Fossil range: Late Jurassic
| Scientific classification
Stokesosaurus (meaning "Stokes' lizard") is a genus of small (around 3 to 4 meters in length) early tyrannosaur from the Late Jurassic period of Utah and England. It was named after Utah geologist William Lee Stokes. Remains possibly referable to Stokesosaurus have been recovered from stratigraphic zone 2. of the Morrison Formation.
The holotype (UUVP 2938) consists of a hip bone, originally thought to belong to the possible early tyrannosaur Iliosuchus, as well as several vertebrae and a partial braincase. Another ilium referred to this dinosaur is lost but may actually belong to the related Aviatyrannis, and a premaxilla thought to belong to Iliosuchus is actually from Tanycolagreus.
A second species, Stokesosaurus langhami, was described by Roger Benson in 2008 based on a partial skeleton. The skeleton consists of an "associated partial skeleton represented by a complete pelvis" as well as a partially complete leg, and neck, back, and tail vertebrae. This second skeleton was discovered in 1984 in Dorset, was mentioned in several papers, but was not formally described until 2008. The new species was named in honor of Peter Langham, who collected the specimen. The new specimen was discovered in strata dating from the Tithonian, the final stage of the Late Jurassic, meaning the fossil is around 150 million years old.
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 known element of Stokesosaurus) of Tanycolagreus has never been recovered, making direct comparison difficult.
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 as an derived member of Proceratosauridae.
The Morrison Formation is a sequence of shallow marine and alluvial sediments which, according to radiometric dating, ranges between 156.3 million years old (Ma) at its base, to 146.8 million years old at the top, which places it in the late Oxfordian, Kimmeridgian, and early Tithonian stages of the Late Jurassic period. This formation is interpreted as a semiarid environment with distinct wet and dry seasons. The Morrison Basin where dinosaurs lived, stretched from New Mexico to Alberta and Saskatchewan, and was formed when the precursors to the Front Range of the Rocky Mountains started pushing up to the west. The deposits from their east-facing drainage basins were carried by streams and rivers and deposited in swampy lowlands, lakes, river channels and floodplains. This formation is similar in age to the Solnhofen Limestone Formation in Germany and the Tendaguru Formation in Tanzania. In 1877 this formation became the center of the Bone Wars, a fossil-collecting rivalry between early paleontologists Othniel Charles Marsh and Edward Drinker Cope.
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 ornithischians Camptosaurus, Dryosaurus, Stegosaurus and Othnielosaurus. Predators in this paleoenvironment included the theropods Saurophaganax, Torvosaurus, Ceratosaurus, Marshosaurus, Ornitholestes and Allosaurus, which accounting 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 fern (gallery forests), to fern savannas with occasional trees such as the Araucaria-like conifer Brachyphyllum.
- ^ a b Madsen (1974). "A new theropod dinosaur from the Upper Jurassic of Utah". Journal of Paleontology 48: 27–31.
- ^ Foster, J. (2007). "Appendix." Jurassic West: The Dinosaurs of the Morrison Formation and Their World. Indiana University Press. pp. 327-329.
- ^ Galton (1976). "Iliosuchus, a Jurassic dinosaur from Oxfordshire and Utah". Paleontology 19: 587–589.
- ^ Chure and Madsen (1998). "An unusual braincase (?Stokesosaurus clevelandi) from the Cleveland-Lloyd Dinosaur Quarry, Utah (Morrison Formation; Late Jurassic)". Journal of Vertebrate Paleontology 18 (1): 115–125.
- ^ Foster and Chure (2000). "An ilium of a juvenile Stokesosaurus (Dinosauria, Theropoda) from the Morrison Formation (Upper Jurassic: Kimmeridgian), Meade County, South Dakota". Brigham Young University Geology Studies 45: 5–10.
- ^ a b Benson, R.B.J. (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.
- ^ Foster, J. (2007). Jurassic West: The Dinosaurs of the Morrison Formation and Their World. Indiana University Press. 389pp.