The biggest problem for Tyrannosaurus rex could have been a single-celled parasite in a paleolithic turn on the tiny-fells-mighty, War of the Worlds story.

A relative of lowly Trichomonas, a microbe commonly found today in pigeons, may have killed off Sue, the famous T. rex skeleton at the Chicago Field Museum, and many other tyrannosaurids, too.

Paleontologist Ewan Wolff of the University of Wisconsin-Madison and colleagues use evidence from modern predatory bird species to argue that the protozoan parasite could have formed lesions along the tyrannosaur mandible, eroding the bone away.

In some cases, the illness might have been bad enough to prevent the animals from feeding, leading to their death from starvation, and creating telltale holes in the jaws of the great beasts.

"I think it's very tempting when you see a hole in a bone to say it's bite marks, but there are innumerable disease you could list that cause holes in bones that have nothing to do with bite wounds," said Ewan Wolff, lead author of a new paper describing the work published Sept. 29 in PLoS One.

There are three pieces of evidence against the bite-wound hypothesis, Wolff said. First, the holes are generally nicely circular or ovoid, not obviously tooth-shaped. Second, dinosaurs obviously had many teeth, not just one, and the holes don't seem to come in groups. Third, there are no smaller marks or scrapes on the bone surrounding the holes.

"We're definitely familiar with what predation traces look like from tooth marks in tyrannosaurus," Wolff said. "And this is not it."

Furthermore, they found evidence in predatory birds that trichomonosis can cause holes in the mandible that look quite similar to those found in nine tyrannosaurid specimens.

The disease could have been passed to dinosaurs from their prey or from tyrannosaur to tyrannosaur during combat, mating, or cannibalism.

"Head or face-biting behavior relating to intraspecific territoriality, social dominance, courtship, feeding or some other unknown aspect of tyrannosaurid behavior would have provided the ideal mechanism for the transmission of this trichomonosis-like disease," Wolff and colleagues wrote.

Other researchers, notably dinosaur anatomy expert Elizabeth Rega of Western University of Health Sciences in Pomona, California, have argued the holes were disease-induced. But Rega's culprit was a bacteria on the model of actinomyces.

Wolff, however, argued that much of the damage that actinomycosis causes in humans is due to pus. Birds don't actually create puss, and the same was likely true of dinosaurs. It's not part of the avian immune response. Instead, birds coat invading organisms with a fibrous protein.

"So instead of getting puss, you get this very cheesy inflammatory area," Wolff said. "You get chronic infection that just doesn't go away."

He said that the actinomycosis-trichomonosis debate showed how his team's approach differed from other paleopathologists.

"In the past, a lot of comparisons of paleopathological specimens have been made to humans or domesticated animals," Wolff said, instead of with dinosaurs' closer relatives, birds and reptiles. "It makes a lot of sense to look at animals that share a close evolutionary relationship to each other and the diseases that they have. The approach in this article is to do just that and that's relatively novel in the paleopathological community."

Still, not everyone is sold on the new idea about Sue's holey jaw.

Rega called Wolff's idea an "intriguing hypothesis" but one that wasn't "a perfect match" for the data. She noted that any disease that can make holes in bone has to stick around for a long time, wearing away at the material. She doesn't see trichomonosis as a good fit on that score.

"What the T-rex symptoms demand is chronically affected animals who don't recover but who don't die. In this way, a long standing disease (acting like actinomycosis) fits the data better," Rega wrote in an email to Wired.com. "And if trichomonas is indeed the culprit, the natural history of trichomonas suggests it was more chronic and less acute in its non-avian dinosaur phase in the late Cretaceous."

And some still contend that bite marks could be responsible for the holes, too.

"My suspicion is that it's not valid," said Bruce Rothschild, a medical doctor and paleopathologist at the University of Kansas.

Rothschild said the holes in Sue's jaw look a lot like the marks made by trepanation — the practice of puncturing of the skull — in ancient humans.

"I'm not saying it was trepanation," Rothschild said. "I'm saying it was a bite."

And he disputed that the modern examples of presumed trichomonas jaw holes matched up "exactly" with the tyrannosaur fossil holes.

"To me, it's not exactly the same," he said.

Rothschild said that the tyrannosaur fossils showed just "a hole with simple infilling," without the more complex features associated with lesions.

What might be next is looking at dinosaur specimens for other signs of starvation, suggested Thomas Holtz, a tyrannosaur specialist at the University of Maryland.

"I don't know if you'd see maybe bone loss as a sign of starvation," Holtz said.

Wolff said his team could follow up with exactly that type of observation. Another line of inquiry could run in the molecular direction, trying to determine if parasites like trichomonas did in fact exist 65 millions of years ago.

"I don't know to what degree anyone can confirm that this particular group of organisms were around during the Cretaceous," Holtz said. "We don't have a fossil record for them."

Update: 9/29, 5:07pm. Added comment from Elizabeth Rega. Updated the whereabouts of Sue to the Field Museum.

Image: Pex Rex. Chris Glen/University of Queensland

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