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Ammonite shell shape covaries with facies and hydrodynamics: Iterative evolution as a response to changes in basinal environment



Variation within an Upper Cretaceous ammonoid species that correlates with facies differences and is consistent with a hydrodynamic explanation is reported. In the Turner Sandy Member of the Carlile Shale (Turonian) of South Dakota and Wyoming, more compressed morphs of Scaphites whitfieldi Cobban are found in nearshore sandy facies, whereas more depressed morphs occur in offshore muds. Thinner, more compressed morphs swam more efficiently at higher velocities and depressed morphs swam more efficiently at low velocities. Higher swimming velocities may be essential for life in nearshore sandy environments. Shelled cephalopods swim most efficiently at low swimming speeds; therefore, lower velocity, more energetically economical swimming should be preferred in more quiescent offshore settings. -from Authors
doi: 10.1130/0091-7613(1994)022<0905:ASSCWF>2.3.CO;2
David K. Jacobs, Neil H. Landman and John A. Chamberlain , Jr.
as a response to changes in basinal environment
Ammonite shell shape covaries with facies and hydrodynamics: Iterative evolution
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Ammonite shell shape covaries with facies and hydrodynamics;
Iterative evolution as a response to changes
in basinal environment
Neìl'hl ^Lar^dman }" DePartment of invertebrates, American Museum of Natural History, New York, New York 10024
John A. Chamberlain, Jr. Department of Geology, Brooklyn College, Brooklyn, New York 11210
Shell shape varies within many ammonoid species, and some ammonoid lineages
appear to have evolved in concert with changes in their environment. We report variation
within an Upper Cretaceous ammonoid species that correlates with facies differences and
is consistent with a hydrodynamic explanation. In the Turner Sandy Member of the Carlile
Shale (Turanian) of South Dakota and Wyoming, more compressed morphs of Scaphites
whitfieldi Cobban are found in nearshore sandy facies, whereas more depressed morphs
occur in offshore muds. We measured drag forces on models of juvenile and adult shells
that differed in lateral compression of the shell. Plots of drag coefficient as a function of
Reynolds number indicate that thinner, more compressed morphs swam more efficiently at
higher velocities and depressed morphs swam more efficiently at low velocities. Higher
swimming velocities may be essential for life in nearshore sandy environments, which have
higher ambient current velocities. Shelled cephalopods swim most efficiently at low swim-
ming speeds; therefore, lower velocity, more energetically economical swimming should be
preferred in more quiescent offshore settings. An analysis of power consumption supports
this interpretation. Correlated changes in shell compression and environmental factors,
here observed within a species, have been documented in numerous ammonite lineages.
These iterative evolutionary changes within lineages may be similarly explained by selec-
tion for shell morphologies appropriate to environments that fluctuate cyclically with sea
Since the time of Hyatt (1889, 1894),
explanations have been sought for the fre-
quently repeated pattern of increased
compression and involution of the shell in
ammonoid lineages. More recently, work-
ers have linked this phenomenon of iter-
ative evolution with repeated patterns of
change in the paleoenvironment. Bayer
and McGhee (1984) examined ammonoid
lineages found in shallowing-upward cy-
cles in the Middle Jurassic of the German
basin and argued that separate lineages in-
volving Leioceratinae and Graphoceratinae
became more involute and compressed
during the regressive higher energy part of
each "Klupfel cycle." Bayer and McGhee
(1984) inferred that environmental influ-
ence was involved, but they were not spe-
cific. A similar pattern emerges in scaphite
lineages from the Cretaceous of the west-
ern interior seaway of North America. The
independent scaphite lineages Hop-
loscaphites and Jeletzkites extend from the
Pierre Shale to the overlying shallower
and sandier Fox Hills Formation; in each
"Present address: Department of Biology,
University of California, 405 Hilgard Avenue,
Los Angeles, California 90024-1606.
case the forms in the higher energy Fox
Hills Formation are more compressed
(Landman and Waage, 1993). Thus, a re-
lation between environment and hydrody-
namics has been suggested but has never
been tested explicitly.
In this work we compare shell shape and
hydrodynamic properties between popula-
tions of a single ammonoid species, Scaph-
ites whitfieldi Cobban. These specimens
are found in the same faunal zone but are
derived from facies with distinctly differ-
ent energy regimes. We examined suites of
specimens collected from these different
environments and measured differences in
thickness ratio (thickness ratio, T.R., is
whorl width/diameter of the coil) in the
populations from the two environments.
