What Tales Do Albertosaurus Injuries Tell?

TMP 2003.45.64 is not exactly a headline-making fossil. The left lower jaw of an Albertosaurus, most of the teeth have fallen out and the bone is only one part of a well-known species represented by many other skeletons. But, for those who know what they are looking for, this specimen bears the traces of ancient interactions between dinosaurs.

The Albertosaurus jaw portion is just one of many bones recovered during the past decade from a Late Cretaceous bonebed in Alberta, Canada's Dry Island Buffalo Jump Provincial Park. This is a very unusual site. Remains from at least 26 Albertosaurus, ranging from about 2 to 24 years old, have been found from this deposit. Such a rich collection of skeletons from a single species has allowed paleontologists to better understand what the local population of Albertosaurus was like around 70 million years ago, including the prevalence of injury and disease.

What makes the lower jaw significant is that it bears a series of gouges. As determined by Phil Bell in his recent assessment of the pathologies in the Dry Island Albertosaurus, these furrows were driven into the bone by another tyrannosaur. This sort of damage has been seen before. Other fossils with pathology have indicated that tyrannosaurs often bit each other on the face while fighting, and this leaves a pattern of damage distinct from that created by microorganisms which open up smooth-walled lesions in the jaws.

Curiously, though, the Albertosaurus jaw Bell described was bitten at two different times. One long groove near the front of the jaw was smooth and relatively fresh, while three parallel toothmarks and a puncture wound further back on the jaw had healed. The repaired wounds showed that the Albertosaurus had survived a fight with another tyrannosaur, but the other bite was made near the time of death or soon afterward. As with a tyrannosaur jaw fragment with the tooth of another tyrannosaur embedded in it, described in 2009, the exact timing of the injury is practically impossible to determine.

The tooth-scored lower jaw was not the only injured bone found in the quarry. Bell listed five other pathological bones, including damaged ribs and toe bones from other individuals. The ribs had been fractured and healed, while the toe bones were marked by bony spurs called enthesophytes. These form at the attachments of ligaments or tendons. What this may mean for the Albertosaurus toe bones represent is unclear—enthesophytes can form for a variety of reasons, from repetitive stress to a simple genetic predisposition for them.

Future studies may identify other pathologies, but Bell points out that the occurrence of pathology among the 26 Albertosaurus individuals was low—only six injuries in as few as two individuals. Bonebeds of the large predatory dinosaurs Allosaurus and Majungasaurus both had higher incidences of pathology. It would seem that the Dry Island Albertosaurus population was not as injury-prone as some of these other dinosaur populations, but why this should be so remains a mystery.

References:

Bell, P. (2010). Palaeopathological changes in a population of Albertosaurus sarcophagus from the Upper Cretaceous Horseshoe Canyon Formation of Alberta, Canada Canadian Journal of Earth Sciences, 47 (9), 1263-1268 DOI: 10.1139/E10-030

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Riley Black is a freelance science writer specializing in evolution, paleontology and natural history who blogs regularly for Scientific American.