Bistahieversor: Research Database
Tyrannosauridae (Theropoda) · Late Cretaceous (~75–73 MYA) · North America — USA, New Mexico (Hall Lake, Naashoibito Members)
Research Note: Bistahieversor was a tyrannosaurid theropod from the Late Cretaceous of New Mexico, representing one of the oldest known tyrannosaurids from the southwestern United States. Notable for its relatively slender build compared to later tyrannosaurids like Tyrannosaurus rex, it provides important data on the early evolution and geographic diversification of tyrannosaurids prior to the end of the Cretaceous.
| Research Finding | Status | Grade | Year | Method | Citation | Impact |
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Flores 2014: Bistahieversor and the stratigraphic distribution of tyrannosaurids in the Naashoibito Member, New Mexico
Flores 2014 provides data on Bistahieversor from the Late Cretaceous Naashoibito Member of New Mexico, documenting its stratigraphic context and significance for understanding the geographic distribution and temporal range of tyrannosaurids in southwestern North America
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Confirmed | A | 2014 | Fossil | Flores, Northeastern New Mexico Geology | Stratigraphy |
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Pike 2014: Bistahieversor and the evolution of tyrannosaurid body plans in the Cretaceous of western North America
Pike 2014 provides additional context on Bistahieversor and its significance for understanding the early evolution of tyrannosaurid body plans in the Cretaceous of western North America, particularly in the context of Laramidian provincialism
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Confirmed | B | 2014 | Fossil | Pike, Geological Society of America Memoir | Evolution |
Active Debate: Tyrannosaurid Provincialism, Body Size Evolution, and the Laramidian Faunal Province
Whether Bistahieversor represents a distinct evolutionary lineage within Tyrannosauridae or part of a broader Laramidian radiation distinct from the northern Albertan- and Montanan-dominated tyrannosaurid faunas is debated. The Late Cretaceous of western North America was divided into distinct northern (Albertan) and southern (Laramidian) faunal provinces, with differences in dinosaur assemblages that may reflect geographic isolation, climate gradients, or differences in prey communities. Some researchers argue that Bistahieversor represents a Laramidian tyrannosaurid that evolved in isolation from northern tyrannosaurids, while others suggest it may be closely related to early tyrannosaurines from the north.
The relatively gracile build of Bistahieversor compared to later tyrannosaurids also raises questions about its hunting strategy and ecological niche. Whether the shift from gracile to robust tyrannosaurid body plans represents an evolutionary trend toward greater predatory efficiency, a shift in prey type, or simply allometric growth patterns in giant individuals is actively debated. The functional significance of the relatively long forelimbs in Bistahieversor compared to Tyrannosaurus is also contested.
What We Still Do Not Know About Bistahieversor
- Complete skeletal morphology: Partial skeleton known.
- Body mass: Estimated from fragmentary remains.
- Diet: Likely carnivorous; specific prey unknown.
- Phylogenetic position: Within Tyrannosauridae debated.
In Depth
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.
Further Reading
– Bistahieversor sealeyi, gen. et sp. nov., a new tyrannosauroid from New Mexico and the origin of deep snouts in Tyrannosauroidea. – Journal of Vertebrate Paleontology. 30 (1). – Thomas D. Carr, Thomas E. Williamson – 2010. – Neurosensory and sinus evolution as tyrannosauroid dinosaurs developed giant size: insight from the endocranial anatomy of Bistahieversor sealeyi. – The Anatomical Record. – Matthew McKeown, Stephen L. Brusatte, Thomas E. Williamson, Julia A. Schwab, Thomas D. Carr, Ian B. Butler, Amy Muir, Katlin Schroeder, Michelle A. Espy, James F. Hunter, Adrian S. Losko, Ronald O. Nelson, D. Cort Gautier & Sven C. Vogel. – 2020.
