Paleobiota of the Morrison Formation

The Morrison Formation is a distinctive sequence of Late Jurassic sedimentary rock that is found in the western United States, which has a wide assortment of taxa represented in its fossil record, including 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.

(mostly from Foster [2003], with additional material in the mammal section after [1]; the higher-level classifications will vary as new finds are made.

Amphibians
According to museum curator John Foster, "frogs are known from several sites in the Morrison Formation but are not particularly well represented."[1] The history of Morrison anuran discoveries began with the recovery of remains from Reed's Quarry 9 near Como Bluff Wyoming. The new genus Eobatrachus was erected for some of these remains by O. C. Marsh, but the material was later considered non-diagnostic. Decades later another dubious anuran genus, Comobatrachus was erected for addition fragmentary remains. Despite the erection of multiple new names, scientists only recognize two legitimate frog species from the Morrison, Enneabatrachus hechti[2] and Rhadinosteus parvus.[3]

In addition to formally named taxa, indeterminate anuran remains have been retrieved from Morrison strata in Colorado, Wyoming, and Utah, with the best specimens found in Dinosaur National Monument and Quarry 9.[1] Stratigraphically speaking, indeterminate anurans have been found in stratigraphic zones 2 and 4.[4] Indeterminate anurans with remains diagnostic down to the family level have also been reported from the Morrison. Pelobatids are represented by the illium of an unnamed, indeterminate species.[5] A specimen has been recovered from Quarry 9 of Como Bluff in Wyoming.[5] Pelobatids are present in stratigraphic zones 5 and 6.[4]

Indeterminate salamander remains are present in stratigraphic zones 2, 4, and 5.[4] A distinctive type of salamander known only as Caudata B is present in stratigraphic zone 6.

Crurotarsans
Crocodiles of a variety of sizes and habitats were common Morrison animals. Cursorial mesosuchians, or small terrestrial running crocs, included Hallopus victor and Fruitachampsa callisoni. More derived crocodilians included Diplosaurus ferox, Amphicotylus, Hoplosuchus kayi, and Macelognathus vagans''.

Fish
Although the paleoclimate of the Morrison formation was semiarid with only seasonal rainfall, there were enough bodies of water to support a diverse ichthyofauna. Although abundant, fish remains are constrained to only certain locations within the formation. Microvertebrate sites in Wyoming are dominated by fish remains. Indeterminate ray-finned fish remains have been recovered from Ninemile Hill and a microvertebrate site in the Black Hills. Found in stratigraphic zones 2, 4, and 5. Morrison actinopterygians generally have no close modern relatives. The Wyoming microvertebrate remains are extracted from the sediment by screenwashing. Paleoniscoid remains are geographically present in the western part of Colorado, where remains have been recovered from "a level above the Mygatt-Moore Quarry." Largely complete remains of small individuals have been consistently recovered for over 15 years. So far, Morrison pycnodontoids are represented by a single specimen from Dinosaur National Monument in Utah. Found in stratigraphic zone 4. Only a single specimen from Dinosaur National Monument in Utah has been recovered. Pycnodontoids were "deep-bodied and laterally compressed fish" whose tooth morphology suggest that they preyed on small contemporary invertebrates. They may have resembled modern butterfly fish. A single tooth is the only known remains. Dipnoan remains found at a fossil site not far from Cañon City, Colorado. Remains usually in a state of rather complete preservation. Halecostome remains are geographically present in the western part of Colorado, where remains have been recovered from "a level above the Mygatt-Moore Quarry." Largely complete remains of small individuals have been consistently recovered for over 15 years. Amiid remains found in stratigraphic zones 2, 3, and 4. Found at a fossil site not far from Cañon City, Colorado. Remains usually in a state of rather complete preservation.

Mammals
Many types of early mammals are known from the Morrison; almost all of them were small, rodent-like animals. Unclassified types include the digger Fruitafossor windscheffelia. Docodonts included the common genus Docodon, represented by D. victor, D. striatus, and D. superbus, and Peraiocynodon sp. Multituberculates, a common type of early mammal, were represented by Ctenacodon serratus, C. laticeps, C. scindens, "C." brentbaatar, Glirodon grandis, Psalodon fortis, ''