To determine if this variation in lateral
compression of the shell was hydrodynam-
ically important, we measured drag forces
on casts of the closely coiled juvenile and
the open-coiled adult forms of these
scaphites from the two distinct energy
regimes. These data permitted calculation
of drag coefficients and power require-
ments of the different morphs at various
swimming velocities. We then compare
these results with indicators of environ-
mental energy, including grain size and
Geographic and Stratigraphic Setting
Scaphites whitfieldi and Inoceramus per-
plexus Whitfield indicate faunal zone 16 in
the upper Turanian sedimentary strata of
the western interior seaway. This zone en-
compasses the lower half of the Turner
Sandy Member of the Carlile Shale in
South Dakota and Wyoming, as well as in
correlative rocks from Montana to New
Mexico (Landman, 1987). The Turner
Sandy Member is interpreted as a near-
shore lowstand transgressive facies ex-
tending into the seaway from the western
shore, on the basis of observations in Wy-
oming (Merewether and Cobban, 1986).
This interpretation is supported by our ob-
servations north and south of the Black
Hills, where a basal transgressive lag
forms a sharp contact with the underlying
silts and fine sands of the Poole Creek
Member. The presence of wood in the
Turner Sandy Member (Jacobs et al., un-
published) also suggests rapid transgres-
sion (Savrda, 1991). The flooding surface
at the base of the Turner provides a se-
quence stratigraphic and temporal bound-
ary across the region of study.
Within the Turner Sandy Member, fa-
cies differences are observable north and
south of the Black Hills (Fig. 1). In the
Belle Fourche region of South Dakota, the
Turner in the Scaphites whitfieldi zone has
often been described as predominantly
shale (e.g., Cobban, 1951). Our field ob-
servations and grain-size counts (Fig. 2)
indicate that this "shale" is composed pre-
dominantly of silt-sized quartz grains.
Southwest of the Black Hills in South Da-
kota the Turner consists of medium- to
fine-grained sand and contains larger frag-
ments of shell and wood. We collected nu-
merous Scaphites whitfieldi from three lo-
calities: Orman Dam near Belle Fourche,
South Dakota (AMNH 3177), in the silty
lower energy facies north of the Black
Hills; Edgemont, South Dakota (AMNH
3179); and the Boner Ranch in eastern
Wyoming (AMNH 3163), located south of
the Black Hills in the higher energy sandy
facies (Fig. 1).
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108°W 106°W 104°W 102°W
Land :
Nearshore sands [--'-"! Fine sands t~-~-~l Silts
Figure 1. Paleogeographic reconstruction of middle Turonian (Zone 16 of Mereweather and
Cobban, 1986) of part of western interior seaway (McGookey, 1972). Note three American Mu-
seum localities, one north of Black Hills in silty facies and two south of Black Hills in sands.
0 Scale
^ smaller Grain Size larger
Figure 2. Grain-size distributions based on uncorrected point counts of matrix samples asso-
ciated with fossils from localities AMNH 3177 (Orman Dam) located north of Black Hills and
AMNH 3179 (Edgemont) and AMNH 3163 (Boner Ranch) south of Black Hills. Note that sample
north of Black Hills is much finer (larger 4>) than two samples from south.
Within-species Variation
Although some specimens of S. whitfieldi
north of the Black Hills are slightly larger
than those to the south, both populations
are virtually identical in shell ornament in
terms of ribbing and other features consid-
ered important in discriminating the Scaph-
ites species of the western interior seaway
(Cobban, 1951). We did not consider closely
related species that differ in minor features
of ribbing and whorl shape (Cobban, 1951;
Landman, 1987), nor did we treat sexual di-
morphism because it primarily involved
characters of the adult body chamber
(Landman, 1987). Whereas the early onto-
genetic stages of Scaphites are closely coiled,
the final body chamber extends outward
from the coil, forming a hook that marks the
attainment of maturity (Landman, 1987;
Landman and Waage, 1993). Body expan-
sion and growth of the hook have been in-
terpreted as modifications for brooding of
young in an organism with a single terminal
phase of reproduction (Landman, 1987).
