They found some specimens difficult to classify. But 26 seemed to group into three species, Mr. Paul said: a robust form from the early Hell Creek with two sets of incisors in the lower jaw, and a robust and graceful later form with only one set of incisors.
These three forms, concluded Mr. Paul and his colleagues, probably differed enough from each other — and appeared over a long enough time — to warrant separate names. Therefore, the earliest bulky Tyrannosaurus to appear in Hell Creek was given the name Tyrannosaurus imperator (“Imperial Tyrant Lizard”), which then – over the course of a million to two million years – split into the robust Tyrannosaurus rex and named the new, relatively slender Tyrannosaurus regina (tyrant lizard queen).
That proposed evolutionary trajectory — from one tank-like population to another relatively lithe — is consistent with ecosystems earlier in the Cretaceous that were dominated by Tyrannosaurus relatives, Mr. Paul said. During that period, bruisers like Daspletosaurus coexisted with long-legged hunters like Gorgosaurus.
According to Dr. Holtz, such an idea is perfectly plausible – but to prove it must be supported by future finds.
“It’s a testable hypothesis, as any statement of species identity should be,” said Dr. holtz. “With additional new instances, we can see if instances that don’t contain them or that we haven’t yet found are consistent with this suggestion, or whether they’re rejecting it.”
Although Dr. Holtz felt that the authors’ argument would be more compelling if the different species they describe were organized more chronologically into specific rock strata, he noted that the Hell Creek Formation contains other examples of species divergence. Other fossils found there led to a general agreement that there was more than one species of triceratops, said Dr. holtz. And there’s a possibility that the hadrosaurs and domed Pachycephalosaurus found at the top of the formation are significantly different from those at the bottom.
“At the same time, organisms of the same size classes move through a succession of species in the same place,” said Dr. holtz. “It’s not inconceivable that Tyrannosaurus did.”
