The Morrison Formation is a distinctive sequence of Upper Jurassic sedimentary rock that is found in the western United States, which has been the most fertile source of dinosaur fossils in North America. It is composed of mudstone, sandstone, siltstone and limestone and is light grey, greenish gray, or red. Most of the fossils occur in the green siltstone beds and lower sandstones, relics of the rivers and floodplains of the Jurassic period.

Fauna comparisons

The fauna of Morrison Formation is similar to one in the coeval rocks of Tendaguru Beds (in Tanzania) and Lourinhã Formation in Portugal, mostly with the second. Some genera are shared in Morrison and Lourinhã, such as Torvosaurus, Ceratosaurus, Stegosaurus, Dryosaurus, and Allosaurus. In sum, Morrison Fm has 37 valid genera of dinosaurs.

Ornithischians

The herbivorous ornithischian dinosaurs were diverse but not as common as sauropods in the Morrison. Fruitadens, previously known as the "Fruita Echinodon", was found to be a heterodontosaurid. Plate-backed stegosaurids included Hesperosaurus mjosi, Stegosaurus ungulatus, S. stenops, and Alcovasaurus longispinus. Armored dinosaurs that weren't stegosaurs were unknown in the formation until the 1990s. Two have been named: Gargoyleosaurus parkpinorum and Mymoorapelta maysi. Ornithopods, bipedal herbivores, came in several types. Small "hypsilophodonts" included Drinker nisti, Laosaurus celer, "L." gracilis, Nanosaurus agilis, Othnielia rex, and Othnielosaurus consors (all of which are now synonymous with Nanosaurus). Larger but similar-looking dryosaurids were represented by two species of Dryosaurus (D. altus and D. elderae) and the camptosaurid Uteodon aphanoecetes, which is currently known only from Dinosaur National Monument. Still larger was the more common Camptosaurus dispar. Dryosaurids and camptosaurids were early iguanodonts, a group that would later spawn the duck-billed dinosaurs.

Neornithischians

Thyreophorans

Ankylosaurs

Stegosaurs

Other Ornithischians

Sauropods

Sauropods, the giant long-necked long-tailed four-legged herbivorous dinosaurs, are among the most common and famous Morrison fossils. A few have uncertain relationships, like "Apatosaurus" minimus (possibly a basal titanosauriform) and Haplocanthosaurus. Sauropods including Haplocanthosaurus priscus, H. delfsi, and the diplodocid Brontosaurus yahnahpin appeared in the early stages of the Morrison. The middle stages were dominated by familiar forms such as the giraffe-like Brachiosaurus altithorax, which were uncommon, but related camarasaurids, like Camarasaurus supremus, C. grandis, C. lentus, and Cathetosaurus/Camarasaurus lewisi, were very common. Also common were long, low diplodocids, like Apatosaurus ajax, A. louisae, Brontosaurus excelsus, B. parvus, Barosaurus lentus, Diplodocus longus, D. carnegii, Galeamopus and Dyslocosaurus polyonychius. By the late Morrison, gigantic diplodocids (or similar species) had appeared, including Diplodocus hallorum (formerly Seismosaurus), Supersaurus vivianae, Amphicoelias altus, and M. fragilimus. Smaller sauropods, such as Suuwassea emiliae from Montana, tend to be found in the northern reaches of the Morrison, near the shores of the ancient Sundance Sea, suggesting ecological niches favoring smaller body size there compared with the giants found further south.

Haplocanthosaurids

Rebbachisaurids

Dicraeosaurids

Diplodocids

Macronarians

Theropods

Theropod dinosaurs, the carnivorous dinosaurs, came in several different types. The less derived types, the ceratosaurs and megalosaurids, included Ceratosaurus nasicornis, C. dentisulcatus, C. magnicornis, and the megalosaur Torvosaurus tanneri (including Edmarka rex). Allosaurids included the common Allosaurus fragilis (including Epanterias amplexus), A. jimmadseni, and the giant Saurophaganax maximus (potentially included in Allosaurus). Indeterminate theropod remains have been recovered in Utah, with indeterminate ceratosaur remains formerly considered referable to Elaphrosaurus recovered in Colorado. Indeterminate theropod tracks have been recovered from both Utah and Arizona.

Allosauroidea

Ceratosaurs

Coelurosaurs

Coelurosaurs, the group of theropods most closely related to and including birds, included Coelurus fragilis, Ornitholestes hermanni, Tanycolagreus topwilsoni, the possible troodontid Koparion douglassi, the definite troodontid Hesperornithoides, and the early tyrannosauroid Stokesosaurus clevelandi.

Megalosauroids

Eggs

Dinosaur eggs have been found in Utah.

Tracks

Ornithopods

Morrison ornithopod trace fossils are represented by three toed tracks which are generally small. The toes of Morrison ornithopod tracks are usually more widely splayed than the theropod tracks preserved in the formation.

Stegosaurs

Stegosaur tracks were first recognized in 1996 from a hindprint-only trackway discovered at the Cleveland-Lloyd quarry, which is located near Price, Utah. Two years later, a new ichnogenus called Stegopodus was erected for another set of stegosaur tracks which were found near Arches National Park, also in Utah. Unlike the first, this trackway preserved traces of the forefeet. Fossil remains indicate that stegosaurs have five digits on the forefeet and three weight-bearing digits on the hind feet. From this, scientists were able to successfully predict the appearance of stegosaur tracks in 1990, six years in advance of the first actual discovery of Morrison stegosaur tracks. Since the erection of Stegopodus, more trackways have been found, however none have preserved traces of the front feet, and stegosaur traces remain rare.

Theropods

Indeterminate theropod tracks have been recovered from both Utah and Arizona.

Footnotes