This form of reproduction is typical of mod-
ern coleoids (squids and ocotopods), the
closest living relatives of ammonoids (e.g.,
Jacobs and Landman, 1993). We assume
that growth of the hooklike body chamber
relates to a temporally limited terminal re-
productive phase in life history where lo-
comotion may be less important. Conse-
quently, our primary argument does not
involve the adult form. We investigate the
hydrodynamics of juveniles, perhaps com-
parable to the active feeding phase of
modern coleoids, where the influence of
hydrodynamic constraints should be more
We measured 69 specimens from Edge-
mont (AMNH 3179), 51 from the Boner
Ranch (AMNH 3163), and 26 from Orman
Dam (AMNH 3177). The measurements
taken included maximum adult length, body
chamber width, and the width and diameter
of the juvenile prior to growth of the hook
(Fig. 3). The parameter of greatest interest
in ammonoid hydrodynamics is the thick-
ness ratio (Jacobs, 1992). The thickness
ratio for juvenile Scaphites whitfieldi is cal-
culated by dividing the greatest whorl thick-
ness (JW) by the diameter of the shell (JD).
Despite their conspecific nature, juveniles
north of the Black Hills have a statistically
significant (p = 0.001) greater thickness ra-
tio than those south of the Black Hills
(AMNH 3177: mean thickness ratio =
= 0.049; AMNH 3163: mean thick-
ness ratio = 0.479, cr = 0.041; AMNH 3179:
mean thickness ratio = 0.495,
= 0.39).
Hydrodynamic Measurements
Drag forces were measured on juvenile
and adult casts from five specimens that rep-
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resent the range of morphology typical of
the variation found throughout Scaphites
whitfieldi (Fig. 4). Juveniles were obtained
by breaking back adult specimens. Drag was
measured in a Vogel-LaBarbera-type flow
tank with a beam-type strain transducer and
wheatstone bridge at velocities ranging from
1 to 50 cm/s (see Jacobs, 1992, for details).
The coefficient of drag (Cd) was then cal-
culated with the formula Cd = DflQ.SpAU2
(Df = drag force, A = an area term here
assumed to be volume2'3, p = density of wa-
ter, and U = velocity). Skin-friction drag is
important at low Reynolds numbers (Reyn-
olds number, Re, is l/L/|x, where U = ve-
locity, L = length in direction of flow, and p.
= kinematic vicosity). Jacobs (1992) dem-
onstrated that compressed ammonoids have
more skin-friction drag as a consequence of
greater surface area. Consequently, thin
compressed ammonoids have higher drag
coefficients than thicker ammonoids at low
Reynolds numbers. Conversely, at higher
Reynolds numbers these same compressed
morphs have lower coefficients of drag due
to smaller cross-sectional area and to lower
pressure drag. Thus, different shell shapes of
coiled ammonites perform optimally at dif-
ferent multiples of size and velocity. Thin
forms will be more efficient at faster swim-
ming, and thick forms will be more efficient
at slower swimming. Shelled cephalopods
will tend to be more efficient at lower swim-
ming, velocities than other swimming orga-
nisms. Thus, if ambient current velocities or
other aspects of the mode of life do not re-
quire rapid swimming, slow swimming
should be preferred.
Comparison of Cd
Re plots of juvenile
Scaphites whitfieldi with those for adults of
the same specimen (Fig. 4A) documents
that growth of the hook increases drag on
the adult form in horizontal swimming. Such
forms with large cross-sectional area, tan-
dem structures, or openings, such as that
produced by the hook, tend to generate
large drag forces at high Reynolds numbers
(e.g., Hoerner, 1965; Chamberlain, 1976).
Some authors (e.g., Westermann, 1990)
have argued that the adult Scaphites form
would have been effective at vertical migra-
tion. The Scaphites discussed here lived in
shallow water where vertical migration was
not likely to be a major issue.
In comparing the Cd vs. Re plot of two
juveniles (Fig. 4B) representative of the
range of thickness ratio variation found in
Scaphites whitfieldi, we found that, at higher
Reynolds numbers associated with higher
swimming speeds in these forms of compa-
rable size, the thinner form gains an increas-
ing advantage in terms of lower Cd. This is
consistent with the distribution of this form
in the sandier sedimentary strata exposed in
the outcrops south of the Black Hills. These
sandier environments suggest a higher en-
ergy regime.
