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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
0°
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%
5° 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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