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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.
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3
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
e-mail: chklug@pim.uzh.ch
B. Kröger
Finnish Museum of Natural History, University of Helsinki, 44 (Jyrängöntie 2), 00014 Helsinki,
Finland
e-mail: bjorn.kroger@helsinki.fi
J. Vinther
Schools of Earth and Biological Sciences, University of Bristol, Woodland Road, BS8 1UG,
Bristol, UK
e-mail: jakob.vinther@bristol.ac.uk
D. Fuchs
Earth and Planetary System Science, Department of Natural History Sciences, Hokkaido
University, Sapporo, Japan
e-mail: dfuchs@mail.sci.hokudai.ac.jp
K. De Baets
GeoZentrum Nordbayern, Fachgruppe PaläoUmwelt, Universität Erlangen, Loewenichstr. 28,
91054 Erlangen, Germany
e-mail: kenneth.debaets@fau.de
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,
narrow
Central, narrow Ventral, narrow Ventral, narrow
Shell shape Cyrtoconic Orthoconic Orthoconic Crioconic
Cross section Subcircular Circular Slightly
compressed
Compressed
Initial chamber Unknown Ovoid, small Ovoid, Small Ovoid, Small
Initial shaft angle Wide Wide Narrow Narrow
Suture line Straight Straight Ventral lobe Ventral + lateral
lobes
Muscle attachment Circular,
serial
Circular Dorsal
concentration
Dorsal
concentration
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
cavity
Present Present Present Present
Carnivorous life
style
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
system
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
shaft.
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.
2013b).
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
1.3.2.1 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.
2010).
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).
1.3.2.2 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.
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... 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". ...
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... 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. ...
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... 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. ...
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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.
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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