They want to split the T. Rex into 3 species. Paleontologists are not happy.

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They found some of the specimens difficult to classify. But 26 seemed to cluster into three types, Paul said: a sturdy form from early Hell Creek with two sets of incisors in its lower jaws, and a sturdy, slender form later with a single set of incisors.

These three forms, Mr. Paul and his colleagues concluded, were probably sufficiently different from each other – and appeared over a sufficient period of time – to warrant separate names. Therefore, the first and bulky Tyrannosaurus to appear in Hell Creek was given the name Tyrannosaurus imperator (“tyrant emperor lizard”), which then – over the course of one million to two million years – split into the sturdy Tyrannosaurus rex and the newly named, relatively slender Tyrannosaurus regina (tyrant lizard queen).

This proposed evolutionary trajectory — from a reservoir-like population to a relatively flexible one — matches ecosystems earlier in the Cretaceous period that were dominated by relatives of Tyrannosaurus, Paul said. At that time, murderers like Daspletosaurus coexisted with long-legged hunters like Gorgosaurus.

According to Dr Holtz, such an idea is entirely plausible – but proving it will require the support of future findings.

“It’s a testable hypothesis, as any statement of species identity should be,” Dr. Holtz said. “With additional new specimens, we can see if the specimens that they haven’t included or that we haven’t yet found are consistent with this suggestion, or if they reject it.”

Although Dr. Holtz felt that the authors’ case would be more compelling if the different species they describe were more chronologically organized in specific rock layers, he noted that the Hell Creek Formation contains other examples of species divergence. Other fossils found there have led to general agreement that there was more than one species of triceratops, Dr. Holtz said. And it’s possible that the domed-headed hadrosaurs and Pachycephalosaurus found at the top of the formation differ significantly from those at the bottom.

“At the same time, in the same place, the same size classes of organisms pass through a succession of species,” Dr. Holtz said. “It’s not inconceivable that Tyrannosaurus did it too.”

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