ChapterPDF Available

Abstract and Figures

In order to put the origin of the Ammonoidea into the broader evolutionary context, we review the hypothesis on the origin of cephalopods in general, the origin of bactritids as well as the origin of bactritids with their respective Bauplan characters. We also list major morphological changes that occurred between the origin of cephalopods until the early evolution of ammonoids.
Content may be subject to copyright.
Chapter 1
Ancestry, Origin and Early Evolution
of Ammonoids
Christian Klug, Björn Kröger, Jakob Vinther, Dirk Fuchs and
Kenneth De Baets
© Springer Science+Business Media Dordrecht 2015
C. Klug et al. (eds.), Ammonoid Paleobiology: From macroevolution to paleogeography,
Topics in Geobiology 44, DOI 10.1007/978-94-017-9633-0_1
C. Klug ()
Paläontologisches Institut und Museum, University of Zurich, Karl Schmid-Strasse 6,
8006 Zurich, Switzerland
B. Kröger
Finnish Museum of Natural History, University of Helsinki, 44 (Jyrängöntie 2), 00014 Helsinki,
J. Vinther
Schools of Earth and Biological Sciences, University of Bristol, Woodland Road, BS8 1UG,
Bristol, UK
D. Fuchs
Earth and Planetary System Science, Department of Natural History Sciences, Hokkaido
University, Sapporo, Japan
K. De Baets
GeoZentrum Nordbayern, Fachgruppe PaläoUmwelt, Universität Erlangen, Loewenichstr. 28,
91054 Erlangen, Germany
1.1 Introduction
The phylogeny of most of the major cephalopod clades has been reconstructed with
some confidence using morphological, developmental and molecular data in the
last decades and some general macroevolutionary patterns are beginning to crystal-
ize (e.g., Dzik 1981, 1984; Woodruff et al. 1987; Engeser 1996; Young et al. 1998;
Peterson et al. 2004; Kröger 2005; Bergmann et al. 2006; Strugnell et al. 2006;
Strugnell and Nishiguchi 2007; Bizikov 2008; Shigeno et al. 2008, 2010; Kröger
et al. 2011; Warnke et al. 2011). Undoubtedly, the sister group of cephalopods lies
within the Mollusca, although the sister group of cephalopods is under debate.
Never theless, it appears like the monoplacophorans are the best candidate as extant
4 C. Klug et al.
sister group (Kröger et al. 2011 and references therein). In contrast to earlier views,
the oldest generally accepted cephalopod fossil is Plectronoceras cambria Walcott,
1905 from the middle Late Cambrian (Glaessner 1976; Dzik 1981; Kröger 2007;
Mutvei et al. 2007; Landing and Kröger 2009; Mazurek and Zatoń 2011; Kröger
et al. 2011). It possessed a small (< 2 cm) simple breviconic (short conical) shell
with a subventral (‘posterior’ sensu Kröger 2007) siphuncle and about ten septa
(Webers and Yochelson 1989). Still in the Late Cambrian, the early cephalopods
underwent an explosive radiation that continued and intensified in the Ordovician
(Kröger 2007). An important clade of cephalopods, the Orthocerida from which
all living cephalopods and the Ammonoidea are derived at the end of the Silurian,
originated already in the Early Ordovician (Kröger et al. 2011).
The transition from the Orthocerida via the Bactritida to the Ammonoidea has been
documented in detail recently (Kröger and Mapes 2007). According to fossil evidence,
the Bactritida had originated already in the earliest Emsian. This phylogenetic event
was followed by the origin and radiation of ammonoids in a geologically abbreviated
amount of time (Erben 1960, 1964a, b, 1965, 1966; Becker and House 1994; House
1996; Klug et al. 2008; Kröger 2008b; De Baets et al. 2010, 2013b; Frey et al. 2014).
In this chapter, we will discuss the origin of cephalopods and ammonoids as well
as their respective Bauplans. Important evolutionary events and morphological in-
novations around these originations are also listed.
1.2 Phylogenetic Position of the Ammonoids
in the Cephalopod Tree
Most cephalopod workers agree on the Cambrian origin of cephalopods, that they
were ectocochleate (externally shelled) and that the shell was chambered (Dzik 1981,
1984; Holland 1987; Engeser 1996; Shigeno et al. 2008, 2010; Kröger et al. 2011).
There is also a wide agreement that the cephalopods evolved in one way or the other
from a group of monoplacophorans (Yochelson et al. 1973; Pojeta 1980; Dzik 1981;
Kröger 2007; Webers and Yochelson 1989). By contrast, Brock and Paterson (2004)
as well as Peel (1991) sought for the origin of cephalopods in the Helcionellida.
Dzik (1981, 2010) thought that possibly, the cephalopods root in the Circothecidae
(Hyolithida), although this hypothesis was rejected by Landing and Kröger (2012).
Thus the origin of cephalopods among Cambrian molluscs is still not settled firmly.
Pojeta (1980) suggested that the snorkel-like process of the curved shell of the
monoplacophoran Yochelcionellidae might have evolved into the plectronocerid sip-
huncle. Dzik (1981) hypothesized that the first cephalopods might have taken off
from the sediment by secreting a salt-depleted and thus lighter liquid in the apex. This
hypothesis is indirectly corroborated by the fact that the water is osmotically removed
from newly formed chambers in nautilids (Ward 1979). Subsequently, the phrag-
mocone evolved by a beginning alternation of liquid- and shell-secretion. The final
physiological step in the phragmocone evolution was according to Dzik (1981) the
increasing chamber pressure produced by the ionic pump, thus allowing gas diffusion.
51 Ancestry, Origin and Early Evolution of Ammonoids
In accordance with Kröger et al. (2011), we favor the hypothesis that the close
ancestors of cephalopods resemble Cambrian monoplacophorans (Fig. 1.1) like
Knightoconus (Yochelson et al. 1973; Webers and Yochelson 1989; Dzik 2010)
or hecionellids like Tannuella (Brock and Paterson 2004). Thus, their shells were
probably slightly curved, high and conical. However, more research on middle and
late Cambrian fossil mollusks is necessary to reliably solve this question.
An additional controversial hypothesis was introduced by Smith and Caron
(2010) with a redescription of Nectocaris from the Burgess Shale (Smith 2013).
This form looks superficially like a derived coleoid cephalopod with its lateral fins,
stalked eyes and a funnel-like structure attached to the head. The profound im-
plication was that the fossil record of cephalopods might be severely biased and
Fig. 1.1 Cephalopod phylog-
eny (modified after Kröger
et al. 2011)
6 C. Klug et al.
that the ancestral cephalopod might have resembled a coleoid. The interpretation
was quickly taken under scrutiny (Mazurek and Zaton 2011; Kröger et al. 2011;
Runnegar 2011) and criticized for several incongruences, which rejected the pre-
sumed primary homologies. Among those, they listed a closed funnel, which is
attached to the head in an organism with a straight gut. Embryology demonstrates
that the funnel evolves from the posterior part of the embryo and attains its position
adjacent to the head by dorsal folding of the body (Kröger et al. 2011). Furthermore,
the funnel is attached to the mantle, while the structure in Nectocaris is attached to
the head. Thus, this is more likely a case of superficial convergence. There is no
single unequivocal molluskan feature in Nectocaris, and it therefore seems more
reasonable to interpret this taxon as a yet unknown lophotrochozoan of unclear sys-
tematic affinity, which developed a mode of life possibly convergent with modern
squids (Kröger et al. 2011; Runnegar 2011).
1.2.1 The Cephalopod Bauplan
Since no fossilized soft parts of plectronocerids or ellesmerocerids are known so far,
all ideas on the cephalopod Bauplan are based on empirical evidence from the shell
and its soft tissue imprints (Kröger 2007) as well as inferences from the phylogenet-
ic context (Fig. 1.2). In the following, we present the autapomorphies of the cepha-
lopod Bauplan (Table 1.1) and shortly discuss the (sometimes weak) evidence for
each character state. The list is based on that of the Hypothetical Ancestral Sipho-
nopodean Cephalopod (HASC) of Engeser (1990a, 1996), which is modified here to
define the last common ancestor (an orthocerid) of the crown group of cephalopods.
1. Chambered shell with straight to slightly cyrtoconic phragmocone for buoyancy
control (see preceding paragraphs).
2. One arm crown, probably with ten arms: Since ten arms represent the ancestral
state of coleoids (e.g., Fuchs 2006; Kröger et al. 2011) and nautilids have ten
arm buds in early embryonic developmental stages (Shigeno et al. 2008, 2010), it
appears reasonable to infer this state also for the shared ancestor of coleoids and
nautilids, i.e., some Paleozoic orthocerids. It is difficult to assess the number of
arms in older forms, and since orthocerids diversified in the early Ordovician it
is not yet possible to conlusively reconstruct the number of arms in the majority
of Palaeozoic forms, although from the above data, ten arms appears to be likely.
In any case, a gastropod-like foot as proposed by Bandel (1982) and Teichert
(1948) appears unlikely. Mehl (1984) reported the possible imprints of ten arms
in Michelinoceras from the Silurian of Bolivia, but this imprint might as well be
something else.
3. Hyponome: There is no direct fossil evidence yet for the presence or absence
of a hyponome (and several other organs listed below) in early cephalopods.
From the extant phylogenetic bracket (Witmer 1995), we can extrapolate that
the hyponome was present in the common ancestor of Nautilida and Coleoidea.
