STL Science Center

STL Science Center

22 October 2017

Cannibalism on Display

How do we know an animal that is now a fossil was a cannibal? As with much evidence of cannibalism, even in extant animals, we must look at the teeth, the wounds, and how these two pieces of evidence are correlated. In the past, ichthyosaurs (specifically Shonisaurus and the ichnospecies Ichthyosaurolites) and Coelophysis were suggested to have been extremely cannibalistic in their behaviors. A little over 11 years ago, the cannibalistic tendencies of Coelophysis were analyzed and refuted; the supposedly cannibalized animasl turned out to be a basal crocodile and outside of the body of the second Coelophysis specimen accused. Ichthyosaurolites, on the other hand, is an ichnospecies based entirely on ichthyosaur coprolites that contain the skeletal remains of other ichthyosaurs; the name literally means "ichthyosaur coprolite (fossilized feces)".

From Rogers, et al. 2003. Caudal tail chevron of Majungasaurus. White arrows indicate drag marks and black arrows indicate impressions initial biting marks from Majungasaurus teeth.
In the skull of Majungasaurus, the teeth are characteristically shaped, spaced, and serrated. All of these identifiers have helped paleontologists to recognize the marks of Majungasaurus feeding on a number of other animals from Madagascar. Chief among the animals possessing numerous bite marks from Majungasaurus teeth are the sauropod Rapetosaurus and Majungasaurus itself. No other theropods are known from Madagascar during the 70 - 66 MYA time frame during which Majungasaurus is known to have lived. This was used as the first inference concerning the tooth marks on Majungasaurus bones. This in turn led Rogers, Krause, and Curry Rogers to look at the teeth of Majungasaurus and at the marks that they had left on Rapetosaurus bones in 2003; Curry Rogers had led the description of Rapetosaurus in 2001. The initial bite marks that we see (look at the black arrows) are spaced and shaped identically to the teeth known from the multiple Majungasaurus skulls that have been recovered. The drag marks (white arrows) indicae areas where the serrated denticles have been dragged across the bones.

The first argument that is made here, often, is that the results of interspecies combat might look something like this. However, there are two important aspects of these wounds that make the case for cannibalism more compelling. The first is that these wounds show no sign of healing; the bone would have likely attempted to heal itself, at least, a little, after a traumatic bite in which humerous teeth insulted and scratched the bone deeply enough to clearly score the skeletal material. The second is that these chevrons, and many other limb and vertebral elements that also contain bite marks like these, are inaccessible during combat as they are in areas that could only be (easily) bitten when other elements of the body were exposed either through decomposition or predation.

The remaining question, however, is whether this cannibalism occurred as a result of scavenging behaviors or if Majungasaurus actively hunted members of its own species. The two activities have been documented in various extant species including lions and chimpanzees, so neither would be exceptionally abnormal or unique to Majungasaurus. For more reading on the exact findings of the Rogers, et al. team, read the following paper:

Rogers, R.R.; Krause, D.W.; Rogers, K.C. (2003) “Cannibalism in the Madagascan dinosaur Majungatholus atopus.” Nature, Vol. 422, pp. 515-518

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