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They discovered among the specimens troublesome to categorise. However 26 appeared to group into three sorts, Mr. Paul mentioned: one strong 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 varieties, Mr. Paul and his colleagues concluded, have been in all probability totally different sufficient from each other — and appeared over a ample size of time — to warrant separate names. Subsequently, the earliest, cumbersome Tyrannosaurus to point out up within the Hell Creek acquired the identify Tyrannosaurus imperator (“emperor tyrant lizard”), which then — over the course of 1 million to 2 million years — cut up into the strong 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 have been dominated by Tyrannosaurus kinfolk, Mr. Paul mentioned. In that interval, bruisers like Daspletosaurus coexisted with long-legged hunters equivalent to Gorgosaurus.
In keeping with Dr. Holtz, such an thought is completely believable — however proving it’s going to require help from future finds.
“It’s a testable speculation, as any assertion of species identification ought to be,” Dr. Holtz mentioned. “With further new specimens, we are able to see if specimens they didn’t embrace or we haven’t discovered but are in step with this suggestion, or in the event that they reject it.”
Whereas Dr. Holtz felt the authors’ case can be extra convincing if the totally different species they describe have been organized extra chronologically in particular rock layers, he famous that the Hell Creek Formation comprises different examples of species divergence. Different fossils discovered there led to a basic settlement that there was a couple of species of triceratops, Dr. Holtz mentioned. And there’s a risk that the hadrosaurs and dome-headed Pachycephalosaurus discovered on the prime of the formation differ considerably from these on the backside.
“On the identical time, in the identical place, the identical measurement lessons of organisms are going by a succession of species,” Dr. Holtz mentioned. “It’s not inconceivable that Tyrannosaurus did in order nicely.”
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