There is some indication for the presence of a hyponomic sinus in the ellesmero-
71 Ancestry, Origin and Early Evolution of Ammonoids
cerids (Kröger 2007) as well as in forms that diverged from orthocerids in the
Early Ordovician. However, fossil evidence for the homologization of the hypo-
nomic sinus between plectronocerids and ellesmerocerids on the one side and
the condition in crown cephalopods on the other side is ridden with some level
of uncertainty. Nautilids have a unfused hyponome. It is derived from posterior
Fig. 1.2 Hypothesized Bauplan of a an ancestral cephalopod like Plectronoceras (based on
Kröger 2007), b the HASC, modified after Engeser (1996) and c an ancestral ammonoid like
Metabactrites fuchsi (De Baets et al. 2013b)
8 C. Klug et al.
mantle folds in the embryo. These folds are not fused in nautilids; this condition
was likely the plesiomorphic condition for the crown cephalopod ancestor.
4. Jaws: Unclear. There is no fossil evidence for cephalopod jaws older than Late
Devonian (e.g., Woodward 1885; Clausen 1969). Hence, it is the question
whether this is a taphonomic problem or whether the cephalopod jaw evolved
Table 1.1 Autapomorphies (in bold) and plesiomorphies (in regular) of the Cephalopoda, the
Siphonopodean Cephalopoda (HASC), the Bactritida and the Ammonoidea, using data from Eng-
eser (1990a, 1996). Character states, which are hypothesized based on the extant phylogenetic
bracket or extrapolations are marked in grey
Trait Cephalopoda HASC Bactritida Ammonoidea
Phragmocone Present Present Present Present
Siphuncle Subventral,
Central, narrow Ventral, narrow Ventral, narrow
Shell shape Cyrtoconic Orthoconic Orthoconic Crioconic
Cross section Subcircular Circular Slightly
Initial chamber Unknown Ovoid, small Ovoid, Small Ovoid, Small
Initial shaft angle Wide Wide Narrow Narrow
Suture line Straight Straight Ventral lobe Ventral + lateral
Muscle attachment Circular,
Circular Dorsal
Hyponomic sinus Deep Shallow or absent Moderately deep Deep
Arm crown 10 Arms 10 Arms 10 Arms 10 Arms
Hyponome Present Present Present Present
Jaws No Real Jaws Present Present Present
Internal fertilization Present Present Present Present
Copulatory organs Present Present Present Present
Brain Present Present Present Present
Direct development Present Present Present Present
Large embryo Present Present Present Present
Large coelomic
Present Present Present Present
Carnivorous life
Present Present Present Present
Crop Present Present Present Present
Nidamental glands Present Present Present Present
Pericardial glands Present Present Present Present
Needham’s sac Present Present Present Present
Crystalline style Present Present Present Present
Partially closed
blood circulatory
Present Present Present Present
91 Ancestry, Origin and Early Evolution of Ammonoids
only in the orthocerids and their phylogenetic successors (see HASC; Engeser
1996) or convergently in the Nautilida and the Bactritida plus their descen-
dants. This was already discussed shortly by Kröger et al. (2011). Presence of
at least jaw-like structures appears likely, because such possibly homologous
structures are also present in scaphopods, monoplacophorans (the supposed
sister-group of cephalopods) and some gastropods (Boletzky 2007). Remark-
ably, the upper and lower jaws are fused in early ontogenetic stages of some
coleoids. It is still conceivable that the cephalopod jaw as it is known from the
crown groups evolved only in the Middle Paleozoic orthocerids and not in the
Early Paleozoic groups. These formed perhaps part of the adaptive radiation of
crown cephalopods in the Devonian as part of the Devonian Nekton Revolu-
tion (Klug et al. 2010) and the sudden diversity of jawed vertebrates, which
they were in an escalatory arms race with. Some authors (e.g., Dzik 1981) have
considered fossils like Aptychopsis to function as both jaws and operculum
in Silurian cephalopods, but there is some indication that these can be treated
as opercula (Turek 1978; Holland et al. 1978; Holland 1987 and references
therein) or that they are homologous with later cephalopod beaks.
5. Internal fertilization and copulatory organs: The presence is inferred from the
extant phylogenetic bracket (same line of reasoning as for the hyponome).
6. Brain: The presence is inferred from the extant phylogenetic bracket (same line
of reasoning as for the hyponome).
7. Direct development of a yolk-rich egg: Although direct evidence is missing, the
record of embryonic and post-embryonic ontogeny in the shell lacks evidence
for true larval stages, thus supporting direct development.
8. Moderately large embryonic conch (compared with other molluscs, especially
monoplacophorans): There is good evidence for this from the preserved embry-
onic shells of several early Paleozoic cephalopod groups (and also monopla-
cophorans), although these are not known yet from plectronocerids.
 9. Relatively large coelomic cavity (compared with other molluscs): Same line of
reasoning as for the hyponome.
10. Carnivorous life style: The presence is inferred from the extant phylogenetic
bracket (same line of reasoning as for the hyponome); at least some injuries on
shelled organisms (Brett and Walker 2002 and references therein) and coprolite
contents point to a predatory mode of life (Botting and Muir 2012 and refer-
ences therein) of Ordovician cephalopods, but these are usually based on the
circular argument that extant and therefore fossil ones were carnivorous.
11. Crop: The presence is inferred from the extant phylogenetic bracket (same line
of reasoning as for the hyponome).
12. Nidamental glands: The presence is inferred from the extant phylogenetic
bracket (same line of reasoning as for the hyponome).
13. Pericardial glands: The presence is inferred from the extant phylogenetic
bracket (same line of reasoning as for the hyponome).
14. Needham’s sac: The presence is inferred from the extant phylogenetic bracket
(same line of reasoning as for the hyponome).
15. Crystalline style: The presence is inferred from the extant phylogenetic bracket
(same line of reasoning as for the hyponome).
10 C. Klug et al.
16. Partially closed blood circulatory system: The presence is inferred from the
extant phylogenetic bracket (same line of reasoning as for the hyponome).
In his contribution on the phylogenetic position of ammonoids, Engeser (1990a,
1996) introduced his model of the Hypothetical Ancestral Siphonopodean Cepha-
lopod (HASC). HASC (modified in Fig. 1.2) is his model of the shared ancestor of
crowngroup (i.e. Recent) cephalopods, which are all Coleoidea and Nautilida of
today. In Table 1.1, give an overview over characters of Engeser’s compilation are
listed with some minor modifications, namely the number of arms.
In his chapter on the phylogenetic position of ammonoids, Engeser (1996) also
listed the plesiomorphies supposedly present in the HASC.
1. Marine habitat. Most cephalopod fossils so far have been found in marine rocks
and such from other deposits were probably reworked.
2. Radula (possibly with nine teeth in a row, four marginalia). Comment by Eng-
eser (1996): “Campitius titanicus from the Lower Cambrian of the Westgard
Pass area, California, is a large isolated radula with 13 elements per row (Firby
and Durham 1974). Although its former “owner” is unknown, it demonstrates
that a group of molluscs with this character lived in the Early Cambrian seas.
This radula might have belonged to a stem lineage representative of the Cepha-
lopoda.” Radulae have become known from Ordovician orthoconic nautiloids
(Gabott 1999) and the Silurian orthoceratid Michelinoceras (Mehl 1984), but
the exact morphology of the radula as well as the number of teeth can not be
confidently reconstructed from these finds due to their poor preservation (Nixon
1988; Gabott 1999; Kruta et al. 2014).
3. Two gills in a pallial cavity, one pair of kidneys, and a heart with one pair of
auricles. Although others have argued that paired pathologies in shell struc-
tures might indicate that ammonoids are tetrabranchiate cephalopods like the
Nautilida as opposed to all other living cephalopods (e.g., De Baets et al. 2011,
p. 172), direct evidence for two or four gills from externally shelled cephalopod
fossils is missing still.
4. One pair of retractor muscles: Kröger (2007) studied the muscle attachment fea-
tures of the Ellesmerocerida. Potentially, the situation was more complicated
in the earliest cephalopods, perhaps including the HASC with multiple paired
muscle scars.
5. Simple pinhole eyes: Fossil evidence is missing. Extant Nautilida have pinhole
eyes which could well represent the plesiomorphic condition for cephalopods as
the outgroup has less complex photoreceptor organs. However, the pinhole cam-
era eye (as suggested by a novel molecular study: Ogura et al. 2013) might be a
specialization of the Nautilida just like the great number of arms (Shigeno et al.
2008, 2010; Sasaki et al. 2010). Ammonoids are stem coleoids and are thus situ-
ated on a lineage that evolved camera type eyes. Ocular sinuses suggest that many
shelled cephalopods had eyes and eye capsules might even be preserved in rare
cases in derived Cretaceous ammonoids (Klug et al. 2012), but these results are
inconclusive as to whether the eye was a camera or a pinhole type.
6. A single, high, conical shell with periostracum, prismatic, and nacreous layers; shell
covering the visceral mass; mineralized parts of the shell consisting of aragonite.
111 Ancestry, Origin and Early Evolution of Ammonoids
7. A pair of statocysts. No support from the fossil record so far, but is justified
based on phylogenetic bracketing.
8. Body bilaterally symmetrical.
9. Sexes separate and of roughly equal size.
10. Salivary glands. No support from the fossil record so far.
11. Two oviducts, two spermiducts. No support from the fossil record so far.
12. (?) r-selected reproductive strategy: The embryonic shells of plectronocerids
are still unknown. Taking the small size of plectronocerids into account and
the smallest known shell diameter of plectronocerid fossils (Kröger 2007),
the number of offspring was potentially not very high, possibly tens to hun-
dreds, following the reasoning for more derived cephalopods in De Baets et al.
(2013a). In the orthocerids as well as the bactritids, this was probably still the
case (De Baets et al. 2012, 2015b). It appears like the reproductive rates rose
significantly in the Ammonoidea and some Coleoidea, but it was low in the
Actinocerida, Endocerida as well as the Nautilida. Therefore, the survivorship
curves of HASC-like cephalopods were probably intermediate, i.e. a moder-
ate number of offspring combined with a moderate number of individuals that
managed to achieve sexual maturity and succeeded with reproduction.
