The first remains now attributed to Bistahieversor, a partial skull and skeleton, were described in 1990 as a specimen of Aublysodon. Additional remains, consisting of the incomplete skull and skeleton of a juvenile, were described in 1992. Another, complete, skull and partial skeleton were found in the Bisti/De-Na-Zin Wilderness of New Mexico in 1998, known colloquially as the "Bisti Beast". In a 2000 paper, Thomas Carr and Thomas Williamson re-examined these four specimens and suggested that they did not belong to Aublysodon, but rather to one or more new species of Daspletosaurus. However, it was not until 2010 that Carr and Williamson published a thorough re-description of the specimens and found that they belonged to a new genus and species of more generalized tyrannosauroid, which they named Bistahieversor sealeyi.
Description
Material from both adolescent and adult individuals has been found in the Kirtland Formation of New Mexico, United States. Adult Bistahieversor are estimated to have been around long, weighing at least a ton. The snout is deep, indicating that the feature is not unique to more derived tyrannosaurs such as Tyrannosaurus. Geographical barriers such as the newly forming Rocky Mountains may have isolated the more southerly Bistahieversor from more derived northern tyrannosaurs. In 2010 Gregory S. Paul estimated a length of 8 m and a weight of 2.5 tonnes. In 2016 Molina-Pérez and Larramendi gave a length of 9 m and a weight of 3.3 tonnes. Bistahieversor differs from other tyrannosaurs in the possession of 64 teeth, an extra opening above the eye, and a keel along the lower jaw. The opening above the eye is thought to have accommodated an air sac that would have lightened the skull's weight. Bistahieversor also had a complex joint at its "forehead" that would have stabilized the skull, preventing movement at the joint.
Classification
Bistahieversor is a genus of derived dinosaur currently classified in the subfamily Tyrannosaurinae. It is more derived than Teratophoneus but less derived than Lythronax. It forms the sister taxon of a group including Lythronax, Nanuqsaurus, Tyrannosaurus, Tarbosaurus and Zhuchengtyrannus. Below is a cladogram illustrating the relationships of all tyrannosaurid genera: The cladogram below is based on a phylogenetic analysis conducted by Voris et al. in 2020. Here, Bistahieversor was recovered as a basal member of Eutyrannosauria rather than as a tyrannosaurine:
