During the Early and Middle Jurassic, Utah was mostly desert, although periodically sea level would rise and the sea would invade this sand dune-covered landscape. The great sandstone cliffs of Zion and Arches National Park are basically fossil sand dunes. Although dinosaur footprints are known from many sites in Utah, no dinosaur skeletons have yet been found in these desert rocks. Marine reptiles have been found in marine rocks near Dinosaur National Monument.

Most of Utah’s well-known dinosaur fossils are Late Jurassic dinosaurs. They are found mainly in a rock unit known as the Morrison Formation which dates to approximately 150 million years ago.

Dinosaurs are divided into two major groups, the saurischian, or “lizard-hipped,” dinosaurs and the ornithischian, or “bird-hipped,” dinosaurs.

• The saurischian dinosaurs include the herbivorous (plant eaters), quadrupedal sauropods, and the carnivorous, bipedal theropods.
• The ornithischian dinosaurs are all herbivorous and are composed of many groups, including stegosaurs, ornithopods, and ceratopsians.

Birds evolved from a group of thesaurischian theropods known as coelurosaurs.

Dinosaur fossils from the Morrison Formation can be found in Utah at the Dinosaur National Monument quarry in northeastern Utah and at the Cleveland-Lloyd Dinosaur Quarry in east-central Utah. Allosaurus is Utah’s State Fossil. Allosaurus is a carnivorous theropod found in abundance at the Cleveland-Lloyd Dinosaur Quarry.