We compare power required for swim-
ming at a range of velocities in the same two
juveniles discussed in the drag analysis. The
power required to overcome drag in swim-
ming ammonoids can be readily calculated
from Cd and power scaling techniques (Ja-
cobs, 1992). In this analysis the power con-
sumption was scaled to an identical volume
of 2 cm3 for the two models (see Jacobs,
1992, for details). Identical volume provides
the best biological grounds for comparison
(Vogel, 1981). Drag force and power con-
sumption were determined for the two
forms at velocities of 1, 5,10,15, 20, and 25
cm/s (Table 1).
These power data indicate that at swim-
ming speeds greater than —12 cm/s the
Figure 3. Lateral and ante-
rior views of adult and lat-
eral view of juvenile Scaph-
ites whitfieldi; LMAX =
maximum adult length,
BCW = maximum width of
adult body chamber, JD =
diameter of juvenile just
prior to uncoiling, JW =
width of
just prior to
uncoiling. Thickness ratio of
juvenile is JW/JD.
D) 1.75
§ 1-25
° 0.75
o Adult
* Juvenile
2* O^
to. ""top0
1.75 "
C 1.25 -
o 0.75
2000 4000 6000 S000 10000 12000 0.25
Sandy Facies
T. R. = 0.463
Silty Facies
A T.R. = 0.614
if A
A" A - V* * A ^
-vs . -.J
A " B
2000 4000
6000 8000 10000 12000
Reynolds number Reynolds number
Figure 4. A: Drag coefficient plotted as function of Reynolds number for adult and for unhooked juveniles of Scaphites whitfieldi. Data indicate
higher drag produced by hook in adult, documenting high drag in hooked heteromorphic forms. Difference is most evident at higher Reynolds
numbers. B: Drag coefficient plotted as function of Reynolds number for unhooked juvenile shells from near ends of range in thickness variation
in Scaphites whitfieldi. Note that, at higher Reynolds numbers, more compressed, lower thickness ratio form (T. R. = thickness ratio) has lower
coefficient of drag. Regressions of data for each of two specimens differ in highly significant manner in terms of slope as determined by the t
distribution (f = 3.25; t0 05(2| a6 =
p < 0.002; Zar, 1984). Three other specimens of intermediate thickness ratio were measured and produced
results intermediate to those shown.
GEOLOGY, October 1994 907
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JUVENILES OF Scaphites whitfieldi
1 5 1Q 15 2(1 25
Thin juvenile 1.53 24.54 74.48 129.70 203.46 250.11
Thick juvenile 0.91 19.15 70.03 150.42 256.94 389.06
Note: Values are ergs per second. Power scaled to volume of
thinner form has lower power require-
ments: however, at speeds less than —12
cm/s the thicker form requires less power.
Thus, if swimming speeds >12 cm/s were
important for survival, the thinner form
would be preferred. Such swimming
speeds may have been of greater impor-
tance in the higher energy inshore envi-
ronment represented by the sandy facies
south of the Black Hills. Conversely, swim-
ming faster than 12 cm/s may not have
been advantageous in the quieter shaley
facies to the north.
As discussed in the Introduction, several
lines of evidence indicate that there is a re-
lation between ammonite evolution and en-
vironment in lineages evolving in epeiric
seas or basins. Here we document the rela-
tion between shell morphology and environ-
ment on a finer scale, within geographically
distinct populations of a single species. We
demonstrate that the covariation of shell
shape in terms of thickness ratio is consist-
ent with a hydrodynamic explanation. Thin-
ner forms permit more efficient rapid loco-
motion and are found in higher energy
environments. Thicker forms permit more
efficient lower speed locomotion and are
found in lower energy offshore environ-
These results have several implications.
