhabdoceras and Peripleurites,
Choristoceratinae with Choristoceras and Vandaites, as well as
Cochloceratinae comprising Cochloceras and Paracochloceras
(Shevyrev, 2005). Krystyn & Wiedmann (1985) regarded
the Clydonitidae (Pseudothetidites praemarshi) from the Late
Norian Hallstattkalk facies as direct ancestors of the Choris-
toceratidae, a viewpoint rejected by Shevyrev (2005). Based
on a quadrilobate suture, coarse ribs, and with Rhabdoceras
as its oldest representative, Shevyrev (2005) suggested the
family Cycloceltitidae (Ophiorhabdoceras a small, monomor-
phic, coarsely ribbed form) as potential ancestors of Triassic
heteromorphs. The rst Rhabdoceras occurred at the base of
the Sevatian, a time interval that is characterised by decreas-
ing ammonoid diversity (Jenks et al., 2015). Wiedmann
(1973a) highlighted the differences in morphological variabil-
ity between the highly variable Jurassic spiroceratids and the
less variable Triassic heteromorphs.
Triassic heteromorphs occur during the Late Norian and
persist into the Rhaetian (Marshi Zone) (Fig. 11). These
occurences range from 209 to 201 Ma (aged according to
Ogg, Ogg & Gradstein, 2016) and are geographically
restricted to the Tethyan realm. Most reports come from
Europe and the Pacic coast of North and South America
(Yukon Territory to central Argentina; Shevyrev, 2005).
The paucity of reports of Late Triassic heteromorphs pre-
vented Brayard et al. (2015) from developing a biogeograph-
ical reconstruction for these ammonoids.
A life habit similar to spiroceratids was suggested for the
choristoceratids (Wiedmann, 1969, 1973a). This viewpoint
changed based on the co-occurrences of heteromorphs
together with monomorphs (phylloceratids, arcestids, mega-
phyllitids), making it likely that choristoceratids were capable
of active swimming. Based on the absence of gastropods,
Laws (1982) concluded that Triassic heteromorphs from
Nevada (USA) were epibenthic scavengers or micropreda-
tors that occupied the same ecological niche as gastropods.
Wiedmann (1973a) argued that reduced selective pressure
on the suture line occurred during the change from a nektic
to a vagile benthic life habit, expressed as reduced complexity
of suture lines. Such reduction in complexity is documented
in all heteromorphs from this period, inferring a benthic life
habit.
The extinction of choristoceratid heteromorphs is likely
related to the Rhaetian marine regression, which is associ-
ated with a sharp negative excursion in the stable carbon iso-
tope curve (Pálfy, Demény & Haas, 2001; Ward, Haggart &
Carter, 2001; Guex et al., 2003, 2004; Ward, Garrison &
Haggart, 2004). This agrees with their gradual disappear-
ance during the Rhaetian (Shevyrev, 2005). The last Triassic
heteromorph Choristoceras, went extinct together with other
monomorph ceratitids at the TriassicJurassic boundary.
Increased volcanism (i.e. ood basalts) at the Central Atlantic
Magmatic Province during the break up of Pangea (Cohen &
Biological Reviews (2021) 000000 © 2020 The Authors. Biological Reviews published by John Wiley & Sons Ltd on behalf of Cambridge Philosophical Society.
Heteromorph ammonoid palaeobiology 23
Coe, 2002; Guex et al., 2004) likely resulted in the release of
sulfate aerosols and other pollutants. These pollutants
reduced the effect of solar radiation, causing short-term cool-
ing, while the release of greenhouse gases (carbon dioxide)
could have led to long-term warming. We assume that these
pertubations had their largest effect in the uppermost water
column causing a plankton crisis. This crisis eventually
affected marine ecosystems on a global scale. Some tropitids
and choristoceratids survived into the earliest Jurassic based
on rare occurrences from basal Hettangian strata
(Longridge & Smith, 2015). Choristoceratids have been
reported from Hettangian strata in North and South Amer-
ica (Guex, 1995; Guex et al., 2004, 2012) and ?Tibet (Yin
et al., 2007).
(3) Jurassic heteromorphs
Parapatoceras distans and other spiroceratids (Dietl, 1978,
1981) have a higher intraspecic variability compared to Tri-
assic heteromorphs (Jain, 2018). Accordingly, their coiling
mode is insufcient to characterise species. Two pathways
from planispiral to orthocone conchs have been documented
for spiroceratids (Fig. 11), including radial and axial uncoil-
ing. The recognition of large intraspecic variability resulted
in a signicant reduction in the number of species from 38 to
12 (Dietl, 1978). Jurassic heteromorphs are often preserved
as fragments or small specimens. Dietl (1978) reported a frag-
ment of 40 mm whorl height indicating that they could grow
to large sizes. Similar to Triassic choristoceratids, their
sutures are simplied, and ornamentation comprises simple
straight ribs (Jain, 2018). Rows of tubercles suggest the pres-
ence of lateral and ventral spines. Mitta (2017) reported on
micro- and macroconchs with preserved apertures (lappets)
in Spiroceras (Jain, 2018).
