Stokesosaurus - History

History

From 1960 onwards Utah geologist William Lee Stokes and his assistant James Madsen excavated thousands of disarticulated Allosaurus bones at the Cleveland-Lloyd Dinosaur Quarry in Emery County, Utah. During the early seventies Madsen began to catalogue these finds in detail, discovering that some remains represented species new to science. In 1974 Madsen named and described the type species Stokesosaurus clevelandi. Its generic name honours Stokes. The specific name refers to the town of Cleveland, Utah.

The holotype (UUVP 2938) was uncovered in the Brushy Basin Member of the Morrison Formation dating from the early Tithonian stage, about 150 million years old. It consists of a left ilium or hip bone, belonging to a juvenile individual. Madsen also assigned a paratype, UUVP 2320, a 50% larger right ilium. Additionally he referred a right premaxilla, UUVP 2999. However, this was in 2005 referred to Tanycolagreus. Stokesosaurus and Tanycolagreus are about the same size, and it is possible that the latter is a junior synonym of the former. However, the ilium (the best and perhaps only known element of Stokesosaurus) of Tanycolagreus has never been recovered, making direct comparison difficult.

In 1976 Peter Malcolm Galton considered Stokesosaurus to be a second species of the British possible early tyrannosauroid Iliosuchus, that he named as Iliosuchus clevelandi. This has found no acceptance among other researchers; in 1980 Galton himself withdrew his opinion.

Some later finds were referred to Stokesosaurus. This included some ischia and tail vertebrae in 1991, and a partial braincase in 1998. Another, very small ilium referred to Stokesosaurus, found in South Dakota, is lost but may actually belong to the related Aviatyrannis. More fragmentary remains possibly referable to Stokesosaurus have been recovered from stratigraphic zone 2 of the Morrison Formation, dated to the late Kimmeridgian age, about 152 million years ago.

A second species, Stokesosaurus langhami, was described by Roger Benson in 2008 based on a partial skeleton from England. However, further study showed that this species should be referred to a new genus, which was named Juratyrant in 2012. Benson and Stephen Brusatte concluded that not a single bone had been justifiably referred to Stokesosaurus, and that not even the paratype could be safely assigned, leaving the holotype ilium as the only known fossil of the taxon.

The holotype ilium is twenty-two centimetres long, indicating a small individual. Madsen in 1974 estimated that the adult body length was about four metres. In 2010 Gregory S. Paul estimated the length at 2.5 metres, the weight at sixty kilogrammes. In 2012 Benson and Brusatte established a single unique derived trait or autapomorphy of Stokesosaurus: the normally vertical ridge on the outer blade surface of the ilium, above the hip joint, was strongly inclined to the back and reached the upper rim of the blade.

In 1974 Madsen assigned Stokesosaurus to the Tyrannosauridae. However, modern cladistic analyses indicate a more basal position. In 2012 the study by Brusatte and Benson recovered Stokesosaurus as a basal member of the Tyrannosauroidea, and closely related to Eotyrannus and Juratyrant.