First, ammonite species appear to be able
to respond relatively plastically to environ-
mental change. This may account for the
variation observed within many ammonite
species (Reeside and Cobban, 1960; West-
ermann, 1966; Reyment and Kennedy,
1991) and may provide an avenue for reas-
sessing the species level taxonomy of many
groups. Second, the relation that we have
demonstrated between environment, shell
shape, and hydrodynamics within the fossil
species Scaphites whitfieldi provides a causal
rationale for the repeated trends of in-
creased shell compression observed in am-
monoid lineages in gradually shallower ba-
sinal settings. However, at the lower
taxonomic level of our observations, the en-
vironments in question are nearly contem-
poraneous, and the differences in genetic
stock are likely to have been small: with
these variables constrained, the argument
for selection can be made forcefully. That
the variation within the species parallels the
trends observed within lineages suggests
that the lineage level trends are a conse-
quence of the same selective processes op-
erating within or between the constituent
Sea-level change is known to mediate the
environmental changes associated with
many of the lineage trends observed. This is
the case in scaphitid lineages in the Creta-
ceous western interior seaway (Landman
and Waage, 1993) and the Jurassic of the
German basin (Bayer and McGhee, 1984),
where facies changes correlate with trans-
gressive-regressive cycles and in turn corre-
spond to episodic changes in the compres-
sion of the shells in ammonite lineages.
Thus, many of the iterative morphologic
changes observed in ammonoids appear to
be hydrodynamic adaptive responses to sea-
level-mediated changes in environment.
Supported by National Science Foundation
grant EAR-9104888 and by grants from the City
University of New York PSC-CUNY Research
Award Program. We thank S. Klofak, K. Sarg,
J. Whitehill, and B. Worcester for assistance with
model construction and flow-tank measurements,
T. Baumiller and M. Droser for their helpful com-
ments, and S. Roos for assistance with the
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evolution of Middle Jurassic ammonite fau-
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drag coefficients of cephalopod shells: Palae-
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the Colorado Group: U.S. Geological Survey
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Manuscript received April 4, 1994
Revised manuscript received June 27, 1994
Manuscript accepted June 30, 1994
908 Printed in U.S.A. GEOLOGY, October 1994
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Full-text available
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We examine temporal and spatial variation in morphology of the ammonoid cephalopod Discoscaphites iris using a large dataset from multiple localities in the Late Cretaceous (Maastrichtian) of the U.S. Gulf and Atlantic Coastal Plains, spanning a distance of 2000 km along the paleoshoreline. Our results suggest that the fossil record of D. iris is consistent with no within-species net accumulation of phyletic evolutionary change across morphological traits or the lifetime of this species. Correlations between some traits and paleoenvironmental conditions as well as changes in the coefficient of variation may support limited population-scale ecophenotypic plasticity; however, where stratigraphic data are available, no directional changes in morphology occur before the Cretaceous/Paleogene (K/Pg) boundary. This is consistent with models of “dynamic” evolutionary stasis. Combined with knowledge of life-history traits and paleoecology of scaphitid ammonoids, specifically a short planktonic phase after hatching followed by transition to a nektobenthic adult stage, these data suggest that scaphitids had significant potential for rapid morphological change in conjunction with limited dispersal capacity. It is therefore likely that evolutionary mode in the Scaphitidae (and potentially across the broader ammonoid clade) follows a model of cladogenesis wherein a dynamic morphological stasis is periodically interrupted by more substantial evolutionary change at speciation events. Finally, the lack of temporal changes in our data suggest that global environmental changes had a limited effect on the morphology of ammonoid faunas during the latest Cretaceous.
Biomechanical analyses provide unique insights on state shifts in the ecology of extinct communities. Ammonoids present a compelling case study for coupling biomechanical analysis with ecology given their robust fossil record of external conchs. We present a trajectory model to evaluate hydrodynamical advantages and challenges associated with ammonoid conch form. The model is a one-dimensional calculation estimating the dynamic feedbacks between different components of an ammonite’s motion including thrust, drag, acceleration, and distance traveled. Computational fluid dynamics simulations were performed on eleven different ammonoid conch morphotypes and integrated into a mathematical model to analyze the dynamics of swimming across a combination of conch diameters (5, 10, and 20 cm) and jet rhythms (a single jet or series of three pulses). We compared the efficacy of short-term bursts of motion to that of long-term cruising and found: inflated shapes (i.e., spherocones) offer the fastest short-term motion, but at the greatest costs; heavily streamlined shapes (i.e., platycones) offer long cruise distances, but with ineffective short-term motion; and visibly-coiled shapes (i.e., serpenticones) appear to offer intermediate performance in both locomotion styles. Size is critical in ranking the performance of different conch shapes in both locomotion styles because ranking is determined predominantly by the amount of thrust an animal is capable of generating. With increasing size, Reynolds number increases and the effects of second-order morphological characters become more pronounced and alter the performance ranking of conchs. Finally, we present a visual analysis of the flow regimes and shape details that may drive these hydrodynamic consequences. We speculate that serpenticone morphologies capitalized on these subtleties with a morphology that provided reasonably high-speed swimming at small sizes relevant to juveniles while maintaining relatively efficient coasting locomotion at the larger sizes relevant to adult animals. We highlight the ubiquitous serpenticones of the Early Jurassic as a case study for applying biomechanical data to a paleoecological context. The broad range of morphotypes expressed by ammonoids in the Late Triassic is dramatically pared down during the End Triassic extinction. In the few million years following the extinction, ammonoids diversify into a suite of shapes with a more restricted range of locomotor performance, and only much later is the full range of morphology recovered.