The Spiroceratidae is composed of the two subfamilies
Spiroceratinae with Spiroceras and Parapatoceratinae, includ-
ing Parapatoceras,Paracuariceras, and Acuariceras. Due to mor-
phological resemblance, Mitta (2017) regarded Bajocia
rarinoda as potentially ancestral to Spiroceras aff. fourneti. Jain
(2018) discussed the descent of the oldest spiroceratids from
either Strenoceras or Bajocia, favouring the former. Jain (2018)
suggests that Spiroceras gave rise to Parapatoceras, and that Para-
patoceras gave rise to Epistrenoceras (a secondary monomorph)
and Paracuariceras, with the latter being the ancestor of Acuar-
iceras. This phylogeny contrasts with the hypothesis that spir-
oceratids and parapatoceratids had independent origins
(Besnossov & Kutuzova, 1990; Mitta, 2017; Galácz, 2019),
but produced morphologically similar convergent forms.
Besnossov & Kutuzova (1990) argued that the late Bajocian
heteromorph genera Apsorroceras and Spiroceras represent
macroconchs of the monomorphs Pseudogarantiana and Streno-
ceras and that all four genera evolved from the monomorph
Caumontisphinctes as members of the Spiroceratidae. Galácz
(2019) introduced the genera Kumetaceras and Sikeliceras, which
he regarded as microconchs and possible ancestors of
Parapatoceras.
The rst Jurassic heteromorphs appear during the late
Middle Bajocian (Arkelli/Bremeri Zone) with the genus Spir-
oceras and disappear with the extinction of Acuariceras during
the Early Oxfordian (Collotiformis Subzone, Athleta Zone).
According to Jain (2018) and Bert & Courville (2016), Juras-
sic heteromorphs range from 169.5 to 163 Ma (aged accord-
ing to Ogg et al., 2016). The Tithonian Protancyloceras is
hypothesised as the ancestor of Early Cretaceous hetero-
morphs (Wierzbowski, 1990; Sarti, 1999). Some species of
the genera Spiroceras and Parapatoceras are distributed world-
wide, such as S. orbignyi and S. annulatum. The latter was
recently reported from Kenya by Galácz (2017), whereas
others are more geographically restricted.
The global distribution of some species has been explained
by a planktic life habit of their hatchlings. Genera such as
Paracuariceras and Acuariceras are only known from France,
Germany, and Romania (Jain, 2018). These heteromorphs
occur frequently in clays (Hamitentone) and their conchs
show substantial morphological variability. This led to the
hypothesis that spiroceratids inhabited calm water (Dietl,
1978). A vagile benthic life habit was inferred by the large
variability of conch morphology from planispiral via helicoid
to rhabdoceratid, i.e. from bilaterally symmetric to asymmet-
ric forms within a single species. Specically, crawling loco-
motion on plants was suggested due to associated ndings
of the bivalve Posidonia(Bositra), which are thought to have
lived in macroalgae forest(Dietl, 1978).
A benthic life of spiroceratids is contradicted by records
of spiroceratids within sideritic beds devoid of benthos
except for abundant Ophiopinna elegans (ophiuroids) in the
clays of La Voulte (Dietl & Mundlos, 1972). Shevyrev
(2005) revived the benthic habitat hypothesis for spirocera-
tids with their biotopes in dense algal thickets. He argued
that their hydrostatic apparatus enabled them to change
their position within these algal mats. Since spiroceratids
rarely co-occur with planispiral ammonoids, Shevyrev
(2005) speculated that they lived in cracks in the sea bottom
in calm water. Mitta (2017), however, found spiroceratids in
Bajocian siltstone nodules together with Baculatoceras,Calli-
phylloceras,Holcophylloceras,Pseudophylloceras,andMegalyto-
ceras, i.e. forms that inhabitat the pelagic realm. Herein,
spiroceratids are regarded as having a pelagic-
nektoplanktic life habit.
The higher-level systematics of Jurassic and Cretaceous
heteromorphs remains problematic. Different schemes are
available but there is no consensus. Arkell, Kummel &
Wright (1957) included all heteromorphs in the Lytoceratina.
