1. Jaw ontogeny of the Late Devonian "T
Jaw ontogeny of the Late Devonian "T. rex", with implications for feeding strategies and life history of the arthrodire Dunkleosteus terrelli
J. Boyle1, M. Ryan2, E. Snively3, and W.J. Hlavin4
1State University of New York at Buffalo Program of Evolution, Ecology, and Behavior , 2Cleveland Museum of Natural History Department of Vertebrate Paleontology, 3University of Wisconsin La Crosse Biology Department, 4Bass Energy, Inc.
Correspondence:
Introduction
Discussion
Changes in the shape of D. terrelli jaws are consistent with a shift from quick soft-bodied prey, such as bony fishes or small sharks toward larger armored prey (placoderms). This ontogenetic niche shift is hypothesized to occur at an overall jaw length of mm based on the change in growth trajectory of the cutting length. This timing (43% of maximum size) is consistent with the ontogenetic niche shift observed in Stenopterygius (38% max. size) and Great White Shark (39% max. size).
The reconstructed life history of D. terrelli shows a typical type I survivorship curve, indicating low juvenile mortality. This might be because even at relatively small sizes juvenile D. terrelli were already larger than most potential predators. However, there is evidence of predation on juvenile D. terrelli by large sharks9. This would fit with another common strategy of apex predators, live birth of one or a few large offspring, which is known to occur in placoderms10. A second explanation for the type I growth curve of D. terrelli would be the lack of juvenile individuals in the deep basins where most D. terrelli jaws are known from as juveniles might have utilized nearshore nurseries until they were large enough to avoid predation in the open basin. Placoderm nurseries are known11,12 and there is at least one occurrence of a small dunkleosteid from a nearshore site13.
10cm
Apex predators often undergo an ontogenetic niche shift, a change in diet at some threshold size, due to the inability of juveniles to overcome the defenses of the preferred prey of adult individuals. This phenomenon is known to occur in modern great white sharks1 and the extinct ichthyosaur Stenopterygius2 based on preserved gut contents and in both cases there is a coincident shift in morphology of the teeth. It seems likely that even more ancient apex predators would have evolved a similar strategy.
Dunkleosteus terrelli was a heavily armored apex predator in the Late Devonian (~360 MYA) seas. Previous work on the shape and biomechanics of D. terrelli has demonstrated that while even juveniles would have been competent predators3 the shape of the jaws differed between juveniles and adults, suggesting they utilized different resources. While there are no known D. terrelli gut contents the morphology of a growth series of jaws might show a shift in morphology that would be consistent with an ontogenetic niche shift.
Figure 4. Examples of D. terrelli inferognathals showing change in shape throughout growth from juvenile (left) to adult (right). All inferognathals are at the same scale.
D. terrelli
Stenopterygius
C. carcharius
Figure 5. Reconstructed survivorship curves of
D. terrelli, Stenopterygius, and the Great White Shark
(C. carcharias).
Methods
Results
Conclusions
Seven morphometric parameters (insets in figures 1-3) were measured from 60 complete D. terrelli inferognathals as well as recording the presence or absence of rear denticles. The total lengths of the inferognathals ranged from 48mm to 560mm, so that the smallest inferognathal is less than ten percent the size of the largest likely representing all stages of the organism’s life.
Linear regressions were fit to each parameter versus overall length and slopes were interpreted as relative growth rates. To test whether there were differences in growth rates that would indicate a shift in jaw shape, and thus diet, we used permutation tests. Each parameter was resampled randomly 100 times from the full data set (allowing duplicates) and fit to a linear regression. If the central 95% of the range of permuted slopes did not overlap between parameters they were considered significantly different.
To examine the life history of D. terrelli and compare it with other know apex predators with niche shifts we compiled size estimates of Stenopterygius (n=50)2 and Great White Sharks (n=244)4-7 and used the data to construct log survivorship curves (fig. 4).
Slopes varied from with the blade and functional lengths growing at significantly greater rates than any of the other parameters. However, these two measures did not vary significantly in growth rates from each other (Fig. 1). Among the latitudinal measures width had a significantly greater slope than both depth and height (Fig. 2). The cutting length had the greatest variance in slopes among all measures and appears to be better represented by two different growth trajectories that split at an overall length of mm (Fig. 3).
Together these show that the jaws of D. terrelli became more elongate with a proportionally larger anterior cusp as size increased (Fig. 4). Together these indicate a concentration of force at the anterior cusp that would have aided in piercing or slicing large and armored prey. The decrease in growth rate of the cutting length at mm would have led to a shift from a scissor-like occlusion toward a more vise-like one, again consistent with piercing tough materials.
The reconstructed life histories of D. terrelli, Stenopterygius, and Great White Sharks show similar shapes that match a classical type I survivorship curve (Fig. 5).
Morphological changes in the jaw of D. terrelli are consistent with an ontogenetic niche shift from small lightly armored prey to large heavily armored prey at an overall jaw length of mm, similar to other apex predators
The reconstructed survivorship curve is similar to other apex predators with ontogenetic niche shifts, although it is not clear if the low juvenile mortality rate of D. terrelli is real or an artifact of preservational bias
Literature Cited
Figure 1 (top). Overall length versus blade and functional lengths. Figure 2 (middle). Overall length versus width, height, and depth. Figure 3 (bottom). Overall length versus cutting length. Shaded areas represent permuted regression lines. Inset left inferognathal of D. terrelli with seven morphometric parameters show, from Hlavin
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