Did Tyrannosaurus rex suffer from a common bird disease?
Paleontologists Ewan Wolff (University of Wisconsin-Madison), Steve Salisbury (University of Queensland), Jack Horner (Museum of the Rockies) and David Varricchio (Montana State University), published new research in the open-access, peer-reviewed scientific journal PLoS ONE that found the 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," Dr Wolff said.
"The cause of these holes in tyrannosaurs has previously been attributed to tooth gouges from biting or bacterial infections, but we think a trichomonosis-type disease is much 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," said Dr. Salisbury.
Tyrannosaurs are thus 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," said Wolff, but he thinks other scenarios were more frequent.
"Fighting and specifically head-biting would have been an ideal mechanism for spreading the disease among tyrannosaurs," said Dr Salisbury.
"We don't think it is a coincidence that a significant number of adult tyrannosaur specimens show both face-biting marks and evidence of a trichomonosis-like disease," Dr Salisbury said. "Previous studies have shown that up to 60% of tyrannosaur specimens display evidence of face-biting."
"In our study we found evidence of face-biting in 30% of the diseased individuals," said Dr Wolff. "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."
"We can see 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," Dr Salisbury said. "This disease may eventually wipe out this iconic Australian mammal."
"It's ironic to think that an animal as mighty as 'Sue' probably died as a result of a parasitic infection. I'll never look at a feral pigeon the same way again." said Salisbury.
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 dinosaurian 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," Dr Wolff said. "These simple holes in tyrannosaur jaws give us a dramatic example of an avian-like defence system in action."