And they found it difficult to classify some samples. Mr. Paul said it appears to be 26 groups in three types: a strong one form from early Hell Creek with two sets of incisors in its lower jaws, and a strong and thin form from later with only one incisor group.
Paul and his colleagues concluded that these three forms were probably different enough from each other – and appeared over a sufficient period of time – to warrant separate names. So, the oldest huge dinosaur that appeared on Hell Creek got the name Tyrannosaurus Imperator (“Tyrant Emperor’s Lizard”), which then split – over the course of one to two million years – into the powerful and modern Tyrannosaurus rex. His name is, the relatively slender Tyrannosaurus Regina (Queen of Tyrant Lizard).
Mr. Ball said this proposed evolutionary path – from one tank-like group to another relatively light one – matches earlier Cretaceous ecosystems dominated by tyrannosaur’s relatives. In that period, bruises like Daspletosaurus coexisted with long-legged hunters like Gorgosaurus.
According to Dr. Holtz, this idea is entirely plausible – but proving it will require support from future discoveries.
“It’s a testable hypothesis, like any statement of species identity,” said Dr. Holtz. “With additional new samples, we can see if samples that were not included or that we haven’t found yet are compatible with this proposal, or if they reject it.”
While Dr. Holtz felt that the authors’ case 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. Dr. Holtz said other fossils found there led to general agreement that more than one species of Triceratops existed. There is a possibility that the hadrosaurs and the domed-headed Pachycephalosaurus at the top of the formation differed greatly from those at the bottom.
“At the same time, and in the same place, the same classes of organisms pass through a continuum of species,” said Dr. Holtz. “It’s not inconceivable that a tyrannosaur did that, too.”
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