These holes, some the diameter of a golf ball, dot the back half of the left lower jaw. It's not clear what caused them, but similar injuries have been found in other T. rex fossils. In a new study published in Cretaceous Research, scientists showed that one of the popular theories—that SUE had suffered an infection from a protozoan parasite—couldn't be true.

"These holes in SUE's jaw have been a mystery for decades," says Jingmai O'Connor, the associate curator of fossil reptiles at Chicago's Field Museum and a co-author of the study. "Nobody knows how they formed, and there have been lots of guesses."

One early hypothesis was that SUE suffered from a fungus-like bacterial infection, but that was later shown to be unlikely. It was re-hypothesized that SUE had a protozoan infection. Protozoans are microbes with more complex cell structures than bacteria. There are lots of protozoan-caused maladies out there; one common such disease is called trichomoniasis, caused by a microbe called Trichomonas vaginalis. Humans can get infected with trichomoniasis as an STD, but other animals can catch it too.

"Trichomoniasis is found in birds, and there's a falcon specimen with damage to its jaw, so some paleontologists thought that a Trichomonas-like protozoan might have caused similar damage to SUE," says O'Connor. "So for this study, we wanted to compare the damage in SUE's jaw with Trichomonas damage in other animals to see if the hypothesis fit."