Morrison Formation



One of the most extensive and well-studied geological formations in the world, the Morrison Formation has large exposures in Colorado, Wyoming and Utah, as well as less extensive exposures in Idaho, Montana, North and South Dakota, Nebraska, Kansas, Oklahoma, Texas, New Mexico, and Arizona. The fossil fauna of the Morrison Formation first became famous in the late 19th Century during the Bone Wars and makes up the classical Late Jurassic American dinosaur fauna.

The Setting

The Morrison Formation is made up of sandstones and mudstones deposited by rivers and streams crossing an extensive plain. The rivers of the Morrison generally flowed from west to east, with headwaters in the highlands of present-day Nevada. The northern part of the Morrison Formation met the Sundance Sea, a Late Jurassic extension of the Arctic Ocean. To the south, the environment became increasingly arid and records extensive sand dune deposits. The oldest rocks of the Morrison Formation were deposited about 155 million years ago, and deposition continued for another ten million years.


Because pterosaur bones are thin-walled and delicate, they are rarely preserved in the river and flood-plain deposits of the Morrison Formation, although that does not mean they are absent. There are numerous indeterminate pterosaur remains, as well as three valid species. Kepodactylus insperatus is a pterodactyloid with a wingspan of 2.5 meters, and may belong to the dsungaripterid family. Harpactoganthus gentryii is a rhamphorhynchid and also had a wingspan of 2.5 meters. Mesadactylus ornithosphyos is known solely from some hip vertebrae, but represents an anurognathid.


The enormous long-necked herbivorous dinosaurs were represented by a huge number of species. The very long-necked and whip-tailed diplodocoids were represented by the dicraeosaurids Dyslocosaurus polyonychus and Suuwasea emileae, and the diplodocids Amphicoelias altus, Apatosaurus ajax, and A. louisae, Brontosaurus excelsus, B. parvus, and B. yahnahpin, Barosaurus lentus, Diplodocus carnegii, and D. hallorum, Galeamopus hayi, Kaatedocus siberi, and Supersaurus vivianae. The tall shouldered macronarians were represented by Brachiosaurus altithorax, Camarasaurus grandis, C. lentus, C. supremus, and Cathetosaurus lewisi. Two species of Haplocanthosaurus, H. delfsi and H. priscus were also present.


The bipedal carnivorous dinosaurs were also well represented in the Morrison Formation. The carnosaur Allosaurus fragilis is very common throughout the Morrison Formation. The carnosaurs Epanterias amplexus and Saurophaganax maximus are also present but much rarer. The large crested theropod Ceratosaurus is known from three species, C. nasicornis, C. magnicornis, and C. dentisulcatus. The long-snouted megalosaurids were represented by Torvosaurus tanneri and Marshosaurus bicentesimus. Numerous coelurosaurs are found in the Morrison Formation, although they are overall rare fossils. They include Coelurus fragilis, Ornitholestes hermanni, Stokesosaurus clevelandi, and Tanycolagreus topwilsoni.


The beaked bird-hipped dinosaurs are also well represented in rocks of the Morrison Formation and include a wide variety of animals. Basal ornithischians are represented by Drinker nisti, Fruitadens haagarorum, and Othnielosaurus consors. The ornithopods Camptosaurus dispar and Dryosaurus altus, are known from many specimens; Uteodon aphanoecetes is only known from one. A large number of thyreophorans are known from the Morrison Formation including the ankylosaurs Gargoyleosaurus parkpinorum and Mymoorpelta maysi, and the stegosaurs Alcovasaurus longispinus, Hesperosaurus mjosi, Stegosaurus armatus, S. stenops, an S. ungulatus.