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Heteromorphs are ammonoids forming a conch with detached whorls (open coiling) or non-planispiral coiling. Such aberrant forms appeared convergently four times within this extinct group of cephalopods. Since Wiedmann's seminal paper in this journal, the palaeobiology of heteromorphs has advanced substantially. Combining direct evidence from their fossil record, indirect insights from phylogenetic bracketing, and physical as well as virtual models, we reach an improved understanding of heteromorph ammonoid palaeobiology. Their anatomy, buoyancy, locomotion, predators, diet, palaeoecology, and extinction are discussed. Based on phylogenetic bracketing with nautiloids and coleoids, hetero-morphs like other ammonoids had 10 arms, a well-developed brain, lens eyes, a buccal mass with a radula and a smaller upper as well as a larger lower jaw, and ammonia in their soft tissue. Heteromorphs likely lacked arm suckers, hooks, tentacles , a hood, and an ink sac. All Cretaceous heteromorphs share an aptychus-type lower jaw with a lamellar calcitic covering. Differences in radular tooth morphology and size in heteromorphs suggest a microphagous diet. Stomach contents of heteromorphs comprise planktic crustaceans, gastropods, and crinoids, suggesting a zooplanktic diet. Forms with a U-shaped body chamber (ancylocone) are regarded as suspension feeders, whereas orthoconic forms additionally might have consumed benthic prey. Heteromorphs could achieve near-neutral buoyancy regardless of conch shape or ontog-eny. Orthoconic heteromorphs likely had a vertical orientation, whereas ancylocone heteromorphs had a near-horizontal aperture pointing upwards. Heteromorphs with a U-shaped body chamber are more stable hydrodynamically than modern Nautilus and were unable substantially to modify their orientation by active locomotion, i.e. they had no or limited access to benthic prey at adulthood. Pathologies reported for heteromorphs were likely inflicted by crustaceans, fish, marine reptiles, and other cephalopods. Pathologies on Ptychoceras corroborates an external shell and rejects the endocochleate hypothesis. Devonian, Triassic, and Jurassic heteromorphs had a preference for deep-subtidal to offshore facies but are rare in shallow-subtidal, slope, and bathyal facies. Early Cretaceous heteromorphs preferred deep-subtidal to bathyal facies. Late Cretaceous heteromorphs are common in shallow-subtidal to offshore facies. Oxygen isotope data suggest rapid growth and a demersal habitat for adult Discoscaphites and Baculites. A benthic embryonic stage, planktic hatchlings, and a habitat change after one whorl is proposed for Hoploscaphites. Carbon isotope data indicate that some Baculites lived throughout their lives at cold seeps. Adaptation to a planktic life habit potentially drove selection towards smaller hatchlings, implying high fecundity and an ecological role of the hatchlings as micro-and mesoplankton. The Chicxulub impact at the Cretaceous/Paleogene (K/Pg) boundary 66 million years ago is the likely trigger for the extinction of ammonoids. Ammonoids likely persisted after this event for 40-500 thousand years and are exclusively represented by heteromorphs. The ammonoid extinction is linked to their small hatchling sizes, planktotrophic diets, and higher metabolic rates than in nautilids, which survived the K/Pg mass extinction event.
