The star specimen, however, was the T. rex nicknamed Sue, the largest and most complete representative of the storied species (and on display at Chicago’s Field Museum). Despite Sue seeming to have extra-long chompers, the paleontologists found its teeth had the same relationship to skull size as seen in today’s monitor lizards, and therefore wouldn’t require any extraordinary lips to cover.

A critical part of the research was finding modern analogs. Today’s birds—descendants of dinosaurs—lack teeth, and crocodiles are specialized reptiles that live in water. Even though they’re not as closely related, however, reptiles such as monitor lizards are more useful in their relatedness to dinosaur anatomy. (Learn why T. rex couldn’t waggle its tongue.)

“I’m very interested in the role soft tissues play when interpreting tooth pathologies,” Brink says, as gums, lips, and other tissues could experience many of the same injuries and diseases that affect living animals.

In a controversial new study in the journal Evolutionary Biology, three paleontologists argue that the fossils assigned to T. rex cluster into three different body types, which they further contend represent three separate species. In addition to T. rex, the researchers propose two new species names: T. regina and T. imperator, from the Latin for “queen” and “emperor.”

“For Tyrannosaurus, all the specimens from North America have been put into the same species, T. rex … and that’s become something of a problem because nobody was testing whether that was really true or not,” says study leader Gregory Paul, a paleoartist and independent researcher who wrote The Princeton Field Guide to Dinosaurs.

If anatomical traits in a given creature’s fossils clearly change over time, the creature may warrant multiple species names to distinguish earlier forms from later ones. In the last few decades, paleontologists have gained a better understanding of the way that such “chronospecies” may succeed one another.

Perhaps the most famous and well-established chronospecies come from western North America’s Hell Creek Formation, where Tyrannosaurus is found. Older sediments from the formation preserve one species of Triceratops, T. horridus, while younger rocks farther up the formation preserve a different species, T. prorsus. Some researchers have suggested a similar evolution for other Hell Creek dinosaurs.