Wiedmann (1966a,b) then included them in the Ancylocera-
tina. Subsequently, they were split into the Turrilitina and
Ancyloceratina (Doguzhaeva & Mikhailova, 1982; Besnos-
sov & Mikhailova, 1983), which was accepted by most subse-
quent workers (see Wright et al., 1996). Later, Shevyrev
(2006) assigned them to the Lytoceratoidea and Ammoni-
tida. Vermeulen (2005) added the Protancyloceratina. These
examples reect the complexity of the interpretation of their
polyphyletic origin and evolutionary record (Matsukawa,
1987; Monks, 1999, 2000a).
Biological Reviews (2021) 000000 © 2020 The Authors. Biological Reviews published by John Wiley & Sons Ltd on behalf of Cambridge Philosophical Society.
24 René Hoffmann et al.
(4) Cretaceous heteromorphs and ammonoid
extinction
Heteromorphs have been classically linked to the extinction
of the ammonoids (Wiedmann, 1969). Their aberrant
conch shapes were often related to degeneration and phylo-
genetic extinction (e.g. Dacqué, 1935; Schindewolf, 1936,
1945; Müller, 1955). The fact that they are a dominant fau-
nal element in many Late Cretaceous ammonoid faunas
might have sustained this hypothesis. However, Mesozoic
heteromorphs were successful, particularly in the Cretaceous
(Fig. 11), as evidenced by their high abundance, generic
diversity, and morphological disparity (Seilacher, 2013). Ele-
vated generic diversity is recognised during the late Hauteri-
vian to Aptian, late Albian, and Coniacian to Campanian,
with peaks in the late Barremian, late Albian, and early Cam-
panian (Klein et al., 2007; Mikhailova & Baraboschkin,
2009). A wide variety of heteromorph conch shapes devel-
oped in the Cretaceous, but the most complex three-
dimensional shapes evolved within the Nostoceratidae with
genera like Nipponites,Pravitoceras,Eubostrychoceras,Didymoceras,
and Anaklinoceras (Wiedmann, 1969; Wright et al., 1996).
Heteromorphs are the most abundant faunal element
(mostly scaphitids and baculitids) documented at 29 out of
31 sites preserving a record from the last 0.5 Myr of the Cre-
taceous (Landman et al., 2014, 2015). New genera within the
Diplomoceratidae, Baculitidae, and Scaphitidae evolved
during the Maastrichtian (Fig. 12). The diversity of Nostocer-
atidae declined during the late Maastrichtian with only two
Nostoceras specimens known from the Maastrichtian type area
(van der Tuuk & Zijlstra, 1979).
Goolaerts (2010) concluded that the cause of the extinction
of ammonoids must have been a catastrophic event affecting
both heteromorph and monomorph diversity on a global
scale, in colder and warmer waters, in shallow and deeper
settings, and for a wide variety of taxa exhibiting different
food and life strategies. The Chicxulub impact at the Creta-
ceous/Paleogene (K/Pg) boundary (66 Ma; Schulte et al.,
2010) is currently regarded as the most probable trigger for
Fig. 12. Ranges of Maastrichtian to earliest Danian heteromorph genera. Taxonomy after Wright et al. (1996), with the omission of
Jeletzkytes and Karlwaageites following Landman et al. (2010b), Ponteixites following Grier et al. (2007), and the subgenus Tovebirkelundites
following Kennedy & Jagt (1998). In the absence of a fully integrated stratigraphic scheme for the subdivision and correlation of all
basins, the division of the Maastrichtian into the ve bins (early early, late early, early late, late late, and last 0.5 million years) is
arbitrary. Phylogenetic relationships between scaphitid genera differ among authors (e.g. Cooper, 1994; Monks, 2000a), and are
therefore not included.
Biological Reviews (2021) 000000 © 2020 The Authors. Biological Reviews published by John Wiley & Sons Ltd on behalf of Cambridge Philosophical Society.
Heteromorph ammonoid palaeobiology 25
ammonoid extinction (Ward, 1996; Goolaerts, 2010; Land-
man et al., 2014, 2015; Petersen, Dutton & Lohmann,
2016; Witts et al., 2016; Tobin, 2017).
Ammonoids have been reported from beds above the
K/Pg boundary. Most of these records reect reworked spec-
imens or resulted from an erroneously placed K/Pg boundary
level (Sadler, 1988; Landman, Johnson & Edwards, 2004a,b;
Landman et al., 2007a). However, some may belong to possi-
ble extinction event victims or even were true short-term sur-
vivors. At Tanis, North Dakota, the monomorph Sphenodiscus
is the only ammonoid recorded from beds directly correlated
to the Chicxulub impact (De Palma et al., 2019).
In Denmark, ammonoids from the Cerithium Limestone
were considered as reworked until Machalski & Heinberg
(2005) reported early Danian age inlling of the conchs,
which favoured the short-term survivor hypothesis for Hoplo-
scaphites constrictus johnjagti and Baculites vertebralis (Machalski
et al., 2009). The Cerithium Limestone was deposited
between 40 and 500 kyr after the K/Pg boundary
(Rasmussen, Heinberg & Håkansson, 2005).