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The biomechanics of uncoiled heteromorph ammonoids with body chambers that terminate in U-shaped hooks (ancylocones) were investigated with virtual and physical models of Audouliceras renauxianum. Virtual models were used to compute the hydrostatic properties of this morphotype. Audouliceras has the capacity for neutral buoyancy and this suggests that other taxa with similar proportions had this ability as well. Hydrostatic stability gradually increases during ontogeny, coincident with the larger degree of uncoiling. The juvenile planispiral stage has a similar stability and apertural orientation to the extant Nautilus. The adult stage, however, undergoes an increase in stability by a factor of over 3, while assuming an upwardfacing posture. Counterintuitively, the stage during the formation of the shaft (before the growth of the U-shaped hook) is oriented horizontally. This intermediate stage would have had poor horizontal mobility due to the positioning of the hyponome below the centre of mass. The juvenile planispiral stage and mature stage, however, would have been well suited to horizontal backward movement with minimal rocking. Ancylocones are generally thought of as quasiplanktic vertical migrants. Thus, their relative horizontal swimming ability has been largely disregarded. Experiments on 3D printed, neutrally buoyant physical models reveal that hydrodynamic drag is indeed larger compared to Nautilus. However, Audouliceras could reach similar maximum horizontal velocities depending on the available thrust. Sepia-like thrusts yield velocities similar to equivalently sized Nautilus (c. 15 cm/s), while Nautilus-like thrusts yield velocities not much lower (c. 11 cm/s). Due to the hydrostatic properties of the ancylocone, the adult model undergoes less rocking (±4.5°) during movement than Nautilus (±10°). The minimal hydrodynamic consequences for ancylocones suggest that stability, orientation and directional efficiency are key selective pressures for some heteromorph shells, which may have primarily served as hydrostatic devices.
Scaphitid ammonoids were ubiquitous and significant components of the Western Interior Seaway during the Late Cretaceous. This group is characterized by a recurved hook at maturity that deviates from the juvenile whorls. Such a modification seems counterproductive to active locomotion and to manage a biologically effective orientation that facilitates efficient feeding and swimming. Virtually reconstructed 3D hydrostatic models reveal that the examined mature scaphitids had the capacity for neutral buoyancy while assuming a stable, upward-facing orientation in the water column during life. Models of juvenile Hoploscaphites nicolletii suggest that scaphitid apertures were oriented only slightly more horizontal than adults. The hydrostatic influence of sexual dimorphism was explored with the species Hoploscaphites crassus. The inflated macroconch has a lower stability and higher hydrodynamic drag compared to its microconch counterpart. The effect of shell compression was investigated by comparing H. crassus and the more compressed H. nicolletii. The latter species has a relatively high stability and much less hydrodynamic drag during movement. The mature U-shaped body chamber distributes organismal mass in a way that increases stability, and simultaneously orients the soft body so that propulsive energy is efficiently transmitted into horizontal backwards movement with minimal rocking. Swimming velocities computed from hydrodynamic drag experiments suggest that scaphitids were relatively slow swimmers with compressed forms attaining slightly higher velocities (when scaled by mass). Hydrodynamic lift was investigated with computational fluid dynamics simulations. These experiments revealed that the overall shape of the shell is responsible for significant lift in the upwards direction, which is not heavily influenced by ornamentation. This explains how a reduced soft body can overcome and manage a slightly negatively buoyant condition during life. Therefore, the seemingly cumbersome shape and orientation of the scaphitid morphotype may not have been a hindrance during locomotion.