13. (?) Planktic early life phase. The small adult size (ca. 5 mm) of Plectronoceras,
relatively great shell thickness, and numerous septa speak against a planktic
early life stage (Landing and Kröger 2012). The apex of Plectronoceras is still
unknown, but the apices of all plectronocerid descendants (with the exception
of Orthoceratida) are Nautilus-like, cap-shaped, and have high initial angles of
expansion, so that a cap shaped apical shell must be assumed for Plectronoceras
based on the similarity of the general conch form with that of ellesmerocerids,
primitive discosorids, and other descendants of plectronocerids. The small size
of embryonic shells in at least some orthocerids, bactritoids and ammonoids as
well as their facies distribution suggests a planktonic early life phase of these
forms with small, spherical initial chambers (Kröger et al. 2009; Mapes and
Nützel 2009; De Baets et al. 2012, 2015b). The oldest known spherical (ortho-
cerid) cephalopod protoconchs occur in the Early Ordovician (Tremadocian)
from Bactroceras (compare Evans 2005; Kröger 2006; Kröger and Evans 2011;
Landing and Kröger 2012).
14. (?) Blood pigment consisting of hemocyanin. Nautilus diverged from other
extant cephalopods around the Siluro-Devonian (Bergmann et al. 2006; Kröger
et al. 2011), so that it might have been present at least since then in cephalopods.
1.2.2 Position of the Bactritida and Ammonoidea
As mentioned above, coleoids and ammonoids are derived from the Bactritida
which root in the Orthocerida in the latest Silurian or earliest Devonian. The or-
thocerids form a long branch down to the earliest Ordovician (Dzik 1984; Kröger
2007, 2008a; Kröger and Mapes 2007; Kröger et al. 2011; Kröger and Lefebvre
12 C. Klug et al.
2012). With respect to synapomorphies of Orthocerida and Bactritida, one can list
the small subspherical to ovoid initial chamber, the straight to slightly bent conical
shell and the narrow siphuncle (Fig. 1.3).
Fig. 1.3 Occurrences of
embryonic shells of ortho-
cerids and bactritids in the
Paleozoic (modified after
Kröger and Mapes 2007)
131 Ancestry, Origin and Early Evolution of Ammonoids
In the course of the Silurian and Early Devonian, a ventral shift of the siphuncle
occurred in two orthocerid lineages (Fig. 1.4), one of them leading to the Bactritida
(Kröger and Mapes 2007). These lineages differ in the shape of their initial chamber
(subspherical vs. ovoid) and shaft (high vs. low apical angle). This phylogenetic
hypothesis of Kröger and Mapes (2007) opposed that of Ristedt (1968). The two
hypotheses mainly differ in the interpretation of the homeomorph evolution of a
ventral siphuncle (Kröger and Mapes 2007) and of a narrow initial shaft (Ristedt
1968). Both character states persist into the early Ammonoidea.
As in other animal groups, the similarities between the newly evolved group
and the sister group are strong close to the bifurcation. This caused a complicated
pattern of apomorphic and plesiomorphic characters in both groups (compare the
much discussed origins of arthropod stemgroups in the Cambrian). For instance,
this is reflected in the contradicting character states in the intensely disputed ge-
nus Pseudobactrites (= Bojobactrites of Horny 1956; Erben 1960; De Baets et al.
2013b), which has “a transversally ornamented shaft with a high angle of expan-
sion adapical to the initial chamber” (Kröger and Mapes 2007: p. 325). It is unclear
whether this checker pattern of character state distribution originated from pheno-
typic plasticity, intraspecific variability (compare De Baets et al. 2015a), some kind
of homoplasy (Monnet et al. 2015) or still something different.
In any case, there is not much doubt that the lineage from Devonobactrites via
Bactrites to Lobobactrites led ultimately to the first ammonoids (Erben 1964a, b,
1966; Dzik 1984; Klug 2001b; Klug and Korn 2004; Kröger and Mapes 2007; Klug
et al. 2008a, b; De Baets et al. 2009; De Baets et al. 2013a, b, 2015b). This is also
not contradicted by stratigraphy (De Baets et al. 2013b, p. 27) as the earliest known
Devonobactrites (Kröger 2008a) and Lobobactrites are found below the earliest
ammonoid finds in the early Emsian of Australia (compare Teichert 1948; Mawson
1987) and Morocco (Kröger 2008b). Further morphological changes occurred at
Fig. 1.4 Morphological changes of the embryonic shell around the origin of bactritids and ammo-
noids (modified after Kröger and Mapes 2007)
14 C. Klug et al.
the transitions from the Bacritida to the Ammonoidea (Erben 1966; De Baets et al.
2013b). With the translocation of the hyponome to the venter, the suture line began
to undulate. This induced the formation of the external lobe. The ventralization of
the hyponome possibly caused a slight dorsoventral imbalance which might have
initiated in one way or the other the increasing curvature of the shell (or vice versa),
the lateral compression of the shell cross section, the formation of lateral lobes in
the suture line (caused by the compressed section) and the tilting of the aperture
(and thus growth lines) with a deepening of the hyponomic sinus. These apomor-
phies are opposed by the plesiomorphic shape of the initial chamber and the narrow
1.3 Origin of the Ammonoidea
The most recent phylogenetic reconstructions of the origin of ammonoids and their
bactritid ancestors were published by Kröger and Mapes (2007) as well as Kröger
et al. (2011). De Baets et al. (2012, 2013a, b) discussed developmental, reproduc-
tional and morphological changes around the origin of ammonoids and their early
evolution. The stratigraphic order of ammonoids and their direct ancestors could
now be stratigraphically corroborated (Kröger and Mapes 2007; De Baets et al.
Irrespective of the phylogenetic relationships, the question for the main apomor-
phies of ammonoids arises. Classically, ammonoids have been separated from their
bactritid ancestors by the presence of at least one full whorl (e.g., House 1988).
This character, however, appears somewhat arbitrary, although coiling undoubtedly
represents an important character in this context (e.g., Kröger 2005).
The systematic positions and levels of Bactritida, Coleoidea and Ammonoidea
need to be critically revised. The phylogenetic position of some curved bactritoids
like Pseudobactrites (Kröger and Mapes 2007; showing also some similarities to
Cyrtobactrites, which might indicate closer affinity or convergence) as well as
Kokenia (Turek and Marek 1986) are still debated (compare Erben 1966; Turek
and Marek 1986; Kröger and Mapes 2007; De Baets et al. 2013b for a review). The
oldest stratigraphic occurrences of these genera are all younger than the earliest am-
monoids (Klug 2001b). In combination with their morphology, this might indicate
that Kokenia and potentially even Cyrtobactrites and Pseudobactrites represent in-
dependent lineages of coiled bactritoids, only resembling the transitional morphol-
ogy (Erben 1966; Klug 2001b; De Baets et al. 2013b). This would indicate iterative
coiling trends in bactritoids around the origin of ammonoids (see also Kröger 2005).
Therefore, only the earliest coiled Anetoceratinae and closely related more derived
ammonoids (excluding bactritoids and Kokenia) would be included in the Ammo-
noidea until better preserved material becomes known and the bactritoid/ammonoid
transition can be further refined. The bactritoids as currently defined are a paraphy-
letic group with a rather conservative morphology, which also gave rise to coleoids.
151 Ancestry, Origin and Early Evolution of Ammonoids
1.3.1 Ammonoid Bauplan and the HASC
Although often used as a model for reconstructing the Ammonoidea Bauplan,
Extant Cephalopoda should be used with caution. Nautilus has a superficial resem-
blance with ammonoids because of the external shell, but was determined as a poor
model for the appearance of ammonoids (Jacobs and Landman 1993; Ritterbush
et al. 2014). Indeed, not many features (pinhole eye, 90 arms, large embryonic shell,
hood etc.) appear to have developed specifically in the lineage leading up to extant
Nautilus (Shigeno et al. 2008, 2010; Sasaki et al. 2010; Ogura et al. 2013), poten-
tially all after their separation from the Orthocerida (Kröger et al. 2011). Coleoids,
which are more closely related to ammonoids, are not necessarily a better model for
ammonoid anatomy considering their evolution since their separation over 400 Ma
ago (Kröger et al. 2011). Even if the extremely limited information from soft-tissue
preservation of Mesozoic ammonoids is included (Klug and Lehmann 2015 Ritter-
bush et al. 2014), no further details can be added to the bauplan of ammonoids. Con-
sequently, it appears like the differences between the bauplan of ammonoids and
those of the Bactritida and the HASC (as shown in Table 1.1) are actually not very
big and limited to a few autapomorphies or slight differences in character states.
The limit between derived Bactritida such as Lobobactrites and Cyrtobactrites
on the one side and the first Ammonoidea (Anetoceratinae) such as Metabactrites,
Ivoites, Anetoceras (senior synonym of Ruanites; De Baets et al. 2009) and Erben-
oceras on the other side is additionally blurred by intraspecific variability (De Baets
et al. 2013a, b, c, 2015a), incomplete preservation (De Baets et al. 2013b, c), as well
as homoplasies (see Monnet et al. 2015 for a discussion of this phenomenon). Thus,
only the crioconic coiling comprising at least one whorl is a trait that separates the
early ammonoids from their bactritid ancestors. Less distinct characters of early am-
monoids are the more strongly sinuous sutures with external and lateral lobes, the
laterally compressed whorls and the distinct hyponomic sinus. However, all of the
latter characters are also known to some degree from a few bactritids such as Lo-
bobactrites, Cyrtobactrites (Erben 1964a, b, 1966; Dzik 1984; Klug 2001b; Kröger
2005, 2008b; Klug et al. 2008a).