In the type Maastrichtian area (The Netherlands, Bel-
gium), ammonoids were found up to 2 m above the K/Pg
boundary (Goolaerts, 2010; Jagt et al., 2013; Vellekoop
et al., 2020). The fauna consists exclusively of heteromorphs:
Eubaculites latecarinatus,Baculites spp. and H. constrictus johnjagti
(Landman et al., 2014, 2015). These are regarded as true sur-
vivors (Vellekoop et al., 2020), but in contrast to Denmark,
the ammonoids range in age from hundreds to thousands of
years after the K/Pg boundary.
In New Jersey (USA), non-reworked ammonoids were
found above an iridium anomaly, which likely represents
the K/Pg boundary (Landman et al., 2007a, 2010a, 2012c,
2014, 2015). In the Pinna Layer, which contains the iridium
anomaly at its base, the fauna is dominated by the hetero-
morphs Discoscaphites and Eubaculites. In the overlying Bur-
rowed Unit, Discoscaphites and Eubaculites have also been
recorded.
Tajika et al. (2018, 2020)discussed the ecological framework
of the ammonoid extinction at the K/Pg boundary. Tajika
et al. (2018) suggested ecological replacement of the micro- to
mesoplanktic ammonoid (and belemnite) hatchlings by holo-
planktic gastropods in the Paleogene. Tajika et al. (2020)
hypothesised that a combination of higher metabolic rates, a
microphageous diet, and small hatchling size (i.e. limited
resources) likely caused the selective extinction of the ammo-
noids. By contrast, nautilids and coleoids with different metab-
olism, macrophageous to scavenging diets, and varying
reproductive strategies survived the K/Pg mass extinction.
VII. CONCLUSIONS
(1) This review of published data on the soft body, conch
organisation, habitat, and palaeoecology of hetero-
morph ammonoids has resulted in a detailed and
robust reconstruction of their palaeobiology. It
should provide a useful background for those study-
ing the life habits of extinct animals and more specif-
ically heteromorphs.
(2) The digestive tract of heteromorphs with an oesoph-
agus, crop, and stomach, follows the molluscan U-
shaped bauplan. Their brain was likely well devel-
oped as in other cephalopods, because of the need
to control their position in the water column and to
process information provided from their lens eyes
and statocysts. Based on phylogenetic bracketing,
we assume that heteromorphs had 10 arms and
lacked suckers, hooks, and tentacles (extendable
arms). Heteromorphs had no hood and ink sac but
probably had ammonia in their soft tissues, which
resulted in the rapid decay of soft parts, explaining
their absence in the fossil record.
(3) The buccal apparatus in heteromorphs shares the
general pattern of ammonoid jaws: a smaller upper
jaw inserted into a longer and larger lower jaw. All
Cretaceous heteromorphs share an aptychus-type
lower jaw with a lamellar calcitic covering. Aptychi
may have fullled multiple functions: lower jaw,
operculum, ltering device, ushing benthic prey,
pumping for jet propulsion, regulating conch posi-
tion, and stabilisation against rocking.
(4) Differences in the morphology and size of radular
teeth reported for heteromorphs suggest preferences
for microphagous prey (i.e. zooplankton).
(5) Based on functional morphological and phylogenetic
constraints, the presence of muscles such as the
cephalic retractors, hyponome retractors, ventral
muscle, and mantle musculature are assumed for het-
eromorphs. Homologisation of ammonoid muscles
and their attachment sites with those of coleoids and
nautilids will require soft tissue preservation.
(6) Heteromorphs could achieve nearly neutral buo-
yancy regardless of conch shape or ontogeny. This
was potentially achieved by adding uid to a posi-
tively buoyant conch or counteracting slight negative
buoyancy through swimming. Encrustation by epizo-
ans like barnacles, brachiopods, and bivalves sup-
ports the buoyant heteromorph hypothesis and
contradicts the idea of a benthic habitat.
(7) A vertical orientation for orthoconic heteromorphs has
been proposed based on physical and virtual models.
Ancylocone conchs likely had a near-horizontal aper-
ture pointing upwards. Heteromorphs with a U-
shaped body chamber (nostocones, scaphiticones,
and ancylocones) are more stable hydrodynamically
than modern Nautilus and were unable to modify sub-
stantially their orientation by active locomotion,
i.e. had no or limited access to benthic prey. These
forms have their hyponomes horizontally aligned with
their centres of rotation, allowing more energy to be
transmitted to translational movement with minimal
rocking, i.e. these morphotypes are well suited for
backwards horizontal swimming at maturity.
