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Evolution of the coiled ammonoid conch from the uncoiled bactritid conch was probably coupled with changes in ma− noeuvrability and swimming velocity. The gradual transformation of uncoiled to coiled ammonoid conchs has essential functional consequences. The radical change in conch geometry during phylogeny but also in ontogeny of early ammonoids implies a shift of the aperture from an original roughly downward, via a downward oblique and an upward oblique to an upward orientation, presuming a neutrally buoyant condition of the ammonoid animal. Similar trends were reconstructed for the three main ammonoid lineages in the Middle Devonian, the agoniatitid, the anarcestid, and the tornoceratid lineages. This allowed an increase in manoeuvrability and in the maximum horizontal swimming speed.
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The origin of ammonoid locomotion
CHRISTIAN KLUG and DIETER KORN
Klug, C. and Korn, D. 2004. The origin of ammonoid locomotion. Acta Palaeontologica Polonica 49 (2): 235–242.
Evolution of the coiled ammonoid conch from the uncoiled bactritid conch was probably coupled with changes in ma
noeuvrability and swimming velocity. The gradual transformation of uncoiled to coiled ammonoid conchs has essential
functional consequences. The radical change in conch geometry during phylogeny but also in ontogeny of early
ammonoids implies a shift of the aperture from an original roughly downward, via a downward oblique and an upward
oblique to an upward orientation, presuming a neutrally buoyant condition of the ammonoid animal. Similar trends were
reconstructed for the three main ammonoid lineages in the Middle Devonian, the agoniatitid, the anarcestid, and the
tornoceratid lineages. This allowed an increase in manoeuvrability and in the maximum horizontal swimming speed.
Key words: Bactritida, Ammonoidea, ontogeny, phylogeny, locomotion, coiling, Devonian.
Christian Klug [chklug@pim.unizh.ch] Paläontologisches Institut und Museum der Universität Zürich, Karl Schmid−Str.
4, 8006 Zürich, Switzerland;
Dieter Korn [dieter.korn@museum.hu−berlin.de] Humboldt−Universität zu Berlin, Museum für Naturkunde, Institut für
Paläontologie, D−10115 Berlin, Germany.
Introduction
Although ammonoids are among the most famous and the
most common fossil invertebrates in the Palaeozoic and Me−
sozoic, little is known about the animals’ ecology. Their
conchs consisted of a body chamber and a gas−filled cham−
bered phragmocone to maintain neutral buoyancy. It is
largely accepted that they possessed a hyponome for propul
sion (Jacobs and Chamberlain 1996). Backward movements
can be achieved in Recent Nautilus by two actions; (1) by os
cillation of the wings of the hyponome and thus generating a
continuous weak stream of water over the gills and out of the
hyponome, inducing a gentle motion, and (2) by contracting
the mantle cavity, they produce a strong jet of water and
move backward at a higher velocity (Packard et al. 1980). It
appears likely, that ammonoids were able to propel them
selves by the same means. Their conch geometry allows cal
culation of flow resistance and swimming velocities (Jacobs
1992; Jacobs and Chamberlain 1996; Seki et al. 2000), septal
strength and maximal diving depths (Westermann 1973,
1975, 1982; Daniel et al. 1997), the positions of the centres of
gravity and buoyancy, and the orientation of the shell in the
water column (Trueman 1941; Raup and Chamberlain 1967;
Saunders and Shapiro 1986; Swan and Saunders 1987; Saun
ders and Work 1996; Westermann and Tsujita 1999).
Several times in their evolutionary history, ectocochleate
cephalopods developed conchs with horizontally aligned
centres of gravity and aperture (and by implication the posi
tion of the hyponome, as in modern Nautilus). The most
common strategy leading to a rotation of the aperture was the
evolution of planispiral (i.e., coiled) shells. More than ten
clades of the Nautiloidea (Dzik 1984), the early Ammonoi
dea, and several additional clades of Mesozoic ammonoids
embarked on this strategy.
It is generally accepted that the coiled ammonoids origi−
nated from a group of uncoiled cephalopods—Bactritida
(Erben 1960; Dzik 1984; Doguzhaeva 1999; Korn 2001),
as documented by numerous transitional Early Devonian
ammonoid species (Schindewolf 1932; Erben 1960, 1964,
1965; Korn 2001). This process was accompanied by signifi−
cant morphological transformations such as the shapes of ap−
ertures and growth parameters (e.g., whorl expansion, umbil
ical width), as well as consequent changes in body chamber
length and orientation of the ammonoid conch within the wa
ter column (Klug 2001; Korn and Klug 2001, 2003). All of
these morphological transformations both during phylogeny
and during ontogeny allow interpretations with regard to
ammonoid manoeuvrability.
The energy cost for achieving a position of the conch suit
able for rapid and directed horizontal movements was lower in
planispiral than in orthoconic conchs. In passive moments, the
orthocones were simply “hanging” in the water column with
the aperture facing downwards (Westermann 1977). During
horizontal swimming manoeuvres in order to reduce drag,
their conchs had to rotate into an inclined or possibly horizon
tal position. In contrast, most cephalopods with planispiral
conchs could maintain the same orientation or slightly rotate
the conch until the hyponome reached the same level as the
centre of gravity. In many ammonoids, this must have resulted
in a rocking movement, as has been observed in Recent Nauti
lus (Chamberlain 1987). In the subsequent paragraphs, we dis
cuss the constraints of these morphological transformations of
conchs in phylogeny and ontogeny of the earliest ammonoids
regarding manoeuvrability and swimming speed.
