A single fossilised bone can change everything. In this case, it might reshape the story of the most famous dinosaur of all time — Tyrannosaurus rex. A new study published in the journal “Scientific Reports” describes a massive tyrannosaur leg bone discovered in New Mexico. At first glance, it looks like just another theropod fossil. However, the implications run much deeper. Researchers report the discovery of a giant fossil tyrannosaur tibia.

The Hunter Wash tyrannosaurid left tibia (NMMNH P-25085) in A, anterior, B, posterior, C, lateral, and D, medial views. Abbreviations: as, astragalus; cc, cnemial crest; fc, fibular crest; fi, fibular facet; lc, lateral condyle; mc, medial condyle. Picture credit: Nicholas R. Longrich et al.

Picture credit: Nicholas R. Longrich et al (Scientific Reports) – Open Access Paper – Creative Commons Attribution 4.0 International License

A Giant Fossil Tyrannosaur Tibia from 74 Million Years Ago

The fossil comes from the Kirtland Formation in New Mexico. It dates to around seventy-four million years ago, (Campanian faunal stage of the Late Cretaceous).  The paper’s authors are from the New Mexico Museum of Natural History and Science (Albuquerque), Montana State University and the University of Bath. They identified a tibia (lower leg bone), it measures nearly a metre in length (96 cm long and 12.8 cm in diameter).  It is a massive tibia, 84% and 78% the dimensions of the largest known Tyrannosaurus tibia. The scientists propose the existence of a five-tonne predator, an animal that would have been far larger than contemporary tyrannosaurs.  This suggests that giant tyrannosaurs evolved earlier than previously thought.

Which Taxon?

The research team compared the fossil leg bone to a large dataset of tyrannosaur anatomy. They analysed over five hundred traits to determine its evolutionary position. Their conclusion is striking. This animal likely belongs to the tribe Tyrannosaurini — the lineage that includes the famous Tyrannosaurus rex.

The fossil from the Hunter Wash Member of the Kirtland Formation (New Mexico) could represent:

• A giant specimen of the tyrannosaur Bistahieversor sealeyi.
• A previously unknown lineage of giant tyrannosaur.
• Or an early representative of the Tyrannosaurini.

Introduced in 2014, a model of the tyrannosaur from southern Laramidia (New Mexico) – Bistahieversor sealeyi.

The image (above) shows a model of the tyrannosaur Bistahieversor (B. sealeyi) from CollectA.

To view the range of not-to-scale CollectA figures in stock: CollectA Prehistoric Life Figures.

The Geographic Origins of Tyrannosaurus

The appearance of a large, derived tyrannosaurine in the late Campanian of the American Southwest also has implications for the geographic origin of Tyrannosaurus. Palaeontologists have debated where the lineage that led to T. rex evolved. Some argued for an Asian origin. Others pointed to North America. However, any theory proposed now needs to consider the presence of large tyrannosaurs in southern Laramidia.

The massive tyrannosaur tibia highlights the differences between the faunas of northern and southern Laramidia.  For example, in those geological formations representing northern Laramidia, slightly smaller tyrannosaurs are found – Gorgosaurus, Daspletosaurus and Teratophoneus. Meanwhile, larger tyrannosaurs may have been present in the southern part of this landmass.

Intriguingly, the Hunter Wash Member hints at previously unrecognized diversity among southern tyrannosaurs, with Bistahieversor coexisting with a larger and more robust tyrannosaur species represented by this fossil tibia. A small tyrannosaur, (specimen number NMMNH P-27469), has previously been identified as a juvenile of Bistahieversor. However, it shows features seen in Nanotyrannus. Therefore, a much smaller, gracile form of tyrannosaur similar to Nanotyrannus could have been coeval with Bistahieversor and a much larger tyrannosaur.

To read Everything Dinosaur’s blog post about confirmation of the Nanotyrannus taxon: Nanotyrannus – A New Chapter in Tyrannosaur Evolution.

The scientific paper: “A large tyrannosaurid from the Late Cretaceous (Campanian) of North America” by Nicholas R. Longrich, Sebastian Dalman, Spencer G. Lucas and Anthony R. Fiorillo published in Scientific Reports.

The award-winning Everything Dinosaur website: Prehistoric Animal Toys and Models.