Discovery and namingEdit
During the 1960s, over fourteen thousand fossil bones were uncovered at the Cleveland-Lloyd Quarry in central Utah. The majority of these belonged to Allosaurus but some were of at least two theropods new to science. In 1974 one of these was named by James Henry Madsen Jr. as the genus Stokesosaurus.
In 1976 the second was by Madsen named as the type species Marshosaurus bicentesimus. The generic name honoured the nineteenth century paleontologist Professor Othniel Charles Marsh, who described many dinosaur fossils during the Bone Wars. The specific name was chosen "in honor of the bicentennial of the United States of America"
The holotype, UMNH VP 6373, was found in a layer of the Brushy Basin Member of the Morrison Formation dating from the late Kimmeridgian, approximately 155 - 150 mya.. It is a left ilium, or upper pelvis bone. The paratypes consisted of three bones: the ischia UMNH VP 6379 and UMNH VP 380 and the pubic 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. In 1993 a partial skeleton, CMNH 21704, from the Dinosaur National Monument was referred because its spines resembled non-identified spines from the Cleveland Lloyd Dinosaur Quarry. This specimen was more completely described in 1997.
Marshosaurus was medium-sized theropod. In 2010 Gregory S. Paul estimated its length at 4.5 metres, its weight at two hundred kilogrammes. The holotype ilium has a length of 375 millimetres. If the cranial material is correctly referred, the skull was about sixty centimetres long.
In 2012, Matthew Carrano established one autapomorphy, a unique derived trait of the holotype: the suture between the pubic peduncle and the pubic bone is convex, curving upwards, at the front and concave at the rear.