They discovered a few of the specimens troublesome to categorise. However 26 appeared to group into three varieties, Mr. Paul mentioned: one sturdy type from early within the Hell Creek with two units of incisors in its decrease jaws, and a strong and gracile type from later with just one incisor set.
These three types, Mr. Paul and his colleagues concluded, had been most likely completely different sufficient from each other — and appeared over a enough size of time — to warrant separate names. Subsequently, the earliest, cumbersome Tyrannosaurus to indicate up within the Hell Creek obtained the title Tyrannosaurus imperator (“emperor tyrant lizard”), which then — over the course of 1 million to 2 million years — break up into the sturdy Tyrannosaurus rex and the newly named, comparatively slender Tyrannosaurus regina (tyrant lizard queen).
That proposed evolutionary trajectory — from one tank-like inhabitants to a different comparatively lithe one — matches ecosystems earlier within the Cretaceous interval that had been dominated by Tyrannosaurus family, Mr. Paul mentioned. In that interval, bruisers like Daspletosaurus coexisted with long-legged hunters comparable to Gorgosaurus.
Based on Dr. Holtz, such an thought is fully believable — however proving it’ll require help from future finds.
“It’s a testable speculation, as any assertion of species id ought to be,” Dr. Holtz mentioned. “With further new specimens, we are able to see if specimens they didn’t embody or we haven’t discovered but are in keeping with this suggestion, or in the event that they reject it.”
Whereas Dr. Holtz felt the authors’ case could be extra convincing if the completely different species they describe had been organized extra chronologically in particular rock layers, he famous that the Hell Creek Formation accommodates different examples of species divergence. Different fossils discovered there led to a common settlement that there was a couple of species of triceratops, Dr. Holtz mentioned. And there’s a chance that the hadrosaurs and dome-headed Pachycephalosaurus discovered on the prime of the formation differ considerably from these on the backside.
“On the similar time, in the identical place, the identical dimension lessons of organisms are going by a succession of species,” Dr. Holtz mentioned. “It’s not inconceivable that Tyrannosaurus did in order nicely.”