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Ammonoid ecology of the Pennsylvanian Epoch
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Erin B. Schuster, Kathleen A. Ritterbush, and David J. Bottjer
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RRH: Ammonoid Ecology
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LRH: Schuster et al
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Abstract. – Ammonoids existed from the Devonian to the end of the Cretaceous before going
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extinct. During this span of 350 million years (Ma), the ammonoids dominated the oceans. In the
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Pennsylvanian epoch, these animals lived both in the Panthalassic Ocean and in the epeiric seas.
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Within the seas, the ammonoids were potentially restricted to pelagic life styles, either
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swimming or drifting, due to anoxic bottom water. To assess the modes of life of these
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ammonoids, the shells were measured for three parameters – umbilical exposure, thickness ratio,
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and whorl expansion – and placed into an empirical morphospace. Within the morphospace, the
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ammonoids would plot into drifters or swimmers based on the size of their shells. Further, based
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upon Jacobs (1992) work, the ammonoids should maintain a hydrodynamically efficient shell
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shape as they grew in diameter. Finally, it would be expected that there would be very little
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variation within the species, assuming selective pressure, and very little variation between the
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two species based upon initial observations. Wewokites venatum and Eoasianites hyattianum,
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two species from this epoch, both had shells that appeared to be nearly spherical, but their
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similarities ended there. Coming from different families, these two species were statistically
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different from each other for umbilical exposure, thickness ratio, and whorl expansion. In
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addition, each species had a high degree of variation which did not correlate with size. Within
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the morphospace plot, the ammonoid shells randomly fell along one side, with no size basis. The
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shells also failed to show any relationship between the thickness ratio and the size of the shell
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and the shells did not maintain hydrodynamically efficient shapes. The results suggest that there
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may have been low selective pressures on these ammonoids, specifically in regards to
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hydrodynamics. However, another explanation could be that a secondary morphospace need s to
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be developed for ammonoids smaller than 10 mm.
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Introduction
Ammonoids have been in existence from the Devonian to the end of the Cretaceous. Over
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this span of 350 Ma, ammonoids dominated the ancient oceans, diversifying into many different
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ecological niches. However, it is difficult to determine these niches, as the organisms are now
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extinct (Westermann, 1996) and they are most closely related to Coleoidea, which lacks the
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external shell of an ammonoid. The Nautilus, a more distant relative, has an external shell,
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however there are differences to prevent the shell from being a direct ammonoid analog, such as
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muscle attachment sites and suture shape and complexity (Westermann, 1996). Finally the outer
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shape is limited and does not correlate with several of the different ammonoid shell shapes. At
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best the Nautilus shell is useful for studying the basics of possible ammonoid physiology and
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physics (Westermann, 1996). Further, it is unknown if and how the soft body of an ammonoid
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would affect hydrodynamics. In the 200 years of cephalopod fossil research, the preservation of
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muscular mantle tissue, as well as other soft body tissues, is very rare (Doguzhaeva et al., 2007).
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Several studies have looked at ammonoids and their morphology. Raup (1967) began by
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developing a mathematical model in order to determine all the potential shapes an ammonoid
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shell could form. However, only a small portion of these exist within the fossil record and Raup
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sought to determine which shapes were not created, and the reasons why (Raup, 1967).
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Westermann (1996) developed a diagram as a way to summarize his review of ammonoid
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research. This inverted triangle proposed ammonoid shapes that exist and how the shells would
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change over time. The diagram showed that the shells found in the fossil record were essentially
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gradations of each other. As opposed to Raup, Westermann sought to show which shell shapes
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existed and from there determine general modes of life. Both of these areas of research were
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combined in the Ritterbush and Bottjer (in review) study. This study used the parameters
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developed by Raup (1967) to quantify ammonoid shells and the Westermann diagram (Figure 1
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A) to create a morphospace, termed the Westermann Morphospace (Figure 1 B), to aid in
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determining ammonoid ecology.
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Other studies have looked at ammonoid ecology from a different perspective. In his study
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on hydrodynamics, Jacobs’ (1992) experiments suggested that the ideal ammonoid shell shape to
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facilitate swimming would be one that has a disc like shape when the shell is larger than 10 mm
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in diameter. Below this threshold the study showed that the shells would be more spherical,
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lending them to drifting, more than swimming. Further, as the ammonoid grew from a 1mm
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hatchling to a 10 mm diameter shell the animal would have developed an optimal thickness ratio,
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or shell width, below spherical in order to maintain hydrodynamic efficiency (Jacobs 1992).