Hardly anything is known with respect to the jaws or soft parts of early ammo-
noids (Korn and Klug 2003). Similarly, only very poor traces of soft tissue attach-
ment structures have become known (Kröger et al. 2005; Klug et al. 2008a, b). Klug
et al. (2008a, b) described an early ammonoid (“Metabactrites ernsti”, now consid-
ered to be belong to Ivoites: De Baets et al. 2013b, p. 35) from the early Emsian
(Devonian) of Morocco, which displays spirally arranged lines in the dorsal part of
the shell, crossing from the mural parts of the last septa onto the body chamber wall.
These track bands witness the anteriorward translocation of a soft tissue attachment
site, possibly of dorsally located muscles. It is unclear whether these dorsal muscles
are homologous to the cephalic retractor muscles of the Nautilida or not. Addition-
ally, some Early and Middle Devonian ammonoids display linear imprints on the
septa and in the plain of symmetry of the body chamber. According to Klug et al.
(2008b), these imprints may represent imprints of arteries of the septal mantle and
another artery (see also Polizzotto et al. 2015).
16 C. Klug et al.
There is no direct evidence for the presence of a hood similar to that of extant
Nautilus, but both the absence or presence of a hood or a homologues structure has
been suggested based on circumstantial evidence (compare Keupp 2000; Lehmann
et al 2015; Ritterbush et al. 2014). Although extant coleoids do not have a hood like
Nautilus, they do have a homologous structure in their early embryonic develop-
ment (compare Shigeno et al. 2008).
1.3.2 Early Evolution of Ammonoids Morphological Changes
The Early Devonian was a time, in which several new cephalopod clades of high
systematic ranks emerged such as the Bactritida, the Nautilida and the Ammonoidea
(Erben 1964a, b, 1966; Klug 2001b; Kröger and Mapes 2007; Kröger 2008b; De
Baets et al. 2009, 2010, 2013b). In addition to these important clades, several less
diverse ones evolved and within the Ammonoidea, the radiation went on at a high
pace, at least as far as shell morphology is concerned (e.g., House 1996; Korn 2001;
Korn and Klug 2003; Monnet et al. 2011; De Baets et al. 2012). This is also shown
by the co-occurrence of openly coiled to tightly coiled ammonoids within the same
beds (De Baets et al. 2010, 2013b).
It appears like the increase in coiling was the most important character complex
in the early evolution of ammonoids (Figs. 1.5, 1.6.). This holds true for the ini-
tial chamber and the shaft included in the ammonitella (Erben 1960, 1964a, 1966;
Bogoslovsky 1969; Klug and Korn 2004; Kröger 2005; Klug et al. 2008; De Baets
et al. 2012, 2013a, b), for the juvenile shell and the neanoconch until the adult shell
(Klug and Korn 2004; Kröger 2005; De Baets et al. 2012, 2013a, b). This evolu-
tionary trend in the increase in coiling is only rarely reversed; extreme examples
for evolutionary trends towards looser coiling are the Mesozoic groups of hetero-
morph ammonites (Cecca 1997; Guex 2006; Monnet et al. 2015). As pointed out by
House (1996) and De Baets et al. (2012), these reversions usually do not include the
embryonic shell (compare De Baets et al. 2015). Once, the fully coiled embryonic
shell had evolved, the umbilical window was closed and the initial chamber had
also evolved a coiled longitudinal axis, no loosely coiled embryonic shell appeared
again later in earth history. The only exception that occurred repeatedly is a certain
variation in size of the initial chamber and the embryonic shell, although the overly-
ing trend is towards a size decrease (De Baets et al. 2015). According to De Baets
et al. (2012), this size decrease of the ammonoid embryo lead to higher reproduc-
tive rates (and low survivorship numbers), because simultaneously, the ratio from
embryo size to body chamber volume decreased (compare House 1996). This latter
hypothesis of an evolutionary trend towards higher reproductive rates actually co-
incides with a number of morphological changes, which will be listed below. Natu-
rally, this is only one hypothesis out of several, which are also summarized below.
171 Ancestry, Origin and Early Evolution of Ammonoids
Fig. 1.5 Phylogeny of Emsian and Eifelian cephalopods, mainly based on Korn (2001) and Klug
(2001b); compare Korn and Klug (2003)
18 C. Klug et al.
In the previous paragraph, we stressed that one of the major morphological
changes of the ammonoid shell during the Devonian was the degree of shell coil-
ing. Coiling of ammonoids (e.g., Raup 1967) can be quantified in various ways
using several ratios and measurements, which have been discussed in Chap. 1.1
of this volume (Klug et al. 2015). Many of these parameters and ratios underwent
profound evolutionary changes already in the Early Devonian, i.e. shortly after the
origin of ammonoids. In the following, we list the main evolutionary changes that
occurred in the post-embryonic shell already within the Emsian (Early Devonian):
1. Whorl expansion rate increase from around 1.5 to values above 2.0 with extreme
values exceeding 4.0 (e.g., in Mimagoniatites and Rherisites; Klug 2001a, b).
2. Decrease of the umbilical width index from around 0.7 (e.g., in Anetoceras,
Borivites or Erbenoceras) to 0.2 (e.g., in Celaeceras or Weyeroceras; Chlupáč
Fig. 1.6 Some ammonoids
from the early Emsian, to
illustrate morphological
change early in ammonoid
phylogeny. a Erbenoceras
solitarium, GPIT 29789,
Ouidane Chebbi, Morocco.
b Metabactrites fuchsi,
PWL2010/5251-LS, Bunden-
bach (Germany). c Erben-
oceras cf. solitarium, GPIT
29806, Ouidane Chebbi,
Morocco, note the wider
space between the whorls.
d Mimosphinctes rudicosta-
tus, PIMUZ 28985, Kodzha
Kurganm Gorge, Zeravshan,
Uzbekistan. e Anetoceras
obliquecostatum, PIMUZ
29637, Achguig, Morocco
191 Ancestry, Origin and Early Evolution of Ammonoids
and Turek 1983; Bogoslovsky 1984; Klug 2001a, b; Klug and Korn 2002; Mon-
net et al. 2011; De Baets et al. 2013b).
3. Increase of the whorl height index from around 0.2 (e.g., in Anetoceras or Erben-
oceras) to 0.5 (e.g., in Celaeceras or Weyeroceras; Klug 2001a, b; De Baets et al.
4. Increase in the ratio body chamber volume to diameter (Klug 2001a).
5. Initial decrease in the strength of ornamentation (De Baets et al. 2013b).
6. Increase in sutural complexity (Wiedmann and Kullmann 1980; García-Ruíz
et al. 1990; Boyajian and Lutz 1992; Saunders and Work 1996; Daniel et al.
1997; Saunders et al. 1999; Gildner 2003; Ubukata et al. 2014).
These evolutionary changes have been discussed to differing degrees by various
authors (Erben 1964a, 1965, 1966; Bogoslovsky 1969; Kutscher 1969; House 1988;
Kröger 2005; Korn 2001; Korn and Klug 2002, 2003, 2012; De Baets et al. 2009,
2012, 2013b). It was Korn (2001), who first analyzed these morphological changes
using cladistics. His study on the phylogeny of Early and Middle Devonian ammo-
noids is still unrivalled. According to his work, these morphological changes were
more or less unidirectional, at least in the Early Devonian. Reversals in the mor-
phological evolution did occur in single parameters, but in most cases, the changes
occurred in the way listed above (Figs. 1.5, 1.6). Potential Consequences for the Mode of Life
Considering the morphological changes listed above, a number of hypotheses have
been proposed to explain these, some of which are linked with each other:
1. Saturation of the demersal habitat and increasing predatory pressure by the
explosive radiation of gnathostome fish (Kröger 2005; Klug 2007; Klug et al.
2010): As documented by Klug et al. (2010), the Early Devonian was a time,
where demersal animals decreased in relative diversity while nektonic forms
began to diversify. They explained this by a saturation of habitats on and near the
sea-floor in combination by the increasing amount of nektonic predators among
the jawed fish. This predatory pressure induced an escalatory feedback.
2. Increase in swimming speed and maneuverability (Klug 2001a; Korn and Klug
2003; Klug et al. 2008a, b; Monnet et al. 2011; De Baets et al. 2013b; Frey et al.
2014; Naglik et al. 2015): The changes in conch morphology of early ammonoids
occurred simultaneously and convergently (or even in parallel) in various clades
(Korn and Klug 2003; Kröger 2005; Monnet et al. 2011, 2015). This supports the
hypothesis that the evolutionary tendency towards tighter coiling was ecologi-
cally driven. In any case, the change in coiling altered the syn vivo shell orienta-
tion in such way that the aperture became horizontally aligned with the center
of mass, enabling the ammonoids of reasonably high swimming speeds (Saun-
ders and Shapiro 1986; Klug 2001a; Klug et al. 2008a; Hoffmann et al. 2015;
Naglik et al. 2015).
20 C. Klug et al.
3. Increase in reproductive rates (Klug 2001a, 2007; De Baets et al. 2012, 2013b,
2015): The temporal correlation of the reduction of embryo size and the increase
of both the absolute body chamber volume and the body chamber volume-diam-
eter ratio suggests that the reproductive rates increased by several orders of mag-
nitude presuming a constant relative size of the gonads (from possibly about 100
in Emsian Erbenoceras to about 100,000 in Frasnian Manticoceras; De Baets
et al. 2012).
Independent of the likelihood, plausibility or correctness of these hypotheses, it
has to be taken into account that many of the morphological changes of the am-
monoid shell (e.g., degree of coiling, whorl expansion rate, sutural complexity, um-
bilical width…) that occurred during the Early Devonian started near a left wall.