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Acta Palaeontol. Pol. 49 (2): 235–242, 2004
Materials and methods
This entire study is based on the premise that the bactritids
and the early ammonoids were neutrally buoyant. We inves−
tigated the phylogenetic change in the orientation of the
conch from bactritids to early ammonoids. For this purpose,
we sculptured simple 3D models out of plastics of the conchs
of a bactritid and a variety of curved and coiled early am
monoids to experimentally identify the centres of gravity and
buoyancy of the entire conch and the separate body chamber
(Figs. 1, 2). These models are based on actual specimens,
measurements of which were taken both from material at
Tübingen and from the literature. Since some of the taxa
(Cyrtobactrites and Kokenia) are only incompletely known,
they were reconstructed. The models of Erbenoceras and
Mimagoniatites were produced at a smaller scale. The orna
ment and the siphuncle were not sculptured in these models.
According to Raup and Chamberlain (1967: 572), “the
center of buoyancy is equivalent to the center of gravity of
the volume displaced by the whole shell and the center of
mass may be estimated as the centre of gravity of the body
chamber”. Consequently, both the complete model and the
isolated body chamber of the model were mounted on a thin
foil. Then they were balanced on a needle, to identify the
centres of masses of the isolated body chamber of the model
and of the complete model.
The result of our experiment for Agoniatites (the most de
rived genus among the studied taxa) confirmed the results of
the theoretical approach of Raup (1967). Raup’s equations
(Raup and Chamberlain 1967; Raup 1966, 1967; Raup and
Michelson 1966), however, cannot be applied to the more
loosely coiled Early Devonian forms because these equa−
tions presumed isometric growth whereas many of these
primitive ammonoids grew allometrically (Kant 1973; Kant
and Kullmann 1980; Klug 2001).
Like all numerical models for the reconstruction of the
orientation of ammonoid conchs, our physical models are
simplified, neglecting all subtle details of the distribution of
mass in the septa and in parts of the ornament. In contrast to
the mathematical models, all aspects of allometric changes
are included. It was our intention to test our hypothesis that a
significant change in life position happened in the course of
the phylogeny of the earliest ammonoids. This was con
firmed by the results on the one hand. On the other hand, the
numerical details certainly lack precision and have to be
understood as approximations.
Results
Within the phylogenetic lineage from the orthoconic Lobo
bactrites (straight conch) to Agoniatites (planispiral with
embracing whorls), several morphological changes took
place. Regarding morphologies in this morphocline, an in
crease in whorl expansion rate and a decrease of umbilical
width can be observed (Fig. 2). We hypothesise that the ap
236 ACTA PALAEONTOLOGICA POLONICA 49 (2), 2004
thrust force
of jet
gravity
restorative
moment
drag above centre of gravity D1
drag below centre of gravity D2
resulting drag D =D1+D2
centre of buoyancy
centre of gravity
orientation of the hyponome
phragmocone
body chamber
area of drag below
center of gravity
lateral posterior anterior
gravity
65°
orientation
of the aperture
body
chamber
length
buoyanbuoyancy oyanbuoyancy
Fig. 1. Forces operating on ammonoids during swimming, parameters, and terminology. A. Forces operating on ammonoids during swimming (modified
from Jacobs and Chamberlain 1996). The thrust force produced by the jet which is expelled by the hyponome acts on the centre of gravity. This causes an
oblique downward momentum which is opposed by the restorative moment (resulting from buoyancy and gravity) and the drag. At relatively high veloci−
ties, this might result in a fairly stable horizontal movement in some derived ammonoids. B. Angles of the body chamber length and of the orientation of the
aperture. C. Terminology.
erture began to move first from a slightly oblique down
ward (Lobobactrites) to a downward more strongly oblique
position (Cyrtobactrites,Kokenia), then to an upward
oblique position (Metabactrites,Anetoceras, Talenticeras,
Chebbites, Mimagoniatites), and finally to a more or less
upward horizontal position in several Middle Devonian
ammonoid lineages including the Agoniatitina (Figs. 2, 3).
The arrangement of the centres of gravity and buoyancy of
these cephalopods, which were identified experimentally,
supports the above hypothesis (Figs. 2, 3). Considering
Lobobactrites, the ventral siphuncle and the oblique aper−
ture are indications for the slightly oblique orientation of
the living animal.
Based on these experiments, the ventral side of the aper
ture (and thus the hyponome) was probably already more or
less aligned in one horizontal plain with the centre of gravity
in Erbenoceras (Fig. 2). This provided stability during hori
zontal motion at moderate velocities. Accordingly, the gen
era Talenticeras,Chebbites, and Mimagoniatites had similar
orientations of the aperture and positions of the centres of
gravity. In some more derived ammonoids with moderate to
high whorl expansion rates and embracing whorls such as
Agoniatites, the position of the hyponome was higher than
the centre of gravity. For moderately rapid movements, they
had to tilt their aperture slightly downwards to avoid a rock
ing movement, as in Nautilus.
With regard to developmental transformations among the
early ammonoids, two major trends can be documented (Fig.
