| Stokesosaurus | |
|---|---|
 | |
| Holotype UMNH VP 7473 | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Superfamily: | †Tyrannosauroidea |
| Clade: | †Pantyrannosauria |
| Family: | †Stokesosauridae |
| Genus: | †Stokesosaurus Madsen, 1974 |
| Species: | †S. clevelandi |
| Binomial name | |
| †Stokesosaurus clevelandi Madsen, 1974 | |
| Synonyms | |
| |
Stokesosaurus (meaning "Stokes' lizard") is agenus of earlytyrannosauroidtheropoddinosaurs from the lateJurassic period ofUtah,United States. They were small,bipedal predators measuring around 3 to 4 meters (9.8 to 13.1 ft) in length.[1]
From 1960 onwards UtahgeologistWilliam Lee Stokes and his assistantJames Henry Madsen excavated thousands of disarticulatedAllosaurus bones at theCleveland-Lloyd Dinosaur Quarry inEmery 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 thetype speciesStokesosaurus clevelandi. Its generic name honours Stokes. Thespecific name refers to the town ofCleveland, Utah.[2]
Theholotype (UMNH 2938, also known as UMNH VP 7473 and formerly known asUUVP 2938) was uncovered in theBrushy Basin Member of theMorrison Formation dating from the earlyTithonian stage, about 150 million years old. It consists of a leftilium or hip bone, belonging to a juvenile individual. Madsen also assigned aparatype, UUVP 2320, a 50% larger right ilium. Additionally he referred a rightpremaxilla, UUVP 2999.[2] However, this was in 2005 referred toTanycolagreus.[3]Stokesosaurus andTanycolagreus 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 ofStokesosaurus) ofTanycolagreus has never been recovered, making direct comparison difficult.[1]
In 1976Peter Malcolm Galton consideredStokesosaurus to be a second species of the British possible early tyrannosauroidIliosuchus, that he named asIliosuchus clevelandi.[4] This has found no acceptance among other researchers;[5] in 1980 Galton himself withdrew his opinion.[6]
Some later finds were referred toStokesosaurus. This included someischia and tail vertebrae in 1991,[7] and a partialbraincase in 1998.[8] Another, very smallilium referred toStokesosaurus, found inSouth Dakota,[9] is lost but may actually belong to the relatedAviatyrannis.[10] More fragmentary remains possibly referable toStokesosaurus have been recovered from stratigraphic zone 2 of the Morrison Formation, dated to the lateKimmeridgian age, about 152 million years ago.[11][12]
A specimen of an indeterminateStokesosaurus species that was discovered in theAlcobaça Formation ofGuimarota,Portugal was identified and described by Rauhut (2000);[13] this specimen was later named as the new genusAviatyrannis in 2003.[10]
A second species,Stokesosaurus langhami, was described byRoger Benson in 2008 based on a partial skeleton fromEngland.[5] However, further study showed that this species should be referred to a new genus, which was namedJuratyrant in 2012. Benson andStephen Brusatte concluded that not a single bone had been justifiably referred toStokesosaurus, 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 uniteStokesosaurus clevelandi andJuratyrant langhami under one genus[5] 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 ofEotyrannus. This leaves only a singleautapomorphy ofStokesosaurus which is not present inJuratyrant or other tyrannosauroids: a swollen rim around the articular surface of the pubic peduncle.[14]
The holotypeilium is 22 centimeters (8.7 in) long, indicating a small individual. Madsen in 1974 estimated that the adult body length was about 4 meters (13 ft).[2] In 2010,Gregory S. Paul estimated the length at 2.5 meters (8 ft 2 in) and the weight at 60 kilograms (130 lb).[15]
In 1974 Madsen assignedStokesosaurus to theTyrannosauridae.[2] However, moderncladistic analyses indicate a more basal position. In 2012 the study by Brusatte and Benson recoveredStokesosaurus as a basal member of theTyrannosauroidea, and closely related toEotyrannus andJuratyrant.[14]
Below is a 2013 cladogram by Loewenet al. that placesStokesosaurus andJuratyrant as derived members of Proceratosauridae, due to sharing withSinotyrannus a narrow preacetabular notch.[16] Many basal tyrannosauroids have incomplete or unknown ilia and this trait may be more widespread than currently known.[14] Various traits support the argument thatSinotyrannus is a proceratosaurid.[16]
However, a 2016 analysis utilizing bothparsimonious andBayesian phylogeny placedStokesosaurus andJuratyrant as tyrannosauroids slightly more advanced thanProceratosauridae andDilong. In addition,Eotyrannus is recovered as a sister taxon of these genera in the parsimonious phylogeny.[17]
The Morrison Formation is a sequence of shallow marine and alluvial sediments which, according toradiometric dating, ranges between 156.3 million years old (Ma) at its base,[18] to 146.8 million years old at the top,[19] which places it in the lateOxfordian,Kimmeridgian, and earlyTithonianstages of the Late Jurassic period. This formation is interpreted as asemiarid environment with distinctwet anddry seasons. The Morrison Basin where dinosaurs lived, stretched from New Mexico to Alberta and Saskatchewan, and was formed when the precursors to theFront Range of the Rocky Mountains started pushing up to the west. The deposits from their east-facingdrainage basins were carried by streams andrivers and deposited inswampy lowlands, lakes, river channels andfloodplains.[20] This formation is similar in age to theSolnhofen Limestone Formation inGermany and theTendaguru Formation inTanzania. In 1877 this formation became the center of theBone Wars, a fossil-collecting rivalry between early paleontologistsOthniel Charles Marsh andEdward Drinker Cope.
The Morrison Formation records an environment and time dominated by gigantic sauropod dinosaurs such asCamarasaurus,Barosaurus,Diplodocus,Apatosaurus andBrachiosaurus. Dinosaurs that lived alongsideStokesosaurus included the herbivorousornithischiansCamptosaurus,Dryosaurus,Stegosaurus andOthnielosaurus. Predators in this paleoenvironment included thetheropodsSaurophaganax,Torvosaurus,Ceratosaurus,Marshosaurus,Ornitholestes and[21]Allosaurus, which accounted for 70 to 75% of theropod specimens and was at the toptrophic level of the Morrisonfood web.[22] Other animals that shared this paleoenvironment includedbivalves,snails,ray-finned fishes,frogs,salamanders,turtles,sphenodonts,lizards, terrestrial and aquaticcrocodylomorphs, and several species ofpterosaur. Examples of early mammals present in this region, weredocodonts,multituberculates, symmetrodonts, andtriconodonts. The flora of the period has been revealed by fossils ofgreen algae,fungi,mosses,horsetails,cycads,ginkgoes, and several families ofconifers. Vegetation varied fromriver-lining forests oftree ferns,ferns,ginkgos,seed ferns andconifers (gallery forests), to fernsavannas with occasional trees such as theAraucaria-like coniferBrachyphyllum.[23]
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