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Copyright This journal is © 2006 The Royal Society Prey choice and cannibalistic behaviour in the theropod Coelophysis *Author for correspondence (Email: nesbitt/at/ldeo.columbia.edu) Received March 20, 2006; Accepted July 15, 2006. | |||||
Abstract Direct evidence of prey choice in carnivorous dinosaurs is rare in the fossil record. The most celebrated example pertains to purported stomach contents in the carnivorous dinosaur Coelophysis bauri, which besides revealing prey choice, also points to cannibalistic behaviour as being commonplace (Colbert 1989, 1995). Here, we test this hypothesis by conducting the first comprehensive anatomical and histological examination of the famed Coelophysis ‘cannibals’. The results unequivocally show that the gut contents derive from early crocodylomorphs rather than juveniles of Coelophysis. These findings suggest that this taxon is not cannibalistic and bring into question the commonality of this behaviour among non-avian dinosaurs. Keywords: Coelophysis, prey choice, cannibalism, palaeobiology, Mesozoic, dinosaur | |||||
In 1947, crews from the American Museum of Natural History (AMNH) unearthed an enormous bonebed composed of hundreds of skeletons of the early theropod Coelophysis bauri from the Upper Triassic (210 ![]() The famed cannibal-Coelophysis hypothesis, although tenable, has never been rigorously examined. Two criteria must be met to unequivocally ascribe cannibalistic behaviour to dinosaurs from ‘stomach’ contents. The remains must be shown to reside in the abdominal cavity and come from the same taxon. This has not occurred. The stomach contents were never prepared to a degree allowing their location in the body to be definitively ascertained. Furthermore, it is well established that most basal archosaurian reptiles, including early dinosaurian taxa, have similar hindlimb morphology, thus leaving open the possibility that the purported stomach contents come from other animals besides C. bauri. Recent access to the Colbert's Coelophysis specimens allowed an unprecedented opportunity to reanalyse the cannibal-Coelophysis hypothesis. We further prepared the specimens (AMNH FR 7223 and AMNH FR 7224) and used detailed morphological and bone histological analyses to critically examine whether the aforementioned criteria supporting cannibalism in Coelophysis are met. AMNH FR 7223 (see figure 1c and electronic supplementary material) is a nearly complete adult skeleton (femoral length=209
The neotype of C. bauri (AMNH FR 7224) consists of a nearly complete mediolaterally crushed skeleton (femoral length=203 The posterodorsal concentration of bones preserves a three-dimensional structure that appears to delimit the stomach cavity (figure 1). Small bone fragments are concentrated near, and the smaller flat bones are oriented parallel to, the surface of the inferred stomach wall. Previous studies (Colbert 1989, 1995; Gay 2002) specifically highlighted the bones of this region as juvenile Coelophysis. We have identified well-preserved remains consisting of left and right proximal ends of femora, a left ilium and a sacral vertebra within the stomach cavity (figure 1). Comparative morphological analysis of this material reveals that these bones lack any synapomorphies of Coelophysis, Theropoda or even Dinosauria (figure 2). However, they are consistent with Crocodylomorpha. For instance, the left ilium bears a closed acetabulum; nearly all dinosaurs (including Coelophysis) possess an open acetabulum. The sacral vertebra shows no indications of fusion with surrounding vertebrae, has an anteroposteriorly short, dorsally expanded, neural spine, and a sacral rib articulating to the centre of the centrum—all characters consistent with crocodylomorphs, but not with Coelophysis. Additionally, the femora lack the typical dinosaurian offset femoral head, anterior trochanter and well-developed articular facet for the antitrochanter (Langer 2004); instead they exhibit proximal condylar folds (C. A. Brochu 1992, Unpublished Master's Thesis; figure 2). A proximal condylar fold is found only in crocodylomorph archosaurs, including Hesperosuchus agilis (figure 2), whose presence is well known from the Chinle Formation and the Ghost Ranch Coelophysis Quarry. Our results show that although stomach contents were remarkably preserved in situ in Ghost Ranch Coelophysis, no evidence for cannibalism exists.