4). In general, the curvature of the shell cone increased
throughout phylogeny as well as ontogeny of many primal
ammonoids. In the embryonic to preadult conch, this ten
dency is recorded in all forms included in this study except
for Lobobactrites and the most derived genus, Agoniatites.In
some of these ammonoids, however, this is reversed in late
ontogeny towards a decrease in conch curvature which
caused the formation of loosely coiled adult whorls. This
means that intermediate growth stages of some forms like
Erbenoceras and Talenticeras display the most derived mor−
phology in their conchs.
Similar reversals in conch growth and geometry through−
out ontogeny of ammonoids also occur among geologically
younger ammonoids; many involute (whorls strongly over−
lapping) ammonoids become more evolute (low whorl over−
lap) or even gyroconic (whorls not in contact) in late ontog−
eny (e.g., Triassic Ceratites, Jurassic Morphoceras, Creta−
ceous Scaphites) or advolute forms become gyroconic (e.g.,
Triassic Choristoceras, Cretaceous Pictetia and, in a broader
sense, Ancyloceras), some evolute forms turn more involute
with maturity (e.g., Devonian Triainoceras, Jurassic Amal
theus, and, in a broader sense, Cretaceous Axonoceras).
Discussion
The following discussion focuses predominantly on the rela
tion between orientation of the cephalopod shell and locomo
tion. Influences of ornamentation and geometric aspects ir
relevant for the orientation were not evaluated (for details on
these aspects see Jacobs and Chamberlain 1996).
According to measurements from our plastic models, the
aperture moved from a downward to an upward orientation
during phylogeny of early ammonoids. Thus, we hypothesise
that a high orientation of the aperture, and even more so that
higher than the centre of mass, was advantageous for more
rapid horizontal movements.
Jacobs and Chamberlain (1996) portrayed the physical
constraints and advantages of an orientation where the hypo
nome and the centre of mass are more or less aligned. Never
theless, it is difficult to explain the functional advantages of a
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KLUG AND KORN—ORIGIN OF AMMONOID LOCOMOTION 237
20° 35° 40° 65° 75° 70° 60° 85°
90° 110° 120° 180°
180° 195° 160° 215°
Lobobactrites
Cyrtobactrites
Kokenia
Metabactrites
Anetoceras
Erbenoceras
Talenticeras
Chebbites
Mimagoniatites
Agoniatites
27%
39%
15°
genus
OA
BCL
relative swimming velocity
low moderate high
reconstructions
scale bar 1 cm
very low
body chamber
Fig. 2. Phylogenetic change in orientation of the conchs and swimming velocity of Bactritida and primitive Ammonoidea. Outlines of the conchs of one
bactritid and nine ammonoids from the Early and Middle Devonian with body chamber lengths (BCL), orientation of the aperture (OA), and relative swim
ming speed. Centre of gravity is indicated by a cross and the centre of buoyancy by a circle (for further explanations see Fig. 1).
posture with the aperture above the horizontal plane that con
tains the centre of gravity. In the latter case, at higher veloci
ties, drag played an increasingly important role. This might
have been one functional advantage of the high position of
the hyponome in Agoniatites because when the ammonoid
animal exceeded a certain velocity, drag became higher
above the centre of gravity and lower below it. This counter
acted the restorative moment produced by the interaction of
buoyancy and gravity (Fig. 1; see Jacobs and Chamberlain
1996 for further references). When the hyponome was hori
zontally aligned with the centre of gravity, it lost stability at
high velocity because the restorative moment became
smaller due to the higher drag above the centre of gravity. In
Agoniatites, however, the level of the hyponome is above the
238 ACTA PALAEONTOLOGICA POLONICA 49 (2), 2004
Fig. 3. Reconstructions and a simplified cladogram of one bactritid and nine primitive ammonoids from Early and Middle Devonian (from left to right:
Lobobactrites, Cyrtobactrites, Kokenia, Metabactrites, Anetoceras, Erbenoceras, Chebbites, Talenticeras, Mimagoniatites, Agoniatites). Note the change
in the orientation of the aperture and the increase of soft body volume in relation to the conch diameter. The morphology of the soft body is largely specula
tive. The number and proportion of arms, however, is here supposed to have been similar to coleoids, because of similarities in embryonic shell, radula and
beak morphology between ammonoids and coleoids (Landman et al. 1997; Tanabe and Fukuda 1999). Additionally, the presence of a hood as in Recent
Nautilus is presumed based on the absence of jaw apparatuses in early ammonoids which were suitable as a lid for the aperture. In the cladogram (modified
after Korn 2001, see this article also for the character matrix) with the most important evolutionary steps among Devonian ammonoids, those taxa not dis
cussed in detail are marked with an asterisk.
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KLUG AND KORN—ORIGIN OF AMMONOID LOCOMOTION 239
Fig. 4. Transformations in conch morphology of eight primitive ammonoids from the Early and Middle Devonian (from bottom to top: Kokenia,
Metabactrites, Anetoceras, Erbenoceras, Chebbites, Talenticeras, Mimagoniatites, Agoniatites). Subdivision of the coiling modes is slightly arbitrary, es
pecially the differentiation between the crioconic and the cyrtoconic state. In that case, it was the intention to clarify the changes in coiling and not to quan
tify the curvature. Consequently, this imprecision appeared justifiable. In the left column, the body chamber length (BCL) is given at the top right, the angle
of the orientation of the aperture (OA) at the bottom left and a code for the coiling mode (ontogeny) at the bottom right. The second column displays the
complete conchs with the colour code for the coiling modes (white—cyrtoconic, subtle curvature; light grey—crioconic, distinctly curved, but whorls not in
contact; medium grey—advolute, whorls close or touching; dark grey—evolute, whorls slightly overlapping). Columns three to six show the isolated conch
parts sorted according to the coiling mode.
centre of gravity and compensates for the lesser restorative
moment. Thus, horizontal apertures in ammonoids probably
allowed higher swimming velocities.
