Tiny tyrannosauroid fills a big gap

The fossils of a newly discovered species of small tyrannosauroid dinosaur that were found in New Mexico, United States, are described in a paper published online this week in Nature Ecology & Evolution. These specimens, two associated juvenile skeletons dated to around 92 million years ago, provide insight into the little-understood origins of Tyrannosaurus rex and its closely related cousins.

Gigantic tyrannosaurs thrived in the Late Cretaceous from 80-66 million years ago. However, their origins are poorly understood because of a global sampling gap associated with high sea levels in what is now North America that occurred earlier in the epoch. Sterling Nesbitt and colleagues uncovered two skeletons of a new tyrannosauroid in the Zuni Basin of New Mexico. These fossils represent the most complete specimens of a mid-Cretaceous tyrannosauroid to date. Named Suskityrannus hazelae, after the local Zuni word for coyote, ‘suski’, this dinosaur had a skull measuring 25-32 cm in total length. Although the specimens were young, the authors estimate that the adult S. hazelae would have been considerably smaller than its Late Cretaceous cousins, such as the T. rex. Despite its size, S. hazelae had a specially adapted running foot and a strong bite force — a combination of characteristics not present in early tyrannosauroids or later, larger tyrannosauroids.

The authors’ analysis places S. hazelae as an intermediate tyrannosauroid, between the smallest species that diverged the earliest and the giants of the Late Cretaceous. They conclude that this new species fills an important gap in tyrannosauroid evolutionary history.