It is therefore unclear if one or more of these ecological changes and selection for
certain character states to deal with them in the best possible way was driving these
changes in coiling. Alternatively, the hypothesis might be valid that these evolution-
ary changes occurred during a random walk of the ammonoid subclades, affected
by left wall-effects; in the latter case, the three ecological explanations listed above
were just side-effects or wrong. Nevertheless, the synchronicity of the mentioned
evolutionary innovations and changes among the ammonoids with each other on the
one hand and with macroecological events on the other hand provides some support
for these three hypotheses which might have worked in concert.
Acknowledgments We thank the Swiss National Science Foundation for funding the research that
produced some of the results reviewed herein (SNF project numbers 200021–113956⁄ 1, 200020-
25029, and 200020-132870). Dieter Korn (Berlin) and Isabelle Kruta (Paris) kindly reviewed the
manuscript and helped to improve it significantly.
Bandel K (1982) Morphologie und Bildung der frühontogenetischen Gehäuse von conchiferen
Mollusken. Facies 8:1–154
Becker RT, House MR (1994) International Devonian goniatite zonation, Emsian to Givetian, with
new records from Morocco. Cour Forsch-Inst Senck 169:79–135
Bergmann S, Lieb B, Ruth P, Markl J (2006) The hemocyanin from a living fossil, the cephalopod
Nautilus pompilius: protein structure, gene organization, and evolution. J Mol Evol 62:362–374
Bizikov VA (2008) Evolution of the shell in Cephalopoda. VNIRO, Moscow, pp 1–444
Bogoslovsky BI (1969) Devonskie Ammonoidei, I. Agoniatity. Trudy Paleont Zh 124:1–341
Bogoslovsky BI (1984) A new genus of the family Auguritidae and the ammonoids accompanying
it from the Lower Devonian of the Zeravshan Range. Paleont J 1:30–36
Boletzky S von (2007) Origin of the lower jaw in cephalopods: a biting issue. Paläontol Z 81:328–
Botting JP, Muir LA (2012) Fauna and ecology of the holothurian bed, Llandrindod, Wales, UK
(Darriwilian, Middle Ordovician), and the oldest articulated holothurian. Palaeont Electronica
Boyajian GE, Lutz TM (1992) Evolution of biological complexity and its relation to taxonomic
longevity in the Ammonoidea. Geology 20:983–986
Brett CE, Walker SE (2002) Predators and predation in Paleozoic marine environments. Paleont
Soc Pap 8:93–118
211 Ancestry, Origin and Early Evolution of Ammonoids
Brock GA, Paterson JR (2004) A new species of Tannuella (Helcionellida, Mollusca) from the
Early Cambrian of South Australia. Mem Ass Aust Palaeont 30:133–143
Cecca F (1997) Late Jurassic and Early Cretaceous uncoiled ammonites: trophism-related Evolu-
tionary processes. Earth Planetary Sci 325, 629–634
Chlupáč I, Turek V (1983) Devonian goniatites from the Barrandian area, Czechoslovakia. Roz-
pravy Ustředniho Ustavu geologickeho 46:1–159
Clausen CD (1969) Oberdevonische Cephalopoden aus dem Rheinischen Schiefergebirge. II. Ge-
phuroceratidae, Beloceratidae. Palaeontogr A 132:95–178
Daniel TL, Helmuth BS, Saunders WB, Ward PD (1997) Septal complexity in ammonoid cephalo-
pods increased mechanical risk and limited depth. Paleobiology 23:470–481
De Baets K, Klug C, Korn D (2009) Anetoceratinae (Ammonoidea, Early Devonian) from the
Eifel and Harz Mountains (Germany), with a revision of their genera. N Jahrb Geol Paläont
Abh 252:361–376
De Baets K, Klug C, Plusquellec Y (2010) Zlíchovian faunas with early ammonoids from Morocco
and their use for the correlation of the eastern Anti-Atlas and the western Dra Valley. Bull
Geosci 85:317–352
De Baets K, Klug C, Korn D (2011) Devonian pearls and ammonoid-endoparasite co-evolution.
Acta Palaeontol Pol 56:159–180
De Baets K, Klug C, Korn D, Landman NH (2012) Evolutionary trends in ammonoid embryonal
development. Evolution 66:1788–1806
De Baets K, Klug C, Monnet C (2013a) Intraspecific variability through ontogeny in early am-
monoids. Paleobiology 39:75–94
De Baets K, Klug C, Korn D, Bartels C, Poschmann M (2013b) Emsian Ammonoidea and the
age of the Hunsrück Slate (Rhenish Mountains, Western Germany). Palaeontogr A 299:1–114
De Baets K, Goolaerts S, Jansen U, Rietbergen T, Klug C (2013c) The first record of Early De-
vonian ammonoids from Belgium and their stratigraphic significance. Geologica Belgica
Dzik J (1981) Origin of the Cephalopoda. Act Palaeont Pol 26:161–191
Dzik J (1984) Phylogeny of the Nautiloidea. Act Palaeont Pol 45:1–219
Dzik J (2010) Brachiopod identity of the alleged monoplacophoran ancestors of cephalopods.
Malacologia 52:97–113
Engeser T (1990a) Major events in cephalopod evolution. In: Taylor PD, Larwood GP (eds) Major
evolutionary radiations, Vol 42. Clarendon Press, Oxford
Engeser T (1996) The position of the Ammonoidea within the Cephalopoda. In Landman NH,
Tanabe K, Davis RA (eds) Ammonoid Paleobiology. Plenum, New York
Erben HK (1960) Primitive Ammonoidea aus dem Unterdevon Frankreichs und Deutschlands. N
Jahrb Geol Paläont Abh 110:1–128
Erben HK (1964a) Die Evolution der ältesten Ammonoidea. N Jahrb Geol Paläont Abh 120:107–
Erben HK (1964b) Bactritoidea. In: Moore RC (ed) Treatise on Invertebrate Paleontology, Part K,
Mollusca 3, Cephalopoda. GSA and the University of Kansas Press, Kansas, pp K491–K519
Erben HK (1965) Die Evolution der ältesten Ammonoidea (Lieferung II). N Jahrb Geol Paläont
Abh 122:275–312
Erben HK (1966) Über den Ursprung der Ammonoidea. Biol Rev of the Cambridge Phil Soc
Evans DH (2005) The lower and middle Ordovician cephalopod faunas of England and Wales.
Monogr Palaeontogr Soc 623:1–8
Firby JB, Durham JW (1974) Molluscan radula from earliest Cambrian. J Paleont 48:1109–1119
Frey L, Naglik C, Hofmann R, Schemm-Gregory M, Fryda J, Kröger B, Taylor PD, Wilson MA,
Klug C (2014) Diversity and palaeoecology of Early Devonian invertebrate associations in the
Tafilalt (Anti-Atlas, Morocco). Bull Geosci 89:75–112
Fuchs D (2006) Fossil erhaltungsfähige Merkmalskomplexe der Coleoidea (Cephalopoda)und ihre
phylogenetische Bedeutung. Berliner paläobiol Abh 8:1–165
Gabbott S (1999) Orthoconic cephalopods and associated fauna from the late Ordovician Soom
Shale Lagerstatte, South Africa. Palaeontology 42:123–148. doi:10.1111/1475-4983.00065
22 C. Klug et al.
García-Ruíz JM, Checa A, Rivas P (1990) On the origin of ammonite sutures. Paleobiology
Gildner RF (2003) A Fourier method to describe and compare suture patterns. Palaeont Electron
Glaessner MF (1976) Early Phanerozoic annelid worms and their geological and biological signifi-
cance. J Geol Soc London 132(3):159–215
Guex J (2006) Reinitialization of evolutionary clocks during sublethal environmental stress in
some invertebrates. Earth Planetary Sci Lett 242:240–253
Holland CH (1987) The nautiloid cephalopods: a strange success. J Geol Soc 144:1–15
Holland B, Stridsberg S, Bergström (1978) Confirmation of the reconstruction of Aptychopsis.
Lethaia 11:144
Horny R (1956) Bojobactrites ammonitans n. gen., n. sp. (Cephalopoda) from the Devonian of Cen-
tral Bohemia. Sbornik Ústředntniho Ústavu Geologického. Oddíl paleontologicky 23:283–305
House MR (1988) Major features of cephalopod evolution. In: Wiedmann J, Kullmann J (eds)
Cephalopods—present and past. Schweizerbart, Stuttgart
House MR (1996) Juvenile goniatite survival strategies following Devonian extinction events.
Geol Soc London Spec Pub 102:163–185
Jacobs DK, Landman NH (1993) Nautilus—a poor model for the function and behavior of am-
monoids? Lethaia 26:101–111
Keupp H (2000) Ammoniten. Paläobiologische Erfolgsspiralen. Thorbecke, Sigmaringen
Klug C (2001a) Life-cycles of Emsian and Eifelian ammonoids (Devonian). Lethaia 34:215–233
Klug C (2001b) Early Emsian ammonoids from the eastern Anti-Atlas (Morocco) and their suc-
cession. Paläontol Z 74:479–515
Klug C (2007) Sublethal injuries in Early Devonian cephalopod shells from Morocco. Acta Pal-
aeont Pol 52:749–759
Klug C, Korn D (2004) The origin of ammonoid locomotion. Acta Palaeont Pol 49:235–242
Klug C, Kröger B, Rücklin M, Korn D, Schemm-Gregory M, Mapes RH (2008a) Ecological
change during the early Emsian (Devonian) in the Tafilalt (Morocco), the origin of the Am-
monoidea, and the firs African pyrgocystid edrioasteroids, machaerids and phyllocarids. Pal-
aeontogr A 283:1–94
Klug C, Meyer E, Richter U, Korn D (2008b) Soft-tissue imprints in fossil and recent cephalopod
septa and septum formation. Lethaia 41:477–492
Klug C, Kröger B, Kiessling W, Mullins GL, Servais T, Frýda J, Korn D, Turner S (2010) The
Devonian nekton revolution. Lethaia 43:465–477
Klug C, Riegraf W, Lehmann J (2012) Soft-part preservation in heteromorph ammonites from the
Cenomanian-Turonian Boundary Event (OAE 2) in the Teutoburger Wald (Germany). Palae-
ontology 55:1307–1331
Korn D (2001) Morphometric evolution and phylogeny of Palaeozoic ammonoids. Early and Mid-
dle Devonian. Acta Geol Pol 51:193–215
Korn D, Klug C (2002) Ammoneae Devonicae. Fossilium Catalogus 138. Backhuys, Leiden
Korn D, Klug C (2003) Morphological pathways in the evolution of Early and Middle Devonian
ammonoids. Paleobiology 29:329–348
Korn D, Klug C (2012) Palaeozoic ammonoids—diversity and development of conch morphology.
