Othnielosaurus is a genus of ornithiscian dinosaur from the Late Jurassic Period (155 to 148 million years ago). It lived in what is now the western United States. It was formerly assigned to the genus Laosaurus. It is now named in honor of famed paleontologist Othniel Charles Marsh.

This genus was coined to contain the fossils formerly included in Othnielia, which is based on remains that may be too sparse to hold a name. Decades have been spent on research to untangle the taxonomy left behind by Marsh and his rival Edward Drinker Cope. Othnielosaurus has been often classified as a hypsilophodont, but recent research calls this into question.

Several other specimens have been named by O.C. Marsh during the late 1800s that have come to be known as hypsilophodont-like animals, including Nanosaurus, Laosaurus, and a number of species grouped within them. Many of his findings have been renamed by newer research that shows his taxonomy was incorrect. Peter Galton and Jim Jensen reviewed many fossils and renamed them from the more accurate findings.

Othnielosaurus is known from multiple fossils from many different discoveries. The skull is still poorly understood based on lack of good material. It was a small dinosaur, less than 6.6 feet in length and weighing no more than 22 pounds. It was bipedal, had short forelimbs and long hindlimbs. The hands were short and broad with short fingers.

Based on a partial skull form the type specimen and one other specimen, the head would have been small. It had small, leaf-shaped cheek teeth and premaxillary teeth with less ornamentation. Othnielosaurus had thin plates lying along the ribs. The plates, called intercostal plates, were made up of a cartilage-like substance.

The Morrison Formation, in which Othnielosaurus lived, was semiarid with distinct wet and dry seasons, and flat floodplains. Vegetation was varied, from gallery river forests of conifers and tree ferns, to open fern savannas with limited trees. Fossil material of many different plants and animals have been unearthed in the formation: bivalves, snails, fish, frogs, salamanders, lizards, crocodylomorphs, and several species of dinosaurs. Early mammals such as docodonts, multituberculates, and triconodonts were also found.

The Morrison Formation has been home to numerous theropods including
Ceratosaurus, Allosaurus, Ornitholestes, and Torvosaurus, the sauropods Apatosaurus, Brachiosaurus, Camarasaurus, and Diplodocus. The ornithischians Camptosaurus, Dryosaurus, and Stegosaurus are also known form the Morrison.

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