For years paleontologists have been finding teeth and isolated scraps of tyrannosaurs in the southwestern United States, but figuring out which specific dinosaurs they belonged to has been another matter. Many of the best-known tyrannosaurs, including the famous Tyrannosaurus rex, lived farther to the north, and without more complete fossils it was difficult to tell whether the southern types were new species or varieties scientists were already familiar with. Now, in the latest issue of the Journal of Vertebrate Paleontology, researchers Thomas Carr and Thomas Williamson have identified one of the southern tyrants.
During the 1990s, several partial skeletons and skulls of an unknown tyrannosaur were found in New Mexico. Early on these remains were attributed to the genus Aublysodon, but this was a "waste basket" name to which many miscellaneous specimens were attributed. When Carr and Williamson re-investigated the material, however, they found that the bones came from an entirely new type of 75-million-year-old tyrannosaur. They named it Bistahieversor sealeyi, and it showed a curious correspondence to some of its North American relatives.
There is an immediately recognizable difference between the early tyrannosaurs and the later tyrannosaurs. While early types, such as Alioramus, had shallow snouts, the later species, such as Tyrannosaurus, had snouts that were very deep from bottom to top. This suggests that the heavy, robust skulls of the later tyrannosaurs were an evolutionary specialization, and the authors of the new study suggest that it evolved among North American tyrannosaurs after the ancient Western Interior Seaway isolated some tyrannosaurs in the western part of the country. In this hypothesis, early, shallow-snouted tryannosaurs would have dispersed throughout North America, but when the prehistoric seaway cut off the western populations, they evolved deeper skulls, and some of these forms went back across to Asia. If this is correct, Bistahieversor might represent what some of the early deep-snouted tyrannosaurs were like.
Thomas D. Carr; Thomas E. Williamson (2010). Bistahieversor sealeyi, gen. et sp. nov., a new tyrannosauroid from New Mexico and the origin of deep snouts in Tyrannosauroidea Journal of Verterbrate Paleontology, 30 (1), 1-16 : 10.1080/02724630903413032