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Geology
doi: 10.1130/0091-7613(1994)022<0905:ASSCWF>2.3.CO;2
1994;22;905-908Geology
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
ABSTRACT
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
level.
INTRODUCTION
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
paleogeography.
SPECIMEN AND ENVIRONMENTAL
INFORMATION
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.
906
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
important.
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 =
0.535,
CT
= 0.049; AMNH 3163: mean thick-
ness ratio = 0.479, cr = 0.041; AMNH 3179:
mean thickness ratio = 0.495,
cr
= 0.39).
Hydrodynamic Measurements
Drag forces were measured on juvenile
and adult casts from five specimens that rep-
GEOLOGY, October 1994
i
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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
vs.
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
juvenile
just prior to
uncoiling. Thickness ratio of
juvenile is JW/JD.
2.25
D) 1.75
S
XI
§ 1-25
0
1
O
° 0.75
0.25
o Adult
* Juvenile
•
2* O^
to. ""top0
• • •• •<
2.25
o>
s
•o
1.75 "
C 1.25 -
<U
0
1
o
o 0.75
2000 4000 6000 S000 10000 12000 0.25
• •
A
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 =
1.989;
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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TABLE 1. POWER REQUIRED FOR SWIMMING BY
JUVENILES OF Scaphites whitfieldi
Velocity
(cmls)
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
2cm3.
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.
IMPLICATIONS
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-
ments.
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
species.
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.
ACKNOWLEDGMENTS
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
manuscript.
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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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