In: Talent J (ed) Extinction intervals and biogeographic perturbations through time: earth and
Life (International Year of Planet Earth). Springer, Netherlands
Kröger B (2005) Adaptive evolution in Paleozoic coiled cephalopods. Paleobiology 31:253–268
Kröger B (2006) Early growth-stages and classification of orthoceridan Cephalopods of the Dar-
riwillian (Middle Ordovician) of Baltoscandia. Lethaia 39:129–139
Kröger B (2007) Some lesser known features of the ancient cephalopod order Ellesmerocerida
(Nautiloidea, Cephalopoda). Palaeontology 50:565–572
Kröger B (2008a) A new genus of middle Tremadocian orthoceratoids and the Early Ordovician
origin of orthoceratoid cephalopods. Act Pal Pol 53:745–749
Kröger B (2008b) Nautiloids before and during the ammonoid origin in a Siluro–Devonian section
of the Tafilalt (Morocco). Spec Pap Palaeont 79:1–110
231 Ancestry, Origin and Early Evolution of Ammonoids
Kröger B, Evans DH (2011) Review and palaeoecological analysis of the late Tremadocian–early
Floian (Early Ordovician) cephalopod fauna of the Montagne Noire, France. Fossil Record
Kröger B, Lefebvre B (2012) Palaeogeography and palaeoecology of early Floian (Early Ordovi-
cian) cephalopods from the Upper Fezouata Formation, Anti-Atlas, Morocco. Fossil Record
Kröger B, Klug C, Mapes RH (2005) Soft-tissue attachment in Orthocerida and Bactritida of Em-
sian to Eifelian age (Devonian). Acta Palaeont Pol 50:329–342
Kröger B, Mapes RH (2007) On the origin of bactritoids. Paläontol Z 81:316–327
Kröger B, Servais T, Zhang Y (2009) The origin and initial rise of pelagic cephalopods in the
Ordovician. PloS One 4 (9):e7262
Kröger B, Vinther J, Fuchs D (2011) Cephalopod origin and evolution: a congruent picture emerg-
ing from fossils, development and molecules. Bioessays 33:602–613
Kruta I, Landman NH, Mapes RH, Pradel A (2014) New insights into the buccal apparatus of the
Goniatitina: palaeobiological and phylogenetic implications. Lethaia 47:38–48
Kutscher E (1969) Die Ammonoideen-Entwicklung im Hunsrückschiefer. Notizbl Hess Landesa
Bodenf 97:46–64
Landing E, Kröger B (2009) The oldest cephalopods from East Laurentia. J Paleontol 83:123–127
Landing E, Kröger B (2012) Cephalopod ancestry and ecology of the hyolith “Allatheca” degeeri
s.l. in the Cambrian Evolutionary Radiation. Palaeogeogr Palaeoclim Palaeoeco 353:21–30
Lehmann J, Klug C, Wild F (2015) Did ammonoids possess opercula? Reassessment of phospha-
tised soft tissues in Glaphyrites from the Carboniferous of Uruguay. Paläontol Z 89:63–77. doi:
Mawson R (1987) Early Devonian conodont faunas from Buchan and Bindi, Victoria, Australia.
Palaeontology 30:251–297
Mapes RH, Nützel A (2009) Late Palaeozoic mollusc reproduction: cephalopod egg-laying behav-
ior and gastropod larval palaeobiology. Lethaia 42:341–356
Mazurek D, Zatoń M (2011) Is Nectocaris pteryx a cephalopod? Lethaia 44:2–4
Mehl J (1984) Radula und Fangarme bei Michelinoceras sp. aus dem Silur von Bolivien. Paläontol
Z 58:211–229
Monnet C, Klug C, De Baets K (2011) Parallel evolution controlled by adaptation and covariation
in ammonoid cephalopods. BMC Evolutionary biol 11(115):1–21
Mutvei H, Zhang Y-B, Dunca E (2007) Late Cambrian plectronocerid nautiloids and their role in
cephalopod evolution. Palaeontology 50:1327–1333
Nixon M (1988) The buccal mass of fossil and recent cephalopoda. In: Clarke MR, Truman ER
(eds) The Mollusca. Paleontology and neontology of cephalopods, vol 12. Academic, San Di-
ego, pp 103–122
Ogura A, Yoshida M-A, Moritaki T, Okuda Y, Sese J, Shimizu KK, Sousounis K, Tsonis PA (2013)
Loss of the six3/6 controlling pathways might have resulted in pinhole-eye evolution in Nau-
tilus. Sci Rep 3:1432 (1–7)
Peel JS (1991) The classes Tergomya and Helcionelloida, and early molluscan evolution. Grønl
Geol Unders Bull 161:11–65
Peterson KJ, Lyons JB, Nowak KS, Takacs C, Wargo MJ, McPeek MA (2004) Estimating meta-
zoan divergence times with a molecular clock. PNAS 101:6536–6541
Pojeta J Jr (1980) Molluscan phylogeny. Tulane Studies in Geology and Paleontology 16:55–80
Raup DM (1967) Geometric analysis of shell coiling: Coiling in ammonoids. J Paleont 41:43–65
Ristedt H (1968) Zur Revision der Orthoceratidae. Akad Wiss Lit Mainz, Abh Math-Naturwiss Kl
Ritterbush KA, Hoffmann R, Lukeneder A, De Baets K (2014) Pelagic palaeoecology: the impor-
tance of recent constraints on ammonoid palaeobiology and life history. J Zool 292:229–241.
doi: 10.1111/jzo.12118
Runnegar B (2011) Once again: is Nectocaris pteryx a stem-group cephalopod? Lethaia 44:373–373
C. Klug et al.
Sasaki T, Shigeno S, Tanabe K (2010) Anatomy of living Nautilus: Reevaluation of primitiveness
and comparison with Coleoidea. In: Tanabe K, Shigeta Y, Sasaki T, Hirano H (eds) Cephalo-
pods—present and past. Tokai University Press, Tokyo
Saunders WB, Shapiro EA (1986) Calculation and simulation of ammonoid hydrostatics. Paleo-
biology 12:64–79
Saunders WB, Work DM (1996) Shell morphology and suture complexity in Upper Carboniferous
ammonoids. Paleobiology 22:189–218
Saunders WB, Work DM, Nikolaeva SV (1999) Evolution of complexity in Paleozoic ammonoid
sutures. Science 286(5440):760–763
Shigeno S, Sasaki T, Moritaki T, Kasugai T, Kasugai T, Vecchione M, Agata K (2008) Evolution
of the cephalopod head complex by assembly of multiple molluscan body parts: evidence from
Nautilus embryonic development. J Morph 269:1–17
Shigeno S, Takenori S, Boletzky SV (2010) The origins of cephalopod body plans: a geometri-
cal and developmental basis for the evolution of vertebrate-like organ systems. In: Tanabe K,
Shigeta Y, Sasaki T, Hirano H (eds) Cephalopods—present and past. Tokai University Press,
Smith MR (2013) Nectocaridid ecology, diversity, and affinity: early origin of a cephalopod-like
body plan. Paleobiology 39: 297–321
Smith, MR, Caron J-B (2010) Primitive soft-bodied cephalopods from the Cambrian. Nature
Strugnell J, Jackson J, Drummond AJ, Cooper A (2006) Divergence time estimates for major ceph-
alopod groups: evidence from multiple genes. Cladistics 22:89–96
Strugnell J, Nishiguchi MK (2007) Molecular phylogeny of coleoid cephalopods (Mollusca:
Cephalopoda) inferred from three mitochondrial and six nuclear loci: a comparison of align-
ment, implied alignment and analysis methods. J Moll Stud 73:399–410
Teichert C (1948) Middle Devonian Goniatites from the Buchan District, Victoria. J Paleont
Turek V (1978) Biological and stratigraphical significance of the Silurian nautiloid Aptychopsis.
Lethaia 11:127–138
Turek V, Marek J (1986) Notes on the phylogeny of the Nautiloidea. Paläontol Z 60:245–253
Ubukata T, Tanabe K, Shigeta Y, Maeda H, Mapes RH (2014) Wavelet analysis of ammonoid
sutures. Palaeont Electron 17(1):9A (1–17
Ward PD (1979) Cameral liquid in Nautilus and ammonites. Paleobiology 5:40–49
Warnke KM, Meyer A, Ebner B, Lieb B (2011) Assessing divergence time of Spirulida and Sepiida
(Cephalopoda) based on hemocyanin sequences. Mol Phylogenet Evol 58:390–394
Webers GF, Yochelson EL (1989) Late Cambrian molluscan faunas and the origin of the Cepha-
lopoda. Geol Soc Lond Spec Pub 47:29–42
Wiedmann J, Kullmann J (1980) Ammonoid sutures in ontogeny and phylogeny. In: House MR,
Senior JR (eds) The Ammonoidea. Academic Press, London
Witmer LM (1995) The extant phylogenetic bracket and the importance of reconstructing soft
tissues in fossils. In: Thomason JJ (ed) Functional morphology in vertebrate paleontology.
