Marshosaurus is a genus of medium-sized carnivorous theropod dinosaur. It was from the Upper Jurassic Morrison Formation of Utah and perhaps Colorado. This carnivore operated in the same territory as the larger and more famous Allosaurus. It was about 4.5 metres long and weighed 200 kg.
The Morrison Formation is a sequence of shallow sea and river delta sediments between 156.3 million years ago at its base, to 146.8 mya at the top. That puts it in the later Jurassic period. The area would have been rather dry, with distinct wet and dry seasons. The Morrison Basin where dinosaurs lived, stretched from New Mexico to Alberta and Saskatchewan. It was formed when the early Rocky Mountains started pushing up in 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.