Biological Reviews (2021) 000000 © 2020 The Authors. Biological Reviews published by John Wiley & Sons Ltd on behalf of Cambridge Philosophical Society.
26 René Hoffmann et al.
(8) Pathologies reported for ancylocone scaphitids were
likely inicted by crustaceans, sh, marine reptiles,
and other cephalopods. Injuries in baculitids have
been assigned to pycnodontid sh, coleoids, and ben-
thic crustaceans. Pathologies of Ptychoceras corrobo-
rate an external shell and reject the endocochleate
hypothesis for this taxon.
(9) Stomach contents of Cretaceous heteromorphs com-
prise planktic crustaceans, gastropods, and crinoids
suggesting a zooplanktic diet. Forms with a U-shaped
body chamber (ancylocone) are regarded as suspen-
sion feeders, whereas orthoconic conchs might have
had access to benthic prey. A carnivorous diet for het-
eromorphs is supported by the fact that all modern
cephalopods are carnivorous.
(10) Heteromorphs are known from a broad range of
litho- and biofacies. These facies associations
change through time among and within respective
clades. Devonian, Triassic, and Jurassic hetero-
morphs show a preference for deep-subtidal to off-
shore facies but are rare in shallow-subtidal, slope,
and bathyal facies. Early Cretaceous heteromorphs
preferred deep-subtidal to bathyal facies but are
rare in shallow-subtidal facies. Late Cretaceous het-
eromorphs are common in shallow-subtidal to off-
shore facies but rare in slope and bathyal facies.
The increase in heteromorph species in shallow-
subtidal settings during the Late Cretaceous likely
reects an evolutionary shift to shallower-water
environments.
(11) Adult Discoscaphites and Baculites show oxygen iso-
tope values supporting an interpretation of rapid
growth and a demersal habitat. Analyses of the ear-
liest whorls of Hoploscaphites suggest a benthic
embryonic stage, planktic hatchlings, and a habitat
change after about one whorl. Carbon isotope
values derived from Baculites from the Western Inte-
rior Seaway indicate that some heteromorphs lived
on or close to cold seeps throughout their lives and
likely exploited the diverse prey available at the
seeps.
(12) Heteromorphs have the smallest hatchlings of all
Cretaceous ammonoids. Constraints imposed by
adult size and mobility potentially drove selection
for more and/or smaller hatchlings adapted to a
planktic life. This implies high fecundity and an eco-
logical role of the hatchlings as micro- and
mesoplankton.
(13) Various environmental factors, partially reected in
the litho- and biofacies, explain the large degree of
intraspecic variation in heteromorphs. So far, only
scaphitid conch characters have been shown to vary
with environment, in accordance with their nekto-
benthic life habit, e.g. whorls being more com-
pressed, more umbilicate, and less nodose in
nearshore facies compared to specimens from off-
shore facies.
(14) The Maastrichtian is noted for the evolution of new
ammonoid genera and at many latest Cretaceous
localities, heteromorphs are the most abundant
macrofaunal element. The cause of the extinction of
ammonoids must have been a catastrophic event
affecting their diversity on a global scale and for a
wide variety of organisms exhibiting different life
habits. The Chicxulub impact at the K/Pg boundary
(66 Ma) is the likely trigger for their nal extinction.
(15) Post-K/Pg-boundary ammonoid survivors are all
heteromorphs, which likely survived up to 40500
kyr after the mass extinction event.
(16) The heteromorph and monomorph ammonoid
extinction is linked to their small hatchling sizes,
planktotrophic diets, and high metabolic rates. The
end-Cretaceous event caused a collapse of marine
primary producers, which allowed only cephalopods
with sufcient energy reserves, macrophageous diet,
and low metabolic rates to survive.
VIII. ACKNOWLEDGEMENTS
We are grateful to Alexander Lukeneder and two anony-
mous reviewers for constructive comments that signicantly
improved the quality of this review, and to John Welch and
Alison Cooper for the handling of this manuscript.
A.M. acknowledges nancial support from RFBR grant
no. 18-05-01070, the Russian amateur palaeontologists
Marina and Andrei Belousko and Dmitry Vartanian for the
donation of Ptychoceras specimens for this study, and Dmitry
Vinogradov for permission to photograph specimens from
his collection. S.G. received funding from the Belspo
BR/175/A2/CHICXULUB project Chicxulub 2016
IODP-ICDP deep drilling: From cratering to mass-extinc-
tion. J.S. acknowledges Garett M. Brown, Peter J. Harries,
Keith P. Minor, Neil H. Landman, and Ashley L. Sandness.
