Marshosaurus

Extinct genus of dinosaurs

speciesbox
| fossil range = Late Jurassic,
| image = Marshosaurus bicentissimus skull cast Natural History Museum of Utah DSC07218. JPG
| image caption = Skull cast
| genus = Marshosaurus
| parent authority = Madsen, 1976

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 155 152 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 dorsal neural spines resembled non identified spines from the Cleveland Lloyd Dinosaur Quarry. This specimen was also the subject of a 1997 SVP abstract.

Marshosaurus was medium sized for a theropod. In 2010, Gregory S. Paul estimated its length at and its weight at The holotype ilium has a length of If the cranial material is correctly referred, the skull was about 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.

Madsen originally was unsure about the phylogenetic position of Marshosaurus, placing it as Theropoda incertae sedis. Some later analyses showed Marshosaurus to be a member of Avetheropoda, a group of more bird like theropods including Tyrannosaurus, Velociraptor and Allosaurus. However, Roger Benson (2010)

One right ilium of a Marshosaurus bicentesimus is deformed by "an undescribed pathology" which probably originated as a consequence of injury. Another specimen has a pathological rib. In a 2001 study conducted by Bruce Rothschild and other paleontologists, five foot bones referred to Marshosaurus were examined for signs of stress fracture, but none were found.

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, Brachiosaurus, Barosaurus, Diplodocus, and Apatosaurus. Dinosaurs that lived alongside Marshosaurus included the herbivorous ornithischians Camptosaurus, Dryosaurus, Stegosaurus and Othnielosaurus. Predators in this paleoenvironment included the theropods Saurophaganax, Torvosaurus, Ceratosaurus, Stokesosaurus, Ornitholestes and Allosaurus, which accounted for 70 to 75% of theropod specimens and was at the top trophic level of the Morrison food web. Stegosaurus is commonly found at the same sites as Allosaurus, Apatosaurus, Camarasaurus, and Diplodocus. Early mammals were present in this region, such as 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 (gallery forests), to fern savannas with occasional trees such as the Araucaria like conifer Brachyphyllum.