Additionally, an approximately horizontal orientation of
the aperture implies the largest possible horizontal distance
from the aperture to the centre of gravity. This causes a de
crease in stability during horizontal motions but an increase
in manoeuvrability. When the hyponome was directed to ei
ther side, the effect on the motion direction was greater than
in other taxa with apertures oriented at lower angles.
Most Nautiloidea (e.g., Devonian Orthoceras, Triassic
Germanonautilus, Recent Nautilus) had (and some still
have) downward to oblique upward oriented apertures and
therefore were possibly slower and less agile swimmers than
some of the regularly coiled ammonoids (for a discussion of
the locomotion of Recent Nautilus see Packard et al. 1980;
Chamberlain 1987; Ward 1987).
The more or less horizontally upward oriented aperture
evolved independently numerous times among ammonoids
(Fig. 5; e.g., the Carboniferous Anthracoceras; Saunders and
Shapiro 1986). The possible extremes of orientation of neu
trally buoyant planispiral cephalopod conchs, i.e., 20° or over
240 ACTA PALAEONTOLOGICA POLONICA 49 (2), 2004
Fig. 5. Changes in the orientation of the aperture of the adult conchs of ten representative Early and Middle Devonian ammonoids and Recent Nautilus
through phylogeny (Erbenoceras,Mimosphinctes,Convoluticeras,Mimagoniatites,Agoniatites,Ponticeras,Cabrieroceras,Holzapfeloceras,Pharci
ceras). The two graphs on the left are based on diagrams figured by Saunders and Shapiro (1985) and Okamoto (1996). Comments on the modifications
of these graphs are given in Klug (2001) and Korn and Klug (2003). Shell thickness is impossible to determine in most Early and Middle Devonian
ammonoids and thus the lines of correlation between WER, BCL, and OA are printed as broad lines in the graphs. Note the shift of the orientation of the
aperture from oblique to more or less horizontal in the agoniatitid, anarcestid, and tornoceratid lineages. In the agoniatitid lineage (E, F), the horizontal
position was achieved by an increase in whorl expansion rate (relatively short body chambers) compared to A and B. In the anarcestid lineage (G, I, K),
the body chamber lengths first increased in the progress of evolution and subsequently decreased again, leading to moderate body chamber lengths (K,
H) and consequently more or less horizontal apertures. The positions of Erbenoceras (A) and Mimosphinctes (B) are shown in grey because in their
cases, the orientation of the aperture does not correlate with the body chamber length and thus whorl expansion rate, as it is the case for advolute, evolute,
and involute species.
90°, certainly both had advantages. A low angle implied that
the arms could more easily reach down− and backwards and
also, the hyponome could be directed backwards with less ef
fort and thus, forward movements were a smaller problem.
High angles and thus an upward orientation of the aperture
means a higher manoeuvrability, possibly higher maximal
swimming velocities but straight forward movements were
difficult. Synchronous with changes of environmental pa
rameters, the one or the other capability was favoured by
natural selection, causing shifts in orientation.
Evaluation of changes in locomotion ability during the
early ontogenetic stages of primitive ammonoids is difficult
because it is influenced by several additional factors (Klug
2001). For instance, with increasing conch size the maxi
mum sustainable swimming velocity rises (Jacobs 1992;
Jacobs and Chamberlain 1996; Seki et al. 2000) and “As size
increases, per−unit [energetic] costs decline, and optimal
speeds occur at slightly higher velocities” (Jacobs and
Chamberlain 1996: 209).
A scaling effect might also have played a role in the early
ammonoids (Jacobs and Chamberlain 1996; Seki et al.
2000). Among the presented forms, several morphological
trends could be recognised such as an overall increase in the
whorl width / diameter ratio, the whorl expansion rate, the
imprint zone rate, the conch volume / diameter ratio, the ab−
solute conch volume and the conch diameter as well as a de−
crease of the umbilical width / diameter ratio and the size of
the umbilical window (for actual values see Appendix 1 in
Korn and Klug 2003).
Additional indications are sometimes yielded by muscle
attachment structures which are, however, not yet known
from these earliest ammonoids. Nevertheless, the morpho−
logical alterations during ontogeny of primal ammonoids
appear to reflect changes in the mode of life because similar
changes developed numerous times independently. Early
growth stages, although comparatively cost−effective
swimmers, certainly did not actively travel far. Long dis
tances could only be covered by means of currents. Presum
ing a semelparous mode of reproduction for ammonoids
(Stephen and Stanton 2002), the juveniles experienced
more or less random selection, resulting in a low number of
surviving individuals. Older premature specimens specula
tively had a stronger influence on their fate; at these growth
stages, conch geometry probably played a more important
role and they could actively swim longer stretches and thus
reach more or less distant aims with their motions. Finally,
among mature specimens of many Devonian ammonoids,
reproductive success remained as the key purpose and
therefore, manoeuvrability and swimming velocity in com
bination with factors like the safety of the eggs and their
spatial requirements within the body chamber became cru
cial in the search for suitable mating partners. Active mo
tions were probably essential and it appears likely that this
requirement also left its traces in the altered conch
geometry of adult ammonoids.
