| Marshosaurus | |
|---|---|
 | |
| Skull cast | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Family: | †Piatnitzkysauridae |
| Genus: | †Marshosaurus Madsen, 1976[1] |
| Species: | †M. bicentesimus |
| Binomial name | |
| †Marshosaurus bicentesimus Madsen, 1976[1] | |
Marshosaurus is a genus of medium-sizedcarnivoroustheropoddinosaur, belonging to the familyPiatnitzkysauridae, from theLate JurassicMorrison Formation ofUtah and possiblyColorado.
During the 1960s, over fourteen thousand fossil bones were uncovered at theCleveland-Lloyd Quarry in central Utah. The majority of these belonged toAllosaurus but some were of at least two theropods new to science. In 1974 one of these was named byJames Henry Madsen Jr. as the genusStokesosaurus.
In 1976 the second was by Madsen named as thetype speciesMarshosaurus bicentesimus. The generic name honoured the nineteenth century paleontologist ProfessorOthniel Charles Marsh, who described many dinosaur fossils during theBone Wars. Thespecific name was chosen "in honor of thebicentennial of the United States of America".[1]
Theholotype,UMNH VP 6373, was found in a layer of theBrushy Basin Member of theMorrison Formation dating from the lateKimmeridgian, approximately 155 - 152 mya. It is a leftilium, or upper pelvis bone. Theparatypes consisted of three bones: the ischia UMNH VP 6379 and UMNH VP 380 and thepubic bone UMNH VP 6387. Three ilia and six jaw fragments were provisionally referred. The material represents at least three individuals.
In 1991,Brooks Britt referred tail vertebrae from Colorado, because they resembled non-identified tail vertebrae fragments from the Cleveland-Lloyd Dinosaur Quarry.[2] In 1993 a partial skeleton, CMNH 21704, from theDinosaur National Monument was referred because its dorsal neural spines resembled non-identified spines from the Cleveland-Lloyd Dinosaur Quarry.[3] This specimen was also the subject of a 1997SVP abstract.[4]
Marshosaurus was medium-sized for a theropod. In 2010,Gregory S. Paul estimated its length at 4.5 meters (15 ft) and its weight at 200 kilograms (440 lb).[5] The holotypeilium has a length of 37.5 centimeters (14.8 in). If the cranial material is correctly referred, the skull was about 60 centimeters (24 in) long.
In 2012,Matthew Carrano established oneautapomorphy, a unique derived trait of the holotype: the suture between the pubic peduncle and thepubic bone is convex, curving upwards, at the front and concave at the rear.[6]
Madsen originally was unsure about the phylogenetic position ofMarshosaurus, placing it asTheropodaincertae sedis. Some later analyses showedMarshosaurus to be a member ofAvetheropoda, a group of more bird-like theropods includingTyrannosaurus,Velociraptor andAllosaurus. However,Roger Benson (2010)[7] found it to be amegalosauroid in a phylogenetic analysis ofMegalosaurus and 40 other theropods.
The position ofMarshosaurus in the evolutionary tree, as a possible member of thePiatnitzkysauridae, is shown by the cladogram below.[7]
One right ilium of aMarshosaurus bicentesimus is deformed by "an undescribed pathology" which probably originated as a consequence of injury. Another specimen has a pathological rib.[8] In a 2001 study conducted by Bruce Rothschild and other paleontologists, five foot bones referred toMarshosaurus were examined for signs ofstress fracture, but none were found.[9]
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,[10] to 146.8 million years old at the top,[11] 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.[12] This formation is similar in age to theSolnhofen Limestone Formation inGermany and theTendaguru Formation inTanzania. In 1877 this formation became the center of the Bone 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,Brachiosaurus,Barosaurus,Diplodocus, andApatosaurus. Dinosaurs that lived alongsideMarshosaurus included the herbivorousornithischiansCamptosaurus,Dryosaurus,Stegosaurus andOthnielosaurus. Predators in this paleoenvironment included thetheropodsSaurophaganax,Torvosaurus,Ceratosaurus,Stokesosaurus,Ornitholestes and[13]Allosaurus, which accounted for 70 to 75% of theropod specimens and was at the toptrophic level of the Morrisonfood web.[14]Stegosaurus is commonly found at the same sites asAllosaurus,Apatosaurus,Camarasaurus, andDiplodocus.[15] Early mammals were present in this region, such asdocodonts,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, andferns (gallery forests), to fernsavannas with occasional trees such as theAraucaria-like coniferBrachyphyllum.[16]
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