As an independent test of our hypothesis that AMNH FR 7224 consumed a crocodylomorph rather than a juvenile Coelophysis, we conducted comparative histological analyses between the purported cannibalized stomach contents, Coelophysis, and the most common Chinle Formation crocodylomorph—Hesperosuchus (see electronic supplementary material for taxonomic assignment). The metaphyses of the femur from the stomach content specimen and cf. Hesperosuchus show nearly identical histological patterning and structure and hence, attest to comparable developmental histories (figure 2). In the stomach content specimen and cf. Hesperosuchus, the transition zone to the primary bone is not distinct; instead it blends with the compacted cancellous bone. Furthermore, the initial primary bone that was deposited across the zone is avascular. Notably, the transition zones in both also show a pronounced concavity. The rest of the primary bone shows longitudinally or locally semi-radiating vascularization. In stark contrast, the transition zone of the dinosaur Coelophysis is very thin, and distinctive—like the structuring seen when lines of arrested growth form. This suggests that bone deposition completely stopped before the capping of primary bone began. The thick avascular zone seen in the stomach content and cf. Hesperosuchus is not present in Coelophysis. The transition zone is straight, unlike the concave pattern seen in the stomach content and cf. Hesperosuchus specimens. Finally, the vascular canals in Coelophysis are relatively long and show pronounced, inclined, radiating patterns—those stemming more medially incline laterally and those originating laterally incline medially. Cannibalistic behaviour is very common among carnivorous animals (Polis 1981), and may be expected in non-avian dinosaurs. Then again, among living dinosaurs (birds) this behaviour is not prevalent, being most common in colonial nesting seabirds (Parsons 1971) and birds of prey (Ingram 1959). This brings into question just how prevalent this behaviour was among non-avian dinosaurs. The phylogenetic distribution of cases supposedly attesting to cannibalism suggests that it was prevalent throughout the Theropoda. This study shows that there is no compelling evidence for this behaviour in the famed Coelophysis AMNH ‘cannibals’. What about the other cases? Recent reports of coprolites and cololites (internal intestinal casts), said to be preserved below the base of a Coelophysis tail (Rinehart et al. 2005) and to contain cannibalized manual elements, also cannot be considered as evidence for cannibalism in Coelophysis as the purported digested skeletal material is taxonomically uninformative. Occurrences of cannibalism in other theropods are also problematic. Jacobsen (1998) reported tyrannosaur bite marks on the remains of tyrannosaurs from the Dinosaur Park Formation of Alberta and used these as evidence for cannibalism. However, given that there are at least two species of tyrannosaurs in the formation (Daspletosaurus and Gorgosaurus; Jacobsen 1998), cannibalism cannot be demonstrated sufficiently. This leaves just bite mark evidence and associated tooth crowns in the large theropod Majungatholus atopus (Rogers et al. 2003) as the only compelling evidence for cannibalism out of the hundreds of known Mesozoic theropod dinosaurs. On the basis of this evidence, cannibalism is not as prevalent as was once supposed in non-avian dinosaurs. | |||||
Acknowledgments We thank Jack Conrad for his insightful comments while preparing the manuscript. Alex Downs supplied the crocodylomorph femur for histology sectioning. Mick Ellison composed the figures. The photograph appearing in figure 1 is courtesy of the American Museum of Natural History. This work was supported by a National Science Foundation Graduate Research Fellowship and the American Museum of Natural History Division of Paleontology. The histological analyses were supported by National Science Foundation, Division of Earth Sciences grant EAR 0207744 presented to G.M.E. and M.A.N. | |||||
Supplementary data to: Prey choice and cannibalistic behaviour in the theropod Coelophysis 1. Further analysis of non-cannibalism in AMNH FR 7223. 2. Taxomonic assigned of Crocodylomorph histological specimen (GR 215). 3. Histological methods. Supplemental figure: the abdominal region of Coelophysis bauri (AMNH 7223) Click here to view. (1.3M) | |||||
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