February 23, 2012
Despite belonging to one of the most famous dinosaur groups of all time, few people have heard of Stokesosaurus clevelandi. This predator, named in 1974 by paleontologist James Madsen, Jr., was a tyrannosauroid dinosaur that roamed North America tens of millions of years before Tyrannosaurus rex.
The bones of Stokesosaurus were initially discovered in the fossil-rich Cleveland-Lloyd Dinosaur quarry in eastern Utah. Although dominated by the remains of at least 46 Allosaurus, rarer traces of other theropod dinosaurs have come out of the quarry. (The mid-size carnivore Marshosaurus and possibly a distinct species of Ceratosaurus have also been recognized from bones found here.) In the case of Stokesosaurus, Madsen had identified two portions of the hip and a piece of the upper jaw, the premaxilla, as belonging to this small theropod. The complete animal probably didn’t stretch longer than 12 feet from nose to tail. While Madsen was tentative about this conclusion, the diminutive predator seemed to represent the early days of the tyrant dinosaurs in North America. Since then, one of the hips has been lost and the jaw fragment is thought to have belonged to a different dinosaur, but the primary hip Madsen relied upon for his description still indicates the presence of the tyrants in Jurassic Utah around 150 million years ago.
By now you may be wondering why I opened a post titled “England’s Jurassic Tyrant” with a note about a tyrannosauroid from Utah. The reason is because, until recently, Stokesosaurus was thought to have been present in Jurassic Europe, too. In 2008, paleontologist Roger Benson described a partial skeleton from the Late Jurassic of England that he attributed to a new species of the dinosaur, Stokesosaurus langhami. There was far more of this animal than the North American species, whose anatomy remains largely a mystery. The new species, on the other hand, was represented by numerous vertebrae, the majority of the hips, and most of a hindlimb.
But the dinosaur Benson described probably wasn’t Stokesosaurus, after all. In a paper to be published at Acta Palaeontologica Polonica, Benson and colleague Stephen Brusatte suggest that the more complete material from England represents a distinct genus of dinosaur. The change was spurred by the discovery of additional archaic tyrannosaurs in recent years. These finds indicated that some of the features Benson had used to link Stokesosaurus from Utah and the British form together were widely distributed among the tyrannosauroids and therefore might not reveal clear relationships. The more complete material from England now seems more distinct from Stokesosaurus than previously understood. Brusatte and Benson have renamed the animal Juratyrant.
But we still know very little about Stokesosaurus, Juratyrant and their close relatives. For Stokesosaurus, most of the skeleton is unknown, and significant portions of Juratyrant—such as the skull and forelimbs—have yet to be found. These tyrants are hardly unique in this respect. Other closely related dinosaurs such as Aviatyrannis are known from frustratingly incomplete remains. We know that these dinosaurs were small predators that set the stage for the later rise of more imposing tyrants, but what they looked like and how they lived remains mysterious.
Benson, R. (2008). New information on Stokesosaurus, a tyrannosauroid (Dinosauria: Theropoda) from North America and the United Kingdom Journal of Vertebrate Paleontology, 28 (3), 732-750 DOI: 10.1671/0272-4634(2008)28[732:NIOSAT]2.0.CO;2
Brusatte, S., & Benson, R. (2012). The systematics of Late Jurassic tyrannosauroids (Dinosauria: Theropoda) from Europe and North America Acta Palaeontologica Polonica DOI: 10.4202/app.2011.0141
Madsen, J. 1974. A new theropod dinosaur from the Upper Jurassic of Utah. Journal of Paleontology, 48 (1), 27-31
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