Acknowledgements
We sincerely thank Adolf Seilacher (Tübingen, New Haven), Hugo
Bucher (Zürich), Jean Guex (Lausanne), and Stuart Watts (Tübingen)
for valuable comments on the manuscript. Even more so, the valuable
comments included in the thorough reviews of Royal H. Mapes (Ath
ens, Ohio) and of Kazushige Tanabe (Tokyo) were a substantial help
for the improvement of the manuscript.
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242 ACTA PALAEONTOLOGICA POLONICA 49 (2), 2004
... Several studies have already examined the morphological evolution of Devonian ammonoids, mostly on a global level Klug 2003, 2012;Monnet et al. 2011;De Baets et al. 2012;Korn et al. 2015;Whalen et al. 2020). Through the Devonian, rapid coiling trends from uncoiled/ straight ancestors to ammonoids with coiled embryonic as well as post-embryonic conchs have been documented (House 1996a;Korn and Klug 2003;Klug and Korn 2004;Klug et al. 2008a;Monnet et al. 2011;De Baets et al. 2012Naglik et al. 2019). Korn and Klug (2012) and Korn et al. (2015) documented the fluctuations in morphological disparity through the Devonian using a standard morphometric method based on a modified version of the Raupian parameters (Korn 2010). ...
... 7B, 8A, B, 9B, 10A, B, 11). This might be related to the rapid morphological evolution from loosely coiled or advolute conchs to those with increasingly overlapping whorls (Klug and Korn 2004;De Baets et al. 2012. This is consistent with the variations of the position-based disparity estimator; a shift is visible between the Emsian and the rest of the studied time interval (Figs. ...
... In ammonoids, the loosely coiled conch constitutes the plesiomorphic state of morphology, since they are interpreted to have descended from bactritoid ancestors with straight conical or slightly curved conchs (Schindewolf 1933;Erben 1964;Korn 2001;Klug and Korn 2004;Kröger and Mapes 2007;Klug et al. 2015b;Cichowolski and Rustán 2017;Naglik et al. 2019). De Baets et al. (2012 documented the simultaneous increase in coiling of the inner whorls and the disappearance of the umbilical window in several Early Devonian ammonoid lineages. ...
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Fossils of Devonian ammonoids are abundant and well-preserved in the Anti-Atlas of Morocco; as such they provide an invaluable record of regional morphological disparity changes (diversity of shapes) that characterise the first steps of ammonoid evolution. However, they were rarely analysed quantitatively with respect to their morphological spectrum. Here, we investigated the morphological disparity of the Early–Middle Devonian ammonoids of the Moroccan Anti- Atlas by analysing the shape of their whorl profile. A geometric morphometric approach based on the acquisition of outline semilandmark coordinates was used to analyse the whorl profiles. For comparison, morphometric ratios based on classical conch measurements were also analysed to investigate the overall conch geometry. Several standard disparity estimators were computed to measure different aspects of morphological disparity fluctuations through time. It appears that a major increase in disparity occurred throughout the Early Devonian, followed by fluctuating disparity during the Middle Devonian constituting a general decreasing trend. Only the end-Eifelian Kačák Event shows a significant decrease in disparity. Thus, the ammonoids explored the range of possible shapes fairly quickly during their initial radiation; however, we found no evidence for an early burst of shape diversity (i.e., the rise does not exceed the expectations given diversity). Nevertheless, correlation tests between diversity and disparity time series support that they are partially decoupled. The highly resolved biozone record highlights that the increase in disparity began earlier than the increase in diversity that characterises the late Emsian.
... The morphological disparity of these extinct cephalopods likely reflects comparable differences in the functional constraints on their life habits. In many cases, the most fundamental swimming capabilities of these animals are poorly known, though decades of research have focused on addressing this issue (Trueman 1940;Denton 1974;Jacobs 1992;Jacobs and Chamberlain 1996;Westermann 1996Westermann , 1998Klug and Korn 2004;Hoffmann et al. 2015;. The current gap in knowledge is unfortunate because these animals were vital components of marine ecosystems for most of the Phanerozoic, and likely occupied diverse ecological niches across the globe (House 1981;Westermann 1998;Kruta et al. 2011;Korn et al. 2015;Tajika et al. 2018). ...
... To maintain a near neutrally buoyant condition, the proportions of void space in the chambered conch should manage organismal mass so that it is close to the mass of water displaced by the living animal. Conch coiling influences the distribution of organismal mass, largely due to the relative positions of the soft body occupying the body chamber and the air-filled chambers (Trueman 1940;Chamberlain 1981; Saunders and Shapiro 1986;Klug and Korn 2004). The static orientation these animals assumed during life can be determined when the total center of mass is vertically aligned under the center of buoyancy (Denton 1974;Saunders and Shapiro 1986;Klug and Korn 2004;Hoffmann et al. 2015;Naglik, Monnet, et al. 2015;Fig. ...