Cambridge University Press, Cambridge
Woodruff DS, Carpenter MP, Saunders WB, Ward PD (1987) Genetic variation and phylogeny in
Nautilus. In: Saunders WB, Landman NH (eds) Nautilus, the biology and paleobiology of a
living fossil. Plenum, New York
Woodward H (1885) On some Palaeozoic phyllopod-shields, and on Nebalia and its allies. Geol
Mag 3:385–352
Yochelson E, Flower RH, Webers GF (1973) Bearing of new Late Cambrian monoplacophoran
genus Knightoconus upon origin of Cephalopoda. Lethaia 6:275–309
Young RE, Vecchione M, Donovan DT (1998) The evolution of coleoid cephalopods and their
present biodiversity and ecology. S Afr J Mar Sci 20:393–420
... An in-depth literature survey has revealed that there is considerable confusion regarding the appropriate name and authorship of the taxon that represents all Devonian to Cretaceous externally shelled (ectocochleate) cephalopods with at least one complete whorl, a marginal siphuncle, and a small protoconch, i.e. the ammonoids (for reviews, see Engeser 1996;Kröger et al. 2011;Klug et al. 2015). In the first ammonoid Treatise, Miller & Furnish (1957: L2) gave the name and authorship as: "Order Ammonoidea Zittel, 1884". ...
For the taxon comprising all Devonian to Cretaceous ammonoids, a variety of conflicting names with different authorship and taxonomic rank are available and have been repeatedly cited. Here, we review the primary literature and suggest the appropriate name, authorship and date of publication; we suggest the rank of a superorder for the ammonoids. The monophylum including all Devonian to Cretaceous ammonoids is here called Ammonoida. For the traditional suborder Ammo-nitina comprising Jurassic and Cretaceous forms the taxonomic rank of an order named Ammonitida is suggested to match the ranking of Palaeozoic ammonoid groups. Although the International Rules of Zoological Nomenclature only cover categories from subspecies to superfamily levels, similar procedures are applied to the higher taxonomic categories described here in order to avoid further inconsistencies in cephalopod taxonomy.
... To date, the middle late Cambrian Plectronoceras cambria 7 is widely accepted as the oldest known cephalopod 1,35,36 . The taxon was assigned to the order Plectronocerida, a basal branch of Cephalopoda characterised by a ventral siphuncle. ...
Full-text available
Although an early Cambrian origin of cephalopods has been suggested by molecular studies, no unequivocal fossil evidence has yet been presented. Septate shells collected from shallow- marine limestone of the lower Cambrian (upper Terreneuvian, c. 522 Ma) Bonavista For- mation of southeastern Newfoundland, Canada, are here interpreted as straight, elongate conical cephalopod phragmocones. The material documented here may push the origin of cephalopods back in time by about 30 Ma to an unexpected early stage of the Cambrian biotic radiation of metazoans, i.e. before the first occurrence of euarthropods.
... During the late Silurian or Early Devonian, a group of orthoconic cephalopods (Bactritida) developed a curved shell (24), leading ultimately to the origin of Ammonoidea (25). The earliest Emsian ammonoids were open-coiled, but more hydrodynamic and maneuverable closed coils soon evolved and species richness rapidly increased ( Fig. 2A) (20,21). ...
Full-text available
The early burst model suggests that disparity rises rapidly to fill empty ecospace following clade origination or in the aftermath of a mass extinction. Early bursts are considered common features of fossil data, but neontological studies have struggled to identify them. Furthermore, tests have proven difficult because factors besides ecology can drive changes in morphology. Here, we document the ecomorphometric evolution of the extinct Ammonoidea at 1-million-year resolution, from their origination in the Early Devonian (Emsian) to the Early Triassic (Induan), over ~156 million years. This time interval encompasses six global extinction events, including two of the Big Five, and incorporates multiple ammonoid radiations. However, we find no evidence for early bursts of eco-morphological disparity. This contradicts arguments that the temporal scope, or traits measured in genomic data, conceal evidence of early bursts. Rather, early bursts may be less prevalent in fossil data than is often assumed.
... Additionally, the interpretation that ammonoids are more closely related to today's coleoids (e.g., Kröger et al., 2011) cautions that Nautilus, alone, is an insufficient model for the soft tissue anatomy, and thus first-order propulsion potential, of ammonoids. Despite their incredible fossil record (Klug et al., 2015b;Ritterbush and Foote, 2017), primary paleoecological significance (Batt, 1989;Ritterbush et al., 2014;Klug et al., 2015a), and continual morphological study (Raup, 1967;Smith, 1986;Tendler et al, 2015), the extent of shell shape's influence on locomotion efficiency is still poorly constrained for ammonoids. These challenges can be overcome using numerical methods. ...
Full-text available
We use three-dimensional (3D) numerical models to examine critical hydrodynamic characteristics of a range of shell shapes found in extinct ammonoid cephalopods. Ammonoids are incredibly abundant in the fossil record and were likely a major component of ancient marine ecosystems. Despite their fossil abundance, we lack significant soft body remains, which has made it historically difficult to investigate the potential life modes and ecological roles that these organisms played. By employing numerical tools to study how the morphology of a shell affected an ammonite’s hydrodynamics, we can build a foundation for hypothesizing and testing changes in the organism’s capabilities through time. To achieve this goal, the study was carried out in two major steps. First, we applied a number of simulation methods to a known problem, the drag coefficient of a half-sphere, to select the most appropriate modeling method that is accurate and efficient. These were further checked against previous experimental results on ammonoid hydrodynamics. Next, we produced 3D models of the ammonoid shells using Blender and Zbrush where each shell model emulated a specific fossil ammonoid, recent Nautilus, or an idealized shell form created by systematically varying shell inflation and umbilical exposure. We test the hypothesis that both the overall shell inflation and umbilical exposure will increase the drag experienced by a similarly sized ammonoid shell as it moves through water relative to other morphologies. ANSYS FLUENT was employed to execute the study. We further compare our simulation results to published experimental measurements of drag on ammonoid fossil replicas and live Nautilus. The simulation results provide accuracy within an order of magnitude of published values, across the tested range of water flow velocities (1 - 50 cm/s). The simulated drag measurements demonstrate a first-order sensitivity to shell inflation, with a second-order effect from umbilical exposure. The impact of a larger umbilical exposure (shells that are more evolute) is minimal at low velocities but substantial at higher velocities. We conclude that the overall shell inflation and umbilical exposure influence an individual shell’s drag coefficient, therefore, influence the hydrodynamic efficiency.
Full-text available
The impact of increasing atmospheric CO2 and the resulting decreasing pH of seawater are in the focus of current environmental research. These factors cause problems for marine calcifiers such as reduced calcification rates and the dissolution of calcareous skeletons. While the impact on recent organisms is well established, little is known about long-term evolutionary consequences. Here, we assessed whether ammonoids reacted to environmental change by changing septal thickness. We measured the septal thickness of ammonoid phragmocones through ontogeny in order to test the hypothesis that atmospheric pCO2, seawater pH and other factors affected aragonite biomineralisation in ammonoids. Particularly, we studied septal thickness of ammonoids before and after the ocean acidification event in the latest Triassic until the Early Cretaceous. Early Jurassic ammonoid lineages had thinner septa relative to diameter than their Late Triassic relatives, which we tentatively interpret as consequence of a positive selection for reduced shell material as an evolutionary response to this ocean acidification event. This response was preserved within several lineages among the Early Jurassic descendants of these ammonoids. By contrast, we did not find a significant correlation between septal thickness and long-term atmospheric pCO2 or seawater pH, but we discovered a correlation with palaeolatitude. Supplementary information: The online version contains supplementary material available at 10.1186/s13358-022-00246-2.
Full-text available
Background: Despite the excellent fossil record of cephalopods, their early evolution is poorly understood. Different, partly incompatible phylogenetic hypotheses have been proposed in the past, which reflected individual author’s opinions on the importance of certain characters but were not based on thorough cladistic analyses. At the same time, methods of phylogenetic inference have undergone substantial improvements. For fossil datasets, which typically only include morphological data, Bayesian inference and in particular the introduction of the fossilized birth-death model have opened new possibilities. Nevertheless, many tree topologies recovered from these new methods reflect large uncertainties, which have led to discussions on how to best summarize the information contained in the posterior set of trees. Results: We present a large, newly compiled morphological character matrix of Cambrian and Ordovician cephalopods to conduct a comprehensive phylogenetic analysis and resolve existing controversies. Our results recover three major monophyletic groups, which correspond to the previously recognized Endoceratoidea, Multiceratoidea, and Orthoceratoidea, though comprising slightly different taxa. In addition, many Cambrian and Early Ordovician representatives of the Ellesmerocerida and Plectronocerida were recovered near the root. The Ellesmerocerida is para- and polyphyletic, with some of its members recovered among the Multiceratoidea and early Endoceratoidea. These relationships are robust against modifications of the dataset. While our trees initially seem to reflect large uncertainties, these are mainly a consequence of the way clade support is measured. We show that clade posterior probabilities and tree similarity metrics often underestimate congruence between trees, especially if wildcard taxa are involved. Conclusions: Our results provide important insights into the earliest evolution of cephalopods and clarify evolutionary pathways. We provide a classification scheme that is based on a robust phylogenetic analysis. Moreover, we provide some general insights on the application of Bayesian phylogenetic inference on morphological datasets. We support earlier findings that quartet similarity metrics should be preferred over the Robinson-Foulds distance when higher-level phylogenetic relationships are of interest and propose that using a posteriori pruned maximum clade credibility trees help in assessing support for phylogenetic relationships among a set of relevant taxa, because they provide clade support values that better reflect the phylogenetic signal.