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We test for the presence of evolutionary stasis in a species of Late Cretaceous ammonoid cephalopod, Hoploscaphites nicolletii, from the North American Western Interior Seaway. A comprehensive dataset of morphological traits was compiled across the entire spatial and temporal range of this species. These were analysed in conjunction with sedimentologically and geochemically derived palaeoenvironmental conditions hypothesized to apply selective pressures. All changes in shell shape were observed to be ephemeral and reversable, that is, no unidirectional trend could be observed in any of the morphological traits analysed. Correlations between palaeoenvironmental conditions and morphological traits suggests ecophenotypic processes were at play; however, either environmental changes were too minor and/or provided no isolating mechanism to drive speciation. These data support mechanisms of stasis such as homogenizing gene flow or stabilizing selection under a fluctuating optimum (probably reflecting spatiotemporally heterogeneous palaeoenvironmental conditions). Finally, changes in shell size were not significantly associated with changes in shell‐specific δ18O, despite a correlation between shell size and δ18O averaged across horizons. This suggests a mismatch in scales of geochemical sampling that supports caution when making broad interpretations based on averaged geochemical data. Raw morphological and isotopic data available on the Dryad digital repository:
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Iterative evolution has proved a difficult evolutionary phenomenon to study and interpret. Inferences of causality vary from study to study and quantitatively based phylogenetic reconstruction has never been attempted. In an effort to better understand iterative evolution we employed stratocladistics, gap analysis and disparity analysis to study the case of the Monograptidae in the aftermath of the late Silurian Cyrtograptus lundgreni extinction event. Our combination of gap analytical and stratocladistic techniques allowed us to elucidate the evolutionary relationships between the studied taxa. Based on our stratocladistic results we recommend the generic reassignment of five monograptid taxa. The stratocladistic results, in conjunction with morphological disparity analysis suggest the presence of a persistent developmental potential for the emergence of iteratively evolving characters. This persistent potential appears to be limited by extrinsic ecological constraints, which would have relaxed in the aftermath of the C. lundgreni extinction event. Our findings indicate that iterative evolution in the late Silurian Monograptidae is a product of the interaction of both intrinsic and extrinsic constraints on the acquisition of the iteratively evolving character, with the exact causality being dependent on the particular character.
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This study assesses swimming potential in a variety of ammonoid shell shapes on the basis of coefficients of drag (Cd) and the power needed to maintain a constant velocity. Reynolds numbers (Re) relevant to swimming ammonoids, and lower than those previously studied, were examined. Power consumption was scaled to a range of sizes and swimming velocities. Estimates of power available derived from studies of oxygen consumption in modern cephalopods and fish were used to calculate maximum sustainable swimming velocities (MSV). Laterally compressed, small thickness ratio (t. r.) ammonoids, previously assumed to be the most efficient swimmers, do not experience the lowest drag or power consumption at all sizes and velocities. At low values of size and velocity associated with Reynolds numbers below 10 ⁴ , less compressed forms have smaller drag coefficients and reduced power requirements. At hatching a roughly spherical shell shape would have minimized drag in ammonoids; with increasing size, hydrodynamic optima shift toward compressed morphologies. The high energetic cost of ammonoid locomotion may have limited dispersal and excluded ammonoids from high current velocity environments.
Allochthonous logs and/or Teredolites, clavate borings produced within xylic (wood) substrates, occur in extraordinary abundance as sedimentary components in transgressive marine shelf deposits of the lower Paleocene Clayton Formation in western Alabama. It is proposed herein that the abundance and preservational state of these components were controlled by 1) an influx pulse of xylic substrates into marine and marginal-marine environments; 2) hydraulic concentration of substrates during ravinement; and 3) condensation associated with sediment starvation, all three of which are induced by sea-level rise. The proposed relations among fossil wood, ichnofossils produced therein, and sea-level dynamics may be of use in the discrimination of sequence stratigraphic packages. -from Author
Inferences drawn from the biology, function, and behavior of closely related living forms facilitate interpretation of the mode of life of groups known only from the fossil record. The choice of phylogenetically relevant modern ‘model organisms’ can have critical bearing on the resulting interpretations. The biology and behavior of fossil ammonoids are often interpreted in the light of evidence derived from the study of modern Nautilus. However, examination of the fossil record and cladistic analyses both indicate that coleoids are much more closely related to ammonoids than is Nautilus. Coleoid biology and behavior differ dramatically from the biology and behavior of Nautilus. Thus, the inclusion of coleoids as examples, rather than reliance on Nautilus alone, produces a strikingly different vision of ammonoid biology and suggests that inferences of ammonoid biology and behavior that rely exclusively on Nautilus should be reviewed. Two features related to swimming ability in Nautilus, static stability and large retractor muscles, are much reduced in many ammonoids, leading to the interpretation that ammonoids were poorer swimmers than Nautilus. However, reexamination of the evidence indicates that static stability should not play a role in the swimming of ammonoids with long body chambers. In addition, functional arguments suggest that a coleoid-like swimming mechanism should have evolved prior to the loss of the body chamber in coleoids. Thus, a coleoid-like swimming mechanism is likely to have evolved prior to the separation of ammonoid and coleoid lineages. A mechanism is proposed by which a coleoid swimming mechanism, independent of retractor muscle size, could function in ammonoids with long body chambers.□Ammonoids, ammonites, evolution, functional morphology, Nautilus, phylogeny.