... Conch coiling influences the distribution of organismal mass, largely due to the relative positions of the soft body occupying the body chamber and the air-filled chambers (Trueman 1940;Chamberlain 1981; Saunders and Shapiro 1986;Klug and Korn 2004). The static orientation these animals assumed during life can be determined when the total center of mass is vertically aligned under the center of buoyancy (Denton 1974;Saunders and Shapiro 1986;Klug and Korn 2004;Hoffmann et al. 2015;Naglik, Monnet, et al. 2015;Fig. 1C). ...
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Synopsis Stability–maneuverability tradeoffs impose various constraints on aquatic locomotion. The fossil record houses a massive morphological dataset that documents how organisms have encountered these tradeoffs in an evolutionary framework. Externally shelled cephalopods (e.g., ammonoids and nautiloids) are excellent targets to study physical tradeoffs because they experimented with numerous conch morphologies during their long-lived evolutionary history (around 0.5 billion years). The tradeoff between hydrostatic stability and maneuverability was investigated with neutrally buoyant biomimetic models, engineered to have the same mass distributions computed for their once-living counterparts. Monitoring rocking behavior with 3D motion tracking reveals how stability influenced the life habits of these animals. Cephalopods with short body chambers and rapid whorl expansion (oxycones) more quickly attenuate rocking, while cephalopods with long body chambers (serpenticones and sphaerocones) had improved pitch maneuverability. Disparate conch morphologies presented broad functional opportunities to these animals, imposing several advantages and consequences across the morphospace. These animals navigated inescapable physical constraints enforced by conch geometry, illuminating key relationships between functional diversity and morphological disparity in aquatic ecosystems. Our modeling techniques correct for differences in material properties between physical models and those inferred for their living counterparts. This approach provides engineering solutions to the obstacles created by buoyancy, mass distributions, and moments of inertia, permitting more lifelike, free-swimming biomechanical models and aquatic robots.
... This complex structure protected their soft parts against predators and functioned as their buoyancy device (Klug & Korn, 2004;Naglik et al., 2015b). The ammonoid conch is subdivided into a body chamber and a phragmocone. ...
... Brevidome forms, in which the soft body angle Ψ occupies less than 90 • of the last whorl, mesodome forms ( 90 • < Ψ < 200 • ), and longidome forms (Ψ > 200 • ). Palaeozoic ammonoids show a long-standing trend of increasing Ψ even though the angle is rarely wider than 360 • [27], while the (probably) better-built ammonoids populating the Mesozoic seas (whose morphology was the most altered from the ancestral state of Palaeozoic forms) are typically mesodome (see, e.g., ref. [28]). Thus, there seems to be a tendency for "large but moderate" angles Ψ. ...
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This work explores the swimming of ammonoids, cephalopods related to living squids, octopuses, and nautilids and, like the latter, equipped with a coiled external shell. A mathematical model is introduced for theoretical ammonoid conchs. The two differential equations of motion (one for the centre of mass, including the drag force and the added mass coefficient, and one for the roll angle) are solved numerically for the theoretical conchs, and the results are analysed in terms of velocity and rocking angle. Destabilising resonances occur when the rocking motion is in phase with the propelling water jet. It is suggested that the ammonoids partly evolved avoiding the occurrence of such resonances in their construction.
... 4D and H, 5D, H, and L). Klug and Korn (2004) examined the shell morphology of several Paleozoic ammonoids and hypothesized that an increase in WER allowed an increase in maneuverability and maximum horizontal swimming speed. They reasoned that this happened because the horizontal alignment of the hyponome relative to the centers of gravity was achieved by increasing the WER, thereby improving swimming abilities. ...
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In this study, we analyzed the ontogenetic trajectories of shell morphology in some Cretaceous tetragonitid ammonoid specimens (Tetragonitidae) collected from the Tomamae, Mikasa, and Hamanaka areas of Hokkaido, Japan. In all examined species, the ontogenetic trajectories of septal spacing between successive chambers had similar characteristics during their early ontogeny: two cycles, each comprising an increase and subsequent decrease in septal spacing until ~ 30th septum. The trends of whorl expansion rate changed at 5–7 or ~ 10 mm in the Gaudryceratinae and ~ 3 mm shell diameter in the Tetragonitinae. Based on these observations, we propose that the planktic phases of Gaudryceratinae and Tetragonitinae ended at those shell diameters. These different shell diameters at the end of the planktic phase suggest slightly differing strategies within the family Tetragonitidae.
... Moreover, the conch formed the animal-environment interface that dictated opportunity or defeat for a range of locomotion strategies 12-16 . By now, over a half-century of intensive paleo-ecological study has crystallized characterizations of ectocochleate swimming opportunities-or lack thereof-based on their external conch shapes 10,12,13,[15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31] . A robust scheme linking morphology to ecological roles seems almost within reach, but an oversimplified approach will obscure how these animals have evolved solutions to the challenges imposed by environmental crises and day-to-day natural selection. ...
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Externally shelled cephalopods with coiled, planispiral conchs were ecologically successful for hundreds of millions of years. These animals displayed remarkable morphological disparity, reflecting comparable differences in physical properties that would have constrained their life habits and ecological roles. To investigate these constraints, self-propelling, neutrally buoyant, biomimetic robots were 3D-printed for four disparate morphologies. These robots were engineered to assume orientations computed from virtual hydrostatic simulations while producing Nautilus-like thrusts. Compressed morphotypes had improved hydrodynamic stability (coasting efficiency) and experienced lower drag while jetting backwards. However, inflated morphotypes had improved maneuverability while rotating about the vertical axis. These differences highlight an inescapable physical tradeoff between hydrodynamic stability and yaw maneuverability, illuminating different functional advantages and life-habit constraints across the cephalopod morphospace. This tradeoff reveals there is no single optimum conch morphology, and elucidates the success and iterative evolution of disparate morphologies through deep time, including non-streamlined forms.