Full-text available
We describe an exceptionally well-preserved vampyropod, Syllipsimopodi bideni gen. et sp. nov., from the Carboniferous (Mississippian) Bear Gulch Lagerstätte of Montana, USA. The specimen possesses a gladius and ten robust arms bearing biserial rows of suckers; it is the only known vampyropod to retain the ancestral ten-arm condition. Syllipsimopodi is the oldest definitive vampyropod and crown coleoid, pushing back the fossil record of this group by ~81.9 million years, corroborating molecular clock estimates. Using a Bayesian tip-dated phylogeny of fossil neocoleoid cephalopods, we demonstrate that Syllipsimopodi is the earliest-diverging known vampyropod. This strongly challenges the common hypothesis that vampyropods descended from a Triassic phragmoteuthid belemnoid. As early as the Mississippian, vampyropods were evidently characterized by the loss of the chambered phragmocone and primordial rostrum—traits retained in belemnoids and many extant decabrachians. A pair of arms may have been elongated, which when combined with the long gladius and terminal fins, indicates that the morphology of the earliest vampyropods superficially resembled extant squids.
Full-text available
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.
Full-text available
Nectocaridids are soft-bodied Cambrian organisms that have been controversially interpreted as primitive cephalopods, at odds with the long-held belief that these mollusks evolved from a shell-bearing ancestor. Here, I document a new nectocaridid from the Whetstone Gulf Formation, extending the group's range into the Late Ordovician. Nectocotis rusmithi n. gen. n. sp. possesses a robust internal element that resembles a non-mineralized phragmocone or gladius. Nectocaridids can be accommodated in the cephalopod total group if the earliest cephalopods (1) inherited a non-mineralized shell field from the ancestral mollusk; and (2) internalized this shell field. This evolutionary scenario would overturn the traditional ectocochleate, Nautilus -like reconstruction of the ancestral cephalopod, and indicate a trend towards increased metabolic efficiency through the course of Cambrian–Ordovician evolution. UUID:
Full-text available
In the central Anti-Atlas (Morocco), the Early Ordovician succession consists of about 1000 m of fossiliferous argillites and siltstones. The Upper Fezouata Formation (Floian) contains a comparatively rich and abundant cephalopod association. A small collection of these cephalopods is described herein for the first time. The cephalopods are interpreted as autochthonous or parautochthonous, representing a fauna, which originally lived nektobenthically in the open water above the sediments or related to the sea bottom. The cephalopod associations of the Upper Fezouata Formation are similar to other contemporaneous assemblages known from higher palaeolatitudes and associated with deeper depositional settings and in siliciclastically dominated deposits. They are composed almost exclusively of slender orthocones, in this case predominantly of Destombesiceras zagorense n. gen., n. sp., which is interpreted as an early discosorid. Bathmoceras australe Teichert, 1939 and Bathmoceras taichoutense n. sp. from the Upper Fezouata Formation are at present the earliest unambiguous occurrences of bathmocerid cephalopods. Epizoans on the shell of a specimen of Rioceras are the earliest evidence of bryozoans growing as potential hitchhikers on cephalopod shells, indicating an early exploitation of a pseudoplanktonic lifestyle in this phylum. doi:10.1002/mmng.201200004
Full-text available
Cambrian Cephalopods are presently reported only from tropical, carbonate platform successions that occur on a number of paleocontinents. Outside of West Gondwanan occurrences on the eastern Sino-Korean Platform in China, the record of Cambrian cephalopods is limited, and information on the early evolution and habitats of this molluscan class has grown slowly over the last century.
Full-text available
Traditional analyses of Early Phanerozoic marine diversity at the genus level show an explosive radiation of marine life until the Late Ordovician, followed by a phase of erratic decline continuing until the end of the Palaeozoic, whereas a more recent analysis extends the duration of this early radiation into the Devonian. This catch-all approach hides an evolutionary and ecological key event long after the Ordovician radiation: the rapid occupation of the free water column by animals during the Devonian. Here, we explore the timing of the occupation of the water column in the Palaeozoic (including certain fish groups) and test the hypothesis that ecological escalation led to fundamental evolutionary changes in the mid-Palaeozoic marine water column. According to our analyses, demersal and nektonic modes of life were probably initially driven by competition in the diversity-saturatedbenthic habitats together with the availability of abundant planktonic food. Escalatory feedback then promoted the rapid rise of nekton in the Devonian as suggested by the sequence and tempo of water-column occupation.
The evolution of septal complexity in fossil ammonoids has been widely regarded as an adaptive response to mechanical stresses imposed on the shell by hydrostatic pressure. Thus, septal (and hence sutural) complexity has been used as a proxy for depth: for a given amount of septal material greater complexity permitted greater habitat depth. We show that the ultimate septum is the weakest part of the chambered shell. Additionally, finite element stress analyses of a variety of septal geometries exposed to pressure stresses show that any departure from a hemispherical shape actually yields higher, not lower, stresses in the septal surface. Further analyses show, however, that an increase in complexity is consistent with selective pressures of predation and buoyancy control. Regardless of the mechanisms that drove the evolution of septal complexity, our results clearly reject the assertion that complexly sutured ammonoids were able to inhabit deeper water than did ammonoids with simpler septa. We suggest that while more complexly sutured ammonoids were limited to shallower habitats, the accompanying more complex septal topograhies enhanced buoyancy regulation (chamber emptying and refilling), through increased surface tension effects.
The Early Ordovician successions of the southern Montagne Noire consist of a thick sequence of predominantly siliciclastic sediments of which the late Tremadocian St. Chinian Formation and the earliest Floian La Maurerie Formation contain a comparatively rich and abundant cephalopod association. The cephalopods of the St. Chinian and La Maurerie Formation are interpreted as generally authochthonous, representing a fauna which originally lived in the open water above the sediments or related to the sea bottom. The cephalopod associations of the St. Chinian and La Maurerie formations are similar to other contemporaneous assemblages known from higher palaeolatitudes and associated with deeper depositional settings. They are composed almost exclusively of longiconic orthocones, in this case predominantly of eothinoceratids and baltocerids. The occurrences of Annbactrocera, and Bactroceras in the St. Chinian Formation are at present the earliest unambiguous reports of the Orthocerida. The available data suggest an initial expansion of orthoceroid cephalopod faunas from open water habitats of high paleo-latitudes, and a subsequent expansion on the carbonate platforms during the Floian. The presence of the eothinoceratid Saloceras in abundance demonstrates the Gondwanan affinity of the assemblage whilst adding further support for the presence of a "Saloceras realm" that may have extended along the margins of East and West Gondwana at least into intermediate latitudes. The following new taxa are proposed: Annbactroceras n. gen., Annbactroceras felinense n. sp., Cyclostomiceras thorali n. sp., Felinoceras n. gen., Felinoceras constrictum n. sp., Lobendoceras undulatum n. sp., Rioceratidae n. fam., Saloceras murvielense n. sp., Thoraloceras n. gen., Thoraloceras bactroceroides n. sp. doi:10.1002/mmng.201000013
Recent Nautilus pompilius from the Fiji Islands and N. macromaphalus from New Caledonia show decreasing cameral liquid volumes relative to total phragmocone volume during ontogeny. A maximal value of 32% of the phragmocone filled with cameral liquid was measured from a 190 g N. pompilius. No specimens of over 500 g total weight of either species exceeded 12%. These figures are in contrast to values derived for seven ammonoid species by Heptonstall (1970), who found values ranging between 19 and 52%. The relationship between cameral liquid volume and salinity within single chambers engaged in the emptying process are examined in N. pompilius and N. macromphalus. Both species start with newly formed chambers filled with cameral liquid isotonic to seawater. Ionic removal by the siphuncular epithelium rapidly reduces the cameral liquid osmolarity, producing osmotic movement of the cameral liquid into the blood spaces of the siphuncle. In both species the lowest cameral liquid salinities occur when the chamber is slightly over half emptied. After this point, which coincides with decoupling of the cameral liquid from the siphuncle, cameral liquid volume continues to decrease, but cameral liquid salinity increases, indicating that the rate of ionic removal slows relative to liquid removal. In N. macromphalus decoupled cameral liquid salinity rises until it is nearly isotonic to seawater when the chamber is nearly emptied. In N. pompilius , however, the rate of ion removal in decoupled cameral liquid is not slowed as much as in N. macromphalus , since it rarely exceeds 40% seawater osmolarity even when the chamber is nearly emptied. The differences in emptying methods demonstrated in these two species are probably related to their different habitat depths: N. pompilius from Fiji is found in much deeper water and must employ more physiologic work to empty chambers at greater depth.
Previous classifications of the major groups of cephalopods are based mainly on features of the shell and the siphuncle and are phenotypic. In a new approach, the classification is reinvestigated by means of cladistic analysis. The Cephalopoda are a monophyletic group of molluscs. Despite obvious differences between, for example, Nautilus and the Coleoidea, there are some unique synapomorphies that are exclusively shared by these two distinct evolutionary lines. The latest common ancestor of the cephalopods will be considered. In contrast to previous studies which have considered only the fossil record for a suitable candidate, the hypothetic ancestral cephalopod (HAC) is here reconstructed using characters of Recent taxa. A plausible scenario for the evolutionary steps leading to HAC will be presented. This includes a discussion of the position of the Cephalopoda within the Mollusca and their probable sister-group. -from Author