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However, for animals smaller than 5 cm it would have been very difficult to remain stationary
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and maintaining swimming speeds of 15 cm/s, as opposed to 10 cm/s tidal flows in today’s
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oceans (Jacobs, 1992), would have been taxing. Ammonoids of this size would have been
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restricted to low energy seas for this reason (Jacobs, 1992).
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While some ammonoids did live in the larger ocean basin, many were confined to low
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energy of epeiric seas. During the Pennsylvanian epoch, many ammonoids lived within a mid-
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continent sea way, located in the middle of modern North America (Algeo and Heckel, 2008).
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This sea would have likely had estuarine-style circulation, based upon freshwater input at one
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end and ocean communication at the other (Algeo and Heckel, 2008). This would cause a
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pycnocline within the sea way, which in turn would cause sudden intensification of benthic
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anoxia during events of deglaciation or increased precipitation (Algeo and Heckel, 2008).
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Further the pycnocline would have limited vertical mixing, therefore maintaining oxygen
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deficient conditions within the sea (Algeo and Heckel, 2008). Within the epeiric seas,
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hypothetically most of the ammonoids would have been pelagic falling equally into the
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swimming, drifting, and vertical migrant patterns based upon their shell shapes and sizes
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(Westermann, 1996). Within this pattern, the larger ammonoids were more likely to be
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swimmers, while smaller ammonoids were more likely to be drifters (Jacobs, 1992). Frequent
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elimination of benthic habitat could have contributed to the high degree of variation of
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ammonoid groups occupying shallow basins (epeiric seas) (Jacobs 1992).
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This study focused on two different Pennsylvanian Epoch species, Eoasianites
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hyattianum and Wewokites venatum, collected from the Wewoka formation in Oklahoma. This
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places the species within the epeiric sea that existed in the middle of modern North America
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(Blakey). Despite the apparent similarity between the shells, the two genera are distantly related.
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Both of these species share the same order, Goniatitida, and suborder, Goniatitina, however E.
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hyattianum belongs to the Neoicoceratidae family and the Neocicoceratoidea super family while
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W. venatum belongs to the Wiedeyoceratidae family and the Gonioloboceratoidea super family
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(Treatise, 2009). While, both general have nearly spherical shells, there are other diagnostic
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differences. Wewokites has a very small conch which is subdiscoidal to subglobular and involute
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throughout. The conch also has a relatively wide umbilicus with a nodose umbilical shoulder that
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sometimes extends as low as ridges toward the ventral side. The growth lines along the shell are
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biconvex, the suture line is primitive, and a ventral furrow may be present. Finally the lobes and
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saddles are rounded, even at their base, and the median saddle is relatively high (Treatise, 2009).
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Eoasianites are subdiscoidal and evolute, with a low aperture height. The shells usually have
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transverse striae, usually with orad salient. Also constrictions may be present, but umbilical
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tubercles are confined to immature stages. Finally the ventral lobe has slightly pouched prongs,
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the median saddle exceeds two-thirds the height of the entire ventral lobe, and the first lateral
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saddle is subacute (Treatise, 2009).
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Despite all the work done with ammonoid shells, there is still a large amount of research
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required to understand ammonoid ecology as related to morphology, particularly in
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Pennsylvanian ammonoids. This study looked at the ecology of two of the species from the
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Pennsylvanian, Wewokites venatum and Eoasianites hyattianum. Both of these species, though
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from different families, looked essentially the same and lived in the same epeiric sea conditions.
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Based upon this, the study sought to determine if the shells filled the life mode proposed in
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previous studies, specifically that they would be pelagic mostly swimming, as larger animals, or
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drifting, as smaller animals. In this same area, the study looked at the variation within and
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between both species. The expected low variation would show that there were selective pressures
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for the shells to form a particular shape, while high variation would suggest little to no selective
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pressure.
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This study then applied Jacobs (1992) study to these ammonoids. Based upon Jacobs
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findings, the ammonoids should be more disk-like if they were swimmers, i.e. larger than 10 mm,
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and spherical if they were drifters, i.e. smaller than 10 mm. Further, this study looked to see if, as
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the shells grew in size, there was trend in decreasing thickness ratio as the shells reached the 10
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mm mark mentioned in Jacobs study. This decreasing thickness ratio would mean that the
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animals would transition from drifters to swimmers as they grew in size.