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The monograph is devoted to ammonites and infrazonal stratigraphy of the Bathonian and Callovian stages of European Russia, Ukraine and adjacent regions of the North Caucasus, as well as high-latitude regions of the Northern Hemisphere (Northern Siberia, Franz Josef Land, Southern Alaska, British Columbia, Eastern Greenland). The work affects upon the fundamental problems of infrazonal biostratigraphy and substantiates the use of infrazonal subdivisions (biohorizons) as minimal biostratigraphic units. Fundamentally new data on the evolution of ammonites in the upper part of the Middle Jurassic have been obtained. The features of evolutionary transformations in several phylogenetic lineages of cardioceratids, the ammonite family, key for the division and correlation of the Bathonian, Callovian, and Lower Oxfordian, have been revealed. As a result of the research, an important scientific problem has been solved: the regularities of the stratigraphic and biogeographic distribution of ammonites in the Bathonian and Callovian deposits of the boreal and highly boreal regions of the Northern Hemisphere have been established. The author has developed infrazonal ammonite scales for the European part of Russia and central Ukraine, as well as for key areas of the Bathonian and Callovian deposits in the Arctic, such as Northern Siberia, Southern Alaska and British Columbia. On the basis of the data obtained, a scheme for the detailed intraboreal correlation of the Bathonian and Callovian infrazonal scales and a new version of the Boreal ammonite standard has been developed. The theoretical part of the work includes the presentation of the concept of infrazonal stratigraphy, the principles of constructing the Boreal standard, methods of phylogenetic reconstructions in ammonites, the concept of morphs of intraspecific variability, conceptual foundations of the study of paleobiogeographic differentiation and invasions of extinct organisms.
Article
In this study, we analysed the ontogenetic trajectories of septal spacing between succeeding chambers of the gaudryceratid ammonoid, Gaudryceras tenuiliratum, which were collected in the Tomamae and Mikasa areas of Hokkaido, Japan. The ontogenetic trajectories of septal spacing in G. tenuiliratum demonstrate a general trend: two cycles of increasing to decreasing septal spacing until about the 30th septum, gradually decreasing septal spacing until about the 70th septum, and then gradually increasing septal spacing at least until about the 110th septum. The ontogenetic trajectories of the whorl expansion rate (WER) in our specimens also demonstrated that a decreasing trend changed into an increasing trend at a conch diameter of c. 5–7 mm. This conch diameter corresponds to the end of the second cycle of increasing to decreasing septal spacing occurring before about the 30th septum, at which point G. tenuiliratum possibly transitioned from planktic to nektobenthic habits. No significant differences were detected in the ontogenetic trajectories of septal spacing and conch shape between the two areas, which implies that the ontogenetic trajectory patterns of septal spacing in the Late Cretaceous ammonoids were taxonomy‐dependent rather than environment‐dependent, although this should be further examined with G. tenuiliratum collected from areas outside of Hokkaido.
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Measuring locomotion tactics available to ancient sea animals can link functional morphology with evolution and ecology over geologic timescales. Externally-shelled cephalopods are particularly important for their central roles in marine trophic exchanges, but most fossil taxa lack sufficient modern analogues for comparison. In particular, phylogenetically diverse cephalopods produced orthoconic conchs (straight shells) repeatedly through time. Persistent re-evolution of this morphotype suggests that it possesses adaptive value. Practical lateral propulsion is ruled out as an adaptive driver among orthoconic cephalopods due to the stable, vertical orientations of taxa lacking sufficient counterweights. However, this constraint grants the possibility of rapid (or at least efficient) vertical propulsion. We experiment with this form of movement using 3D-printed models of Baculites compressus , weighted to mimic hydrostatic properties inferred by virtual models. Furthermore, model buoyancy was manipulated to impart simulated thrust within four independent scenarios ( Nautilus -like cruising thrust; a similar thrust scaled by the mantle cavity of Sepia ; sustained peak Nautilus -like thrust; and passive, slightly negative buoyancy). Each model was monitored underwater with two submerged cameras as they rose/fell over ~2 m, and their kinematics were computed with 3D motion tracking. Our results demonstrate that orthocones require very low input thrust for high output in movement and velocity. With Nautilus -like peak thrust, the model reaches velocities of 1.2 m/s (2.1 body lengths per second) within one second starting from a static initial condition. While cephalopods with orthoconic conchs likely assumed a variety of life habits, these experiments illuminate some first-order constraints. Low hydrodynamic drag inferred by vertical displacement suggests that vertical migration would incur very low metabolic cost. While these cephalopods likely assumed low energy lifestyles day-to-day, they may have had a fighting chance to escape from larger, faster predators by performing quick, upward dodges. The current experiments suggest that orthocones sacrifice horizontal mobility and maneuverability in exchange for highly streamlined, vertically-stable, upwardly-motile conchs.
