Diseases of dinosaurs
Wednesday, September 30th, 2009
Paleontologists have found that Tyrannosaurus rex and its close relatives suffered from a potentially life-threatening infectious disease similar to one that occurs in living birds known as trichomonosis. Trichomonas gallinae infections are most prevalent in pigeons which are generally immune. Birds of prey are particularly susceptible to trichomonosis if they eat infected pigeons. Adult birds can then pass the disease to their nestlings through beak-to-beak contact. Tell-tale symptoms of trichomonosis include swellings and holes in the back of the lower jaw. The disease is prevented from infecting the entire interior of the bone by the innate immune response that localizes infections as a result of the actions of a unique avian white blood cell called the heterophil. Some of the world’s most famous T. rex specimens, such as “Sue” at the Field Museum in Chicago, and the holotype specimen at the Carnegie Museum of Natural History in Pittsburgh have holes like these in their lower jaw. The holes in tyrannosaur jaws occur in exactly the same place as in modern birds with trichomonosis. The shape of the holes and the way that they merge into the surrounding bone is very similar in both animals. The cause of these holes in tyrannosaurs has previously been attributed to tooth gouges from biting or bacterial infections, but a trichomonosis-type disease is more likely given the position and nature of the holes.
The disease appeared to be quite common in tyrannosaurs and could have been deadly to those that were infected. As the parasites take hold in serious infections, lesions form around the jaw and inside the throat, eventually eating away the bone. As the lesions grow, the animal has trouble swallowing food and may eventually starve to death. Tyrannosaurs are so far the only dinosaurs that appear to have had this disease. The researchers therefore faced the problem of explaining how it was spread. In addition to other routes through which infection may have spread, tyrannosaurs might have facilitated infection by biting each other or even through cannibalism. Cannibalism has been tentatively suggested in other studies of theropod behaviour and this certainly could have been a route of transmission for the infection, but other scenarios were more frequent. Fighting and specifically head-biting would have been an ideal mechanism for spreading the disease among tyrannosaurs. It is unlikely to be a coincidence that a significant number of adult tyrannosaur specimens show both face-biting marks and evidence of a trichomonosis-like disease. Previous studies have shown that up to 60% of tyrannosaur specimens display evidence of face-biting. Bone pathology is hard to find in any specimen, and bone diseases are relatively uncommon. Finding both types of pathologies in a high proportion of individuals strongly suggests that they could be linked.
There are similarities with what has been happening to Tasmanian devils recently, where a debilitating oral cancer is being spread by animals fighting and biting each other’s faces. This disease may eventually wipe out this iconic Australian mammal. It is ironic to think that an animal as mighty as T. rex probably died as a result of a parasitic infection. The link in disease is not surprising given the evolutionary relationship of dinosaurs to birds. But the discovery of a likely candidate for such a disease represents a major step forward in our understanding of disease origins in birds and their dinosaur precursors. The discovery gives us an insight into the dinosaur immune system. The response of tyrannosaurs to this trichomonosis-like disease is almost identical to that found in living birds. These simple holes in tyrannosaur jaws give us a dramatic example of an avian-like defence system in action.
Common Avian Infection Plagued the Tyrant Dinosaurs. PLoS ONE 4(9): e7288 doi:10.1371/journal.pone.0007288
Tyrannosaurus rex and other tyrannosaurid fossils often display multiple, smooth-edged full-thickness erosive lesions on the mandible, either unilaterally or bilaterally. The cause of these lesions in the Tyrannosaurus rex specimen FMNH PR2081 (known informally by the name ‘Sue’) has previously been attributed to actinomycosis, a bacterial bone infection, or bite wounds from other tyrannosaurids. We conducted an extensive survey of tyrannosaurid specimens and identified ten individuals with full-thickness erosive lesions. These lesions were described, measured and photographed for comparison with one another. We also conducted an extensive survey of related archosaurs for similar lesions. We show here that these lesions are consistent with those caused by an avian parasitic infection called trichomonosis, which causes similar abnormalities on the mandible of modern birds, in particular raptors. This finding represents the first evidence for the ancient evolutionary origin of an avian transmissible disease in non-avian theropod dinosaurs. It also provides a valuable insight into the palaeobiology of these now extinct animals. Based on the frequency with which these lesions occur, we hypothesize that tyrannosaurids were commonly infected by a Trichomonas gallinae-like protozoan. For tyrannosaurid populations, the only non-avian dinosaur group that show trichomonosis-type lesions, it is likely that the disease became endemic and spread as a result of antagonistic intraspecific behavior, consumption of prey infected by a Trichomonas gallinae-like protozoan and possibly even cannibalism. The severity of trichomonosis-related lesions in specimens such as Tyrannosaurus rex FMNH PR2081 and Tyrannosaurus rex MOR 980, strongly suggests that these animals died as a direct result of this disease, mostly likely through starvation.
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