Fossil analysis shows that Tyrannosaurus rex is not one species, but three at once. Differences are in the femurs and jaws.

The researchers found that known Tyrannosaurus rex fossils may actually belong to several different but closely related species.

For example, some skeletons have only one incisor on each side of the jaw, while others have two. More researchers drew on the difference in the structure of the hips. During the new work, the authors analyzed 37 Tyrannosaurus rex specimens to see if there was any particular pattern in these variations.

It turned out that the number of strong femurs exceeds the number of thin ones twice. The team emphasized that this difference is not due to different sexes, in which case the ratio would be 50 to 50. It also does not seem to be related to age.

The team examined these differences in the context of which sediment layers the samples were found in. Strong femurs appeared in all layers, but thin ones only in the middle and upper ones. This suggests that there was one species of tyrannosaurs, which eventually differentiated into several.

Researchers have even suggested names and characteristics for these potential species. They named the oldest species Tyrannosaurus imperator. It is found in the lower and middle sediments and typically had strong femurs and two incisors, features that appear to have been preserved in earlier ancestors.

The second species, Tyrannosaurus regina, was found in the upper and possibly middle layers. He has thin thighbones and only one incisor. And the third species – Tyrannosaurus rex – was in the upper and middle layers. He has strong hips and one incisor.