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Ammonoid soft parts have been rarely described. Here, we document the soft parts of a perisphinctid ammonite from the early Tithonian of Wintershof near Eichstätt (Germany). This exceptional preservation was enabled by the special depositional conditions in the marine basins of the Solnhofen Archipelago. Here, we document this find and attempt to homologize its parts with various organs such as the digestive tract, reproductive organs, the mantle cavity with gills, and the hyponome, with differing degrees of reservation. Alternative interpretations are also taken into account. We suggest that the soft parts were separated from the conch either taphonomically (following necrolytical processes affecting the attachment structures) or during a failed predation, where a predator (fish or coleoid) removed the soft parts from the conch but then dropped them. This find is interesting because it adds to the knowledge of ammonite anatomy, which is normally hidden in the conch. The reproductive organs show traces of what might have been spermatophores, thus supporting the hypothesis that the microconchs represented the males.
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Article
Models of “ideal” orthoconic shells having simple concave septa with minimal weight and maximal strength and analysis of 72 species of fossil orthocones and cyrtocones yield important insights into the physical principles underlying cephalopod shell design. The ideal septum is a spherical cap weighing only 77% of a hemispherical septum of equal strength. The septa of most longicones approximate this ideal shape while those of brevicones are less curved, probably owing to buoyancy problems. Increase in septal strength leads to weight increase unless the shell becomes more logiconic or septal spacing increases or both. However, increased spacing requires more cameral liquid for septum formation, thus reducing buoyancy. In ideal longicones, septal spacing resembles the cone radius for thick, strong septa but declines to half of the cone radius for thin, weak septa. In ideal intermediates and brevicones, spacings are respectively reduced by factors of about 2 and 4, with similar additional dependence on septal thickness. Most real septa resemble these ideal models. The relative length of the body chamber to the phragmocone varies greatly between about 0.2 and 1.5, depending mainly on the wall thickness and to a lesser degree on the septal thickness, apical angle and body density. Removal of cameral liquid in the adult must be compensated for by additional growth to retain neutral buoyancy. The conditions for neutral equilibrium calculated for longicones with different “counterweights” indicate that: (1) cameral liquid only is least feasible; (2) half-and-half calcium carbonate and liquid results in one-third length and one-quarter volume reduction of the body chamber; (3) with calcium carbonate only, body chamber reduction is minimal. Real 'counterweights' appear to be intermediate between (2) and (3), providing the animal with horizontal stability, which is missing in (3). Most uncalcified siphuncles reduce the body chamber only slightly although they improve horizontal stability. If the wall attains full thickness only at the apical end of body chamber, the liquid-only 'counterweight' becomes feasible.
Chapter
Theoretical morphology, which was first developed by Raup and Michelson (1965). is a means of describing the morphological spectra of extant and fossil organisms using a mathematical growth model. Raup (1966, 1967) simulated the three-dimensional morphology and growth pattern of marginally growing molluscan shells by several simple parameters and reproduced these shell shapes with the aid of computer graphics. His approach can be applied not only to interpret the functional and adaptive constraints of morphology but also to analyze morphogenesis. With the recent development of the computer and its graphic techniques, the theoretical morphological approach becomes useful for understanding the morphology of extant and extinct animals including ammonoids.
Article
The swimming potential of Late Cretaceous desmoceratine ammonites was analyzed experimentally. Resin casts were made of well-preserved specimens representing seven species of four genera [Desmoceras (Pseudouhligella), Tragodesmoceroides, Damesites, and Hauericeras] from Hokkaido (Japan) and Sakhalin (Russia). A model of a "living" ammonite was created by attaching an artificial body extension at the shell aperture of each cast. The drag force acting on each model was measured in a flow-tank over a range of water velocities from 0.03 to 0.48 m/s. In addition, flow patterns around the models were visualized and compared between species. Based on the relationship between drag coefficient and Reynolds number, power consumption was scaled to a range of sizes and swimming velocities for each specimen. Power consumption was compared between juvenile and adult shells of the same species. Our results show that the Cretaceous Desmoceratinae have similar hydrodynamic properties to other Mesozoic ammonites with low shellthickness ratios. Modest differences in hydrodynamic efficiency were detected among the species examined consistent with modest differences in shell geometry. In every species, the adult or subadult shell is hydrodynamically more efficient at higher swimming velocities than the juvenile shell.
Article
Information pertaining to the function of ammonoid shells is generated by analogy to living cephalopods, by measurement or experiment designed to elucidate the properties of the ammonoid shell in life situations, and by examination of the distribution and sedimentary environments in which ammonoid fossils are preserved. Virtually all discussions of ammonoid shell function implicitly or explicitly incorporate more than one of these approaches. The combination of analogy with empirical work in the field and laboratory makes the reconstruction of the function of ammonoid shells and interpretation of ammonoid life habits and mode of life particularly intriguing. These interpretations have led to many lively debates among paleobiologists. In this chapter, we examine ammonoid buoyancy and locomotion. We evaluate arguments that have been used to reconstruct the buoyancy and locomotor properties of these extinct cephalopods, discuss recent advances in the understanding of ammonoid locomotion, and suggest directions in which the study of these aspects of ammonoid paleobiology may proceed in the future. Other chapters of this book explore aspects of the structural issues pertaining to the implosion strength of ammonoid shells (Hewitt, Chapter 10, this volume) as well as the environmental information that can be brought to bear on the subject (Westermann, Chapter 16, this volume).