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Methods
The ammonoids for this study were on loan from the University of Oklahoma’s Sam
Noble Natural History Museum. The entire collection was comprised of 328 specimens, from
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four different species – Wewokites venatum, Wewokites newelli, Eoasianites angulatus, and
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Eoasianites hyattianum. Wewokites venatum and Eoasianites hyattianum were selected for the
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study as they had the largest number of measureable specimens. A total of 57 W. venatum and 75
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E. hyattianum shells were preserved in a state where all the relevant measurements could be
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made. The remaining 52% of the specimens were damaged in such a way that at least one
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measurement could not be made.
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In order to analyze the species within the Westermann Morphospace the shells of each
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species were measured to create ratios which express the basic exterior shell shape. For each
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shell there were five different measurements. These were the diameter of the shell, the whorl
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width (b), the final whorl height (a), the whorl height 180° from the final whorl (a’), and the
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umbilical diameter (ud) (Figure 2). Of the measurements, two, diameter and b, were measured by
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hand before each of the specimens were photographed twice, once to view the coiling of the
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conch and once for either the aperature or venter view. From the photographs all five
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measurements, including diameter and b, were made digitally using the program ImageJ. Using
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the diameter measurements, both hand (mm) and digital (pixels), a ratio was generated which,
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when applied to the other digital measurements changed them from pixels to millimeters. These
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measurements were then used to generate the dimensionless ratios which plot into the
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Westermann Morphospace.
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These ratios characterized the umbilical exposure (U), thickness (TH), and whorl
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expansion (W) of each shell. Each of these ratios changed the way the shell moved within the
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water, thereby changing the way the ammonoid could potentially live. The umbilical exposure
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ratio was generated by dividing ud by diameter (Table 1), and showed how the umbilicus
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changed through ontogeny (Smith, 1986). This calculation, as opposed to previously proposed
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rations, described the impact of umbilical exposure on the whole shell. Further, the umbilical
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exposure was more clearly measureable on most specimens than the interior of the umbilicus.
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Thickness ratio, generated by dividing b by diameter (Table 1), related to the amount of drag,
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dependent on shell size, produced by water on the shell (Jacobs, 1992). Finally, the whorl
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expansion rate, a divided by a’ (Table 1), related to how the shell grew with each rotation.
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The ratios then needed to be scaled because they have different ranges. For example, U
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was generally less than one and W generally greater than one, forcing all the points to plot within
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the W corner of the ternary diagram. Each ratio was scaled based upon maxima and minima
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developed by the Ritterbush and Bottjer (in review) study. These values were created based upon
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a group of ammonoids identified in the Treatise (2009) (Ritterbush and Bottjer, in review). This
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scaling process required the minimum, or two standard deviations from the mean, to be
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subtracted from the ratio value. This was then divided by the maximum, or mean, minus the
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minimum (Table 2). A Mann Whitney U test was used to show if the ratios were statistically
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different between the two species, W. venatum and E. hyattianum. Also, a Wilcoxon signed-
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ranks test was used between the hand (mm) generated TH and the digitally generated TH
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because it established if the photography method was accurate.
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In addition to using the U, W, and TH ratios to place the shells into Westermann
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Morphospace, the ratios were compared to determine if relationships existed between them and
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the shell size. For each species it was determined if diameter and the different ratios fell into
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normal, or Gaussian distributions. Finally each ratio was compared to diameter to determine if
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shape changed with increasing size.
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Results
With this study, ammonoid shells from two different species were examined using a
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specialized method of photographing the specimens and generating digital measurements. In
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order to determine the accuracy of this method the thickness ratio derived from hand
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measurements and from digital measurements were compared using a Wilcoxon signed-ranks
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test. This test generated a p-value greater than 0.05, indicating that the difference between the
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hand and digital ratios were not significantly different.
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After testing the feasibility of the method, the ratios for each species were compared
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using a Mann Whitney U Test. For each of the ratios: umbilical radius, thickness ratio, and whorl
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expansion rate, there was a very strong statistical difference (p < 0.001) between the two species
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for umbilical exposure and thickness ratio. However there was no statistical difference (p > 0.05)
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between the two species for whorl expansion rate. When the two species were compared again,
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excluding first the shells 11mm and larger and then again excluding shells 10 mm and larger, the
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ratios all showed strong statistical difference (p < 0.001) between the species.
