From 1960 onwards UtahgeologistWilliam Lee Stokes and his assistant James H. Madsen excavated thousands of disarticulated Allosaurus bones at the Cleveland-Lloyd Dinosaur Quarry in Emery County, Utah. During the early 1970s, 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 speciesStokesosaurus clevelandi. Its generic name honours Stokes. The specific name refers to the town of Cleveland, Utah. The holotype 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 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. In addition, many traits initially believed to unite Stokesosaurus clevelandi and Juratyrant langhami under one genus could not be conclusively disproven to exist on other tyrannosauroids. In fact, one of the traits, a posterodorsally inclined ridge on the lateral side of the ilium, was found on the undescribed left ilium of the holotype of Eotyrannus. This leaves only a single autapomorphy of Stokesosaurus which is not present in Juratyrant or other tyrannosauroids: a swollen rim around the articular surface of the pubic peduncle. The holotype ilium is long, indicating a small individual. Madsen in 1974 estimated that the adult body length was about. In 2010, Gregory S. Paul estimated the length at and the weight at.
Classification
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. Below is a 2013 cladogram by Loewen et al. that places Stokesosaurus and Juratyrant as derived members of Proceratosauridae, due to sharing with Sinotyrannus a narrow preacetabular notch. Many basal tyrannosauroids have incomplete or unknown ilia and this trait may be more widespread than currently known. Various traits support the argument that Sinotyrannus is a proceratosaurid. However, a 2016 analysis utilizing both parsimonious and Bayesian phylogeny placed Stokesosaurus and Juratyrant as tyrannosauroids slightly more advanced than Proceratosauridae and Dilong. In addition, Eotyrannus is recovered as a sister taxon of these genera in the parsimonious phylogeny.
The Morrison Formation records an environment and time dominated by gigantic sauropod dinosaurs such as Camarasaurus, Barosaurus, Diplodocus, Apatosaurus and Brachiosaurus. Dinosaurs that lived alongside Stokesosaurus included the herbivorous ornithischiansCamptosaurus, Dryosaurus, Stegosaurus and Othnielosaurus. Predators in this paleoenvironment included the theropodsSaurophaganax, Torvosaurus, Ceratosaurus, Marshosaurus, Ornitholestes and Allosaurus, which accounted for 70 to 75% of theropod specimens and was at the top trophic level of the Morrison food web. Other animals that shared this paleoenvironment included bivalves, snails, ray-finned fishes, frogs, salamanders, turtles, sphenodonts, lizards, terrestrial and aquatic crocodylomorphans, and several species of pterosaur. Examples of early mammals present in this region, were docodonts, multituberculates, symmetrodonts, and triconodonts. The flora of the period has been revealed by fossils of green algae, fungi, mosses, horsetails, cycads, ginkgoes, and several families of conifers. Vegetation varied from river-lining forests of tree ferns, and ferns, to fern savannas with occasional trees such as the Araucaria-like coniferBrachyphyllum.
