Name: Bistahieversor‭ (‬Bistahi Destroyer‭)‬.
Phonetic: Bis-tah-hee-eh-ver-sore.
Named By: Thomas Carr‭ & ‬Thomas Williamson‭ ‬-‭ ‬2010.
Classification: Chordata,‭ ‬Reptilia,‭ ‬Dinosauria,‭ ‬Saurischia,‭ ‬Theropoda,‭ ‬Tyrannosauroidea.
Species: B.‭ ‬sealeyi‭ (‬type‭).
Type: Carnivore.
Size: Estimated‭ ‬9‭ ‬meters long.
Known locations: USA,‭ ‬New Mexico‭ ‬-‭ ‬Kirtland Formation.
Time period: Campanian of the Cretaceous.
Fossil representation: Two sets of remains representing juvenile and adult individuals.

       The skull of Bistahieversor has many features that are considered primitive for the Tyrannosaurid group,‭ ‬but most importantly,‭ ‬a noticeable depth that is lacking in other,‭ ‬and later,‭ ‬tyrannosaurid species.‭ ‬This is significant because it was once thought that only the later and more advanced tyrannosaurids,‭ ‬like Tyrannosaurus itself,‭ ‬had deeper snouts.‭ ‬The fact that the fossil record can show us tyrannosaurid snouts that were deep in Bistahieversor,‭ ‬getting narrower in others like Gorgosaurus and Albertosaurus,‭ ‬and going deep again in Tyrannosaurus,‭ ‬suggests that skull depth was an evolutionary adaptation to predatory styles and prey animals,‭ ‬as opposed to tyrannosaurid advancement.
       Increased skull depth is often taken as an indication for larger jaw muscles,‭ ‬in turn capable of inflicting significantly more bite force.‭ ‬The skull also features an extra opening above the orbital fenestra,‭ ‬which has considered to‭ ‬have‭ ‬been an air sac for reducing skull weight.‭ ‬This opening is absent from the juvenile specimen,‭ ‬suggesting that it was a sign of maturity.‭ ‬This also reinforces the weight reduction theory,‭ ‬as it would have helped to lighten the skull as it grew bigger,‭ ‬and heavier.‭ ‬Bistahieversor had around sixty-four teeth,‭ ‬a lot for a tyrannosaurid when you consider that Tyrannosaurus had fifty-four.
       At roughly nine meters long,‭ ‬Bistahieversor would have been comparable to the tyrannosaurid Daspletosaurus in size,‭ ‬which was active in the Northern reaches of Campanian USA,‭ ‬while Bistahieversor was in the south.‭ ‬Bistahieversor was also joined by the tyrannosaurid Teratophoneus,‭ ‬another of its kind that seems to have been restricted to the Southern US even though it also lived during the Campanian.‭
       Both Bistahieversor and Teratophoneus display more basal tyrannosaurid morphology,‭ ‬and both are known only from the southern area of what was once called Laramidia.‭ ‬This was the western half of North America that was separated from the eastern half by the Western Interior Seaway.‭ ‬Rising sea levels combined with mountain ranges being pushed up could have isolated the southern tyrannosaurids from the North,‭ ‬causing the retention of the more basal features seen in Bistahieversor,‭ ‬so late in the geological timescale.


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