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Following this, the shells were tested to determine the type of distribution for each of the
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ratios. For both species diameter, umbilical exposure, thickness ratio, and whorl expansion rate
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all fell into a normal or Gaussian distribution. However, several of the distributions are skewed
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slightly to the left or right, or have a longer tail (Figure 5).
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In accordance to Jacobs (1992) study, the thickness ratios of the species were compared
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to diameter. For W. venatum this comparison generated a r2 value of 0.115 (Figure 3 A) and E.
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hyattianum a r2 value of 0.1207 (Figure 3 B). Following this, all of the other ratios were
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compared to diameter. W. venatum had r2 values of 0.1598 for umbilical exposure and 0.019 for
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whorl expansion rate (Figure 3 C, D). E. hyattianum had r2 values of 0.0217for umbilical
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exposure and 0.0014 for whorl expansion rate (Figure 3 E, F).
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Finally both species were placed into Westermann Morphospace. Both W. venatum and E.
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hyattianum fall randomly along the right leg of the ternary diagram (Figure 4 A). W. venatum
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showed a wide scatter within the plot, from the top leg to about 60% of the way down the right
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leg with little dependence on size of the shell (Figure 4 B). E. hyattianum also was scattered,
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however, the points only reached from the top let to about 50% of the way down the right leg
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(Figure 4 C). Again there was no dependence on shell size.
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Discussion
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The results generated by this study are different than expected. It was hypothesized that
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there would be a low degree of variation between the two species, due to the fact that the shells
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appear to have similar exterior shape and were collected from the same region. However, there
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was a very strong statistical difference for each of the parameters between the species. The lack
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of statistical difference for W when all the specimens were compared could be due to the
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presence of larger specimens for E. hyattianum, of which there were no comparable specimens in
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the W. venatum data set. In addition, there seemed to be no relation, within each species ,
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between the parameters and size.
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Jacobs’ (1992) study stated that as ammonoid shells grew from 1 mm to 10 mm, the
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shells should change from spherical to a smaller thickness ratio to remain hydrodynamically
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efficient. However, this was not the case in either species. Based upon the r2 values, 0.115 for W.
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venatum and 0.1207 for E. hyattianum (Figure 3 A, B), there was no statistical relationship
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between thickness ratio and diameter. This could potentially be due to the fact that shells smaller
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than 25 mm would have difficulty generating swimming speeds high enough to swim against sea
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currents (Jacobs, 1992). This would mean that there would be no selection for a smaller
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thickness ratio as the shells grew, because there would be no need for the animals to be
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hydrodynamically efficient at such a small size. Furthermore, there was no statistical relationship
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between any of the parameters and diameter. This would suggest that umbilical exposure, whorl
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expansion , and thickness ratio change, in these species, without any basis on the size of the shell.
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Looking at the specimens within the morphospace, this lack of relationship becomes very clear.
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Within Westermann Morphospace, it would be expected for ammonoid shells to plot in
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certain modes of life based upon the external shell shape and the size of the shells. However, for
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both species, this is not the case. Both species plot on both sides of the lines that would denote a
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nektonic (swimming) life style and a vertical migrant life style (Figure 4 A,B,C). This would not
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be unusual if there was a trend with size, of the larger shells plotting more in the nektonic
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category. However, there is no basis on size for either species in either category. Small and mid-
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range shell sizes plot at random within the upper right hand corner of the ternary diagram and the
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larger specimens for both species remain within the vertical migrant category. It should be noted,
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however, that the use of this morphospace is relatively new, and the categories are hypothetical,
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derived from the inset on the Westermann diagram (Westermann, 1996) (Figure 1A). In addition,
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the shells are not plotting randomly within the space, there are some constraints on the shell
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parameters which control the placement.
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Conclusion
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Based upon the high degree of variation within and between the two species, it would
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seem that there is low selective pressure on the shell shapes of these ammonoids. This would
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indicate that in both W. venatum and E. hyattianum, the relationship proposed by Jacobs (1992)
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would not be accurate. However, more specimens of a greater size range would be needed to
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conclusively show this. In addition, the ammonoids did not plot into a specific category of life
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mode based upon size. While this could be due to lack of selective pressure based upon life
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modes, another explanation could be based upon the inability to swim against currents at the size
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presented in the study. In this case, a morphospace would need to be developed specifically for
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smaller ammonoid shells, one in which the nekton category would be eliminated. Again, more
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specimens would be needed in order to provide conclusive evidence of this.
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Acknowledgements
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This research was conducted at the University of Southern California, in the department
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of Earth Sciences for my senior thesis project. I would like to Kathleen A. Ritterbush and David
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J. Bottjer for their support over the course of the study. I would also like to thank the University
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of Oklahoma’s Sam Noble Natural History Museum for the loan of their Pennsylvanian
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ammonoid collection.
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References
Algeo, Thomas J. and Philip H. Heckel (2008). – The Late Pennsylvanian Midcontinent Sea of
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North America: a review. – Palaeogeography, Palaeoclimatology, Palaeoecology., 268,
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205-221.
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Doguzhaeva, Larisa A., Royal H. Mapes, Herbert Summesberger, and Harry Mutvei (2007). The
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preservation of body tissues, shell and mandibles in the Ceratitid ammonoid
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Austrotrachyceras (Late Triassic), Austria. – Cephalopods Present and Past: New
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Insights and Fresh Perspectives, 221-238. Springer.
250
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Raup, David M. (1967). – Geometric analysis of shell coiling: coiling in ammonoids. – Journal
of Paleontology., 41, 43-65.
Westermann, Gerd E. G. (1996). – Ammonoid life and habitat. Ammonoid Paleobiology. Volume
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13, 607-707, Topics in Geobiology, edited by Neil Landman et al., Plenum Press, New
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York.
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A.
B.
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Figure 1. The Westermann Morphospace associates different ammonoid shell shapes with
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different potential life modes. A. From Westermann (1996), the figure summarizes ammonoid
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mobility and shell shape gradations. The inset triangle indicates the life modes associated with
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the shapes in the larger diagram. B. Westermann Morphospace. Three parameters – umbilical
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exposure, thickness ratio, and whorl expansion – are used to plot the shells within the ternary
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diagram and determine their hypothetical mode of life.
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diameter
ud
a’
A.
b
B.
a
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Figure 2. Photographs of ammonoid
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fossil shell used to illustrate
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measurements. A. Measurements on
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side view. The height of the final
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whorl is a, the height of the preceding
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whorl is a’. The umbilical diameter is
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ud. Final measurement is diameter. B.
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Measurement on venter view. Whorl
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width is b.
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Parameter
Calculation
Umbilical Exposure (U)
ud/diameter
Thickness Ratio (TH)
b/diameter
Whorl Expansion (W)
a/a’
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Table 1. Outer shell shape, in Westermann Morphospace is described by three parameters, or
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ratios (Ritterbush and Bottjer, in review).
Scaling Values
Min (mean - 2σ)
Max (mean)
Scaling Equation
U
0
0.52
(U-0)/(0.52-0)
TH
0.14
0.68
(TH-0.14)/(0.68-0.14)
W
1.0
1.77
(W-1.0)/(1.77-1.0)
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Table 2. Scaling values associated with the three parameters. Max and min values generated by
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Ritterbush and Bottjer (in review).
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Figure 3. Relationships between parameters and diameter. A) For W. venatum there is no
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relationship between thickness ratio and diameter (r2 0.115). B) There was no relationship
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between thickness ratio and E. hyattianum (r2 0.1207). There was no relationship between C) U
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and diameter (r2 0.1598) and D) W and diameter (r2 0.019) for W. venatum. There was no
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relationship between E) U and diameter (r2 0.0217) and F) W and diameter (r2 0.0014).
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A.
B.
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Figure 4. Data in Morphospace. A.
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Westermann Morphospace plot of both
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species. The species both plot within the
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nekton and vertical migrant categories with
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no basis on size. Ammonoid shells smaller
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than 5 mm are red,. Ammonoid shells larger
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than 10 mm are green. B. Westermann
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Morphospace plot of Wewokites venatum.
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These ammonoids plot within both the
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nekton and vertical migrant categories. The
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10 mm and larger ammonoid shells are
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indicated by green points and the shells
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smaller than 5 mm are in red. C.
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Westermann Morphospace plot of
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Eoasianites hyattianum. The shells of this
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species all plot in the vertical migrant and
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nekton categories. The ammonoids larger
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than 10 mm are in green, the ones smaller
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than 5 mm are in red.
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C.
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Figure 5. Distributions of parameters for both species. All are similar in shape to Gaussian or
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normal distributions, however some are slightly skewed to the left (E. hyattianum diameter) or
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right (E. hyattianum U and diameter). Some of the distributions also have a slight tail (E.
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hyattianum diameter and U and W. venatum U and W).