R.H. is grateful to Kevin Stevens for helpful comments on
an early draft, Phyllis Mono for help with formatting, Gernot
Nehrke for the Raman images shown in Fig. 2, and M. Rogov
for helping us to nd the originator of the term hetero-
morph. Images of injured specimens were provided by Sönke
Simonsen for Aegocricoeras raricostatum, Wolfgang Grulke for
Pictetia, Kurt Wiedenroth for Aegocrioceras. R.H. thanks the
DFG for nancial support of this project (DFG HO
4674/4-1). C.K. received nancial support from the Swiss
National Science foundations (project nr. 200020_169847,
200021_149119, and 200020_169627). Open access funding
enabled and organized by Projekt DEAL. Open access fund-
ing enabled and organized by Projekt DEAL.
IX. REFERENCES
Arkadiev, V. V. (2008). Representatives of the family Bochianitidae (Ammonoidea)
from the Lower Cretaceous of the Crimean Mountains. Paleontological Journal 42,
1826.
Biological Reviews (2021) 000000 © 2020 The Authors. Biological Reviews published by John Wiley & Sons Ltd on behalf of Cambridge Philosophical Society.
Heteromorph ammonoid palaeobiology 27
Arkell, W. J.,Kummel, B. &Wright, W. (1957). Mesozoic Ammonoidea. In Treatise
on InvertebratePalaeontology, Part L, Mollusca4. Cephalopoda, Ammonoidea (ed.R. C. MOORE),
pp. 80465. Geological Society of America & University of Kansas Press, Boulder.
Arkhipkin, A. I. (2014). Getting hooked: the role of a u-shaped body chamber in the
shell of adult heteromorph ammonites. Journal of Molluscan Studies 80, 354364.
Auclair, A.-C.,Lecuyer, C.,Bucher, H. &Sheppard, S. M. F. (2004). Carbon
and oxygen isotope composition of Nautilus macromphalus: a record of thermocline
waters off New Caledonia. Chemical Geology 207,91100. https://doi.org/10.1016/
j.chemgeo.2004.02.006.
Balinski, A. (2010). First colour-patterned strophomenide brachiopod from the
earliest Devonian of Podolia, Ukraine. Acta Palaeontologica Polonica 55, 695700.
Baraboschkin, E. Y. &Enson, K. V. (2003). Paleobathymetry of the Valanginian-
Aptian Basin in the Crimean Mountains based on strength indices of ammonoid
shells. Vestnik Moskov University Series 4 Geology 4,817.
Barnard, W. &de Waal, D. (2006). Raman investigation of pigmentary molecules in
the molluscan biogenic matrix. Journal of Raman Spectroscopy 37, 342352.
Barragan, R.,Gonzalez-Arreola, C. &Villasen
˜or, A. B. (2004).
Palaeoecological signicance of Barremian ammonite assemblages and facies
variations from Southwest Mexico. Lethaia 37, 223234.
Batt, R.J. (1986). A test of the effects of paleoecological factors on the distribution of
ammonite shell morphotypes. Greenhorn Cyclothem,Cretaceous Western Interior Seaway.
Cretaceous Biofacies of the Central Part of Western Interior Seaway: A Field Guidebook,1652.
Batt, R. J. (1989). Ammonite shell morphotype distributions in the Western Interior
Greenhorn Sea and some paleoecological implications. PALAIOS 4,3242.
Bayer, U. (1970). Anomalien bei Ammoniten des Aaleniums und Bajociums und ihre
Beziehung zur Lebensweise. Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen
135,19
41.
Berry, K. (2018a). Baculites taphonomy: a new technique for identifying potential cases
of long-distance post-mortem dispersal in Cretaceous ammonites. In Fossil Record 6:
New Mexico Museum of Natural History and Science Bulletin (Volume 79, eds S. G. LUCAS
and R. M. SULLIVAN Albuquerque, NM: New Mexico Museum of Natural History
and Science), pp. 5154.
Berry, K. (2018b). On the palaeoecology of the index ammonites Baculites (Lamarck,
1799) and Scaphites (Parkinson, 1811) as revealed by a study of relatively rare
epizoans. Neues Jahrbuch für Geologie und Paläontologie Abhandlungen 288, 221226.
Bert, D. (2013). Factors of intraspecic variability in ammonites, the example of
Gassendiceras alpinum (dOrbigny, 1850) (Hemihoplitidae, upper Barremian). Annales
de Paléontologie 100, 217236.
Bert, D. &Courville, P. (2016). First record of late Callovian to early Oxfordian
heteromorph ammonites. Annales de Paléontologie 102, 117121.
Besnossov, N. V. &Kutuzova, V. V. (1990). Systematic of the Middle Jurassic
heteromorphic ammonites. Paleontological Zhurnal 1990,2330.
Besnossov, N. V. &Mikhailova, I. A. (1983). Evolution of the Jurassic-Cretaceous
Ammonoids. Doklady Akademii Nauk SSSR 269, 733737.
Birkelund, T. (1965). Ammonites from the Upper Cretaceous of West Greenland.
Meddelser om Gronland, Kommisionen for Videnskabelige Undersögelser I Grönland 179,1192.
Bragg, W. H. &Bragg, W. L. (1913). The reexion of x-rays by crystals. Proceedings of
the Royal Society of London A 88, 428438.
Brayard, A.,Escarguel, G.,Monnet, C.,Jenks, J. F. &Bucher, H. (2015).
Biogeography of Triassic ammonoids. In Ammonoid Paleobiology: From Macroevolution
to Paleogeography. Topics in Geobiology (Volume 44, eds C. KLUG,D.KORN,K.DE
BAETS,I.KRUTA and R. H. MAPES), pp. 359426. Springer, Dordrecht.
Breitkreutz, H.,Diedrich, R. &Metzdorf, R. (1991). Fossilfunde aus der
Schwarz-bunten Wechselfolge (Ob. Cenoman bis Unter Turon) des
Ostwestfalendammes bei Bielefeld. Bericht des Naturwissenschaftlichen Vereins für
Bielefeld und Umgegend 32,3748.
Casey, R. (1980). A monograph of the Ammonoidea of the Lower Greensand, part 9.
Monograph of the Palaeontographical Society 133, 633660.
Cecca, F. (1997). Late Jurassic and Early Cretaceous uncoiled ammonites; trophism-
related evolutionary processes. Comptes rendus de lAcadémie des Sciences 325, 629634.
Chamberlain, J. A.,Ward, P. D. &Weaver, J. S. (1981). Post-mortem ascent of
Nautilus shells: implications for cephalopodpaleobiogeography.Paleobiology 7,494509.
Checa, A. G.,Okamoto, T. &Keupp, H. (2002). Abnormalities as natural
experiments: a morphogenetic model for coiling regulation in planispiral
ammonites. Paleobiology 28, 127138.
Chlupa
´cˇ,I.&Turek, V. (1983). Devonian goniatites from the Barrandian area,
Czechoslovakia. Rozpravy, dstredniho dstavu geologickkho 46,1159.
Clarke, M. R. &Maddock, L. (1988). Beaks of Living Coleoid Cephalopoda, pp. 123131.
Academic Press, Cambridge.
Clements, T.,Colleary, C.,De Baets, K. &Vinther, J. (2017). Buoyancy
mechanisms limit preservation of coleoid cephalopod soft tissues in Mesozoic
Lagerstätten. Palaeontology 60,114.
Closs, D. (1967a). Goniatiten mit Radula und Kieferapparat in der Itararé Formation
von Uruguay. Paläontologische Zeitschrift 41,19
37.
Closs, D. (1967b). Upper Carboniferous anaptychi from Uruguay. Ameghiniana 5,
145148.
Cobban, W. A. (1993). Diversity and distribution of Late Cretaceous ammonites,
Western Interior,United States.Geological Association of Canada, Special Paper 39, 435451.
Cobban, W. A.,Kennedy, W. J. &Scott, G. R. (1993). Upper Cretaceous
heteromorph ammonites from the Baculites compressus zone of the Pierre shale in
north-Central Colorado. US Geological Survey Bulletin 2024,A1A11.
Cohen, A. S. &Coe, A. L. (2002). New geochemical evidence for the onset of
volcanism in the Central Atlantic Magmatic Province environmental changes at
the Triassic-Jurassic boundary. Geology 30, 267270.
Company, M. (1987). Los ammonites del Valanginiense del sector oriental de las
Cordilleras Bética (SE de Espana). Tesis Doctoral (Univ. Granada, 1987).
Cooper, M. R. (1994). Towards a phylogenetic classication of the Cretaceous
ammonites. III. Scaphitaceae. Neues Jahrbuch für Geologie und Paläontologie,
Abhandlungen 193, 165193.
Dacque
´,E.(1935). Organische Morphologie und Paläontologie, p. 476. Borntraeger, Berlin.
Davis, R. A.,Mapes, R. H. &Klofak, S. M. (1999). Epizoa on externally shelled
cephalopods. In Fossil Cephalopods: Recent Advances in their Study (eds A. V. ROZANOV
and A. A. SHEVYREV), pp. 3251. Russian Academy of Sciences, Palaeontological
Institute, Moscow.
De Baets, K.,Klug, C.,Korn, D. &Landman, N. H. (2012). Early evolutionary
trends in ammonoid embryonic development. Evolution 66, 17881806.
De Baets, K.,Klug, C.,Korn, D.,Bartels, C. &Poschmann, M. (2013a). Emsian
Ammonoidea and the age of the Hunsrück slate (Rhenish Mountains, Western
Germany). Palaeontographica, Abteilung A 299,1113.
De Baets, K.,Klug, C. &Monnet, C. (2013b). Intraspecic variability through
ontogeny in early ammonoids. Paleobiology 39,7594.
De Baets, K.,Bert, D.,Hoffmann, R.,Monnet, C.,Yacobucci, M. M. &
Klug, C. (2015a). Ammonoid intraspecic variability. In Ammonoid Paleobiology:
From Anatomy to Ecology. Topics in Geobiology (Volume 43, eds C. KLUG,D.KORN,
K. DEBAETS,I.KRUTA and R. H. MAPES), pp. 359426. Springer, Dordrecht.
De Baets, K.,Keupp, H. &Klug, C. (2015b). Parasites of ammonoids. In Ammonoid
Paleobiology: From Anatomy to Ecology. Topics in Geobiology (Volume 43, eds C.
KLUG,D.KORN,K.DEBAETS,I.KRUTA and R. H. MAPES), pp. 837875.
Springer, Dordrecht.
De Baets, K.,Landman, N. H. &Tanabe, K. (2015c). Ammonoid embryonic
development. In Ammonoid Paleobiology: From Anatomy to Ecology. Topics in
Geobiology (Volume 43, eds C. KLUG,D.KORN,K.DEBAETS,I.KRUTA and
R. H. MAPES), pp. 113205. Springer, Dordrecht.
De Palma, R. A.,Smit, S.,Burnham, D. A.,Kuiper, K.,Manning, P. L.,
Oleinik, A.,Larson, P.,Maurrasse, F. J.,Vellekoop, J.,Richards, M. A.,
Gurche, L. &Alvarez, W. (2019). A seismically induced onshore surge deposit
at the KPg boundary, North Dakota. Proceedings of the National Academy of Sciences of
the United States of America 116, 81908199.
Denton, E. J. &Gilpin-Brown, J. B. (1966). On the buoyancy of the pearly nautilus.
Journal of the Marine Biological Association of the United Kingdom 46, 723759.
Dietl, G. (1978). Die heteromorphen Ammoniten des Doggers. Stuttgarter Beiträge zur
Naturkunde Serie B (Geologie und Paläontologie) 33,197.
Dietl, G. (1981). Über Paracuariceras und andere heteromorphe Ammoniten aus dem
Macrocephalen-Oolith (Unter-Callovium, Dogger) des Schwabischen Juras.
Stuttgarter Beiträge zur Naturkunde Serie B (Geologie und Paläontologie) 76,115.
Dietl, G. &Mundlos, R. (1972). Ökologie und Biostratinomie von Ophiopinna elegans
(Ophiuroidea) aus dem Untercallovium von La Voulte (Südfrankreich).Neues
Jahrbuch für Geologie und Paläontologie Monatshefte 1972, 449464.
Doguzhaeva, L. A. &Mapes, R. H. (2015). The body chamber length variations
and muscle and mantle attachments in ammonoids. In Ammonoid Paleobiology
Volume: From Anatomy to Ecology. Topics in Geobiology (Volume 43, eds C. KLUG,
D. KORN,K.DEBAETS,I.KRUTA and R. H. MAPES), pp. 545584. Springer,
Dordrecht.
Doguzhaeva, L. A. &Mikhailova, I. (1982). The genus Luppovia and the phylogeny
of Cretaceous heteromorphic ammonoids. Lethaia 15,5565.
Doguzhaeva, L. A. &Mikhailova, I. (2002). The jaw apparatus of the
heteromorphic ammonite Australiceras whitehouse, 1926 (Mollusca: Cephalopoda)
from the Aptian of the Volga region. Doklady Biological Sciences 382,3840.
Doguzhaeva, L. A. &Mutvei, H. (1989). Ptychoceras - a heteromorphic lytoceratid
with truncated shell and modied ultrastructure (Mollusca: Ammonoidea).
Palaeontographica A 208,91121.
Doguzhaeva, L. A. &Mutvei, H. (1993). Shell ultrastructure, muscle-scars, and
buccal apparatus in ammonoids. Geobios 26, 111119.
Doguzhaeva, L. A. &Mutvei, H. (2015). The additional external shell layers
indicative of endocochleate experimentsin some ammonoids. In Ammonoid
Paleobiology: From Anatomy to Ecology. Topics in Geobiology (Volume 43, eds C.
KLUG,D.KORN,K.DEBAETS,I.KRUTA and R. H. MAPES), pp. 585609.
Springer, Dordrecht.
Doguzhaeva, L. A.,Mapes, R. H. &Mutvei, H. (2002). Shell morphology and
ultrastructure of the early Carboniferous Coleoid Hematites Flower & Gordon,