Stokesosaurus

Phylogeny: Dinosauria > Saurischia > Theropoda > Neotheropoda > Tetanurae > Avetheropoda > Coelurosauria > Tyrannosauroidea > Pantyrannosauria > Stokesosauridae

Overview: In the 1960’s, a team of fossil hunters excavated a vast number of remains from a major bone bed in Utah. This quarry is a part of the larger Morrison Formation – the most famous North American geological formation dated to the Late Jurassic. The majority of the fossils secured were recognized as having come from the genus Allosaurus, but the bones of some other dinosaurs were also noted. In 1974, a portion of a theropod pelvis was described by researcher James H. Madsen, which he recognized as having come from a tyrannosaur. Madsen named it Stokesosaurus, after a colleague by the name of William L. Stokes, who helped excavate part of the quarry where it was found. Finding a tyrannosaur within the Morrison Formation was quite significant, as at that time, Stokesosaurus was the geologically oldest tyrannosaur yet to be described. Most other confirmed tyrannosaurs lived in the Late Cretaceous and were significantly larger than Stokesosaurus. It grew to be about three or four meters long and was a relatively lightly built theropod.

Madsen specifically assigned Stokesosaurus to the family Tyrannosauridae, which would’ve made it closely related to dinosaurs like Albertosaurus, Gorgosaurus and even Tyrannosaurus itself. In most modern phylogenetic studies, however, Stokesosaurus is classified outside of the Tyrannosauridae, as a more basal member of the larger superfamily Tyrannosauroidea. Some recent studies suggest the creature may belong to its own specific family, the Stokesosauridae, which it possibly shared with a few other dinosaurs, most notably Eotyrannus from the United Kingdom. Jurassic tyrannosaurs, in general, were usually small or mid-tier predators at best. More fossils were found after Madsen’s description, though Stokesosaurus is still based on limited fossils. We can infer that it probably had longer arms than its later relatives, as well as three clawed digits on each hand. Stokesosaurus very likely had proto-feathers in life, based on what we know about basal tyrannosauroids. It coexisted with dinosaurs like Allosaurus, Ceratosaurus, Diplodocus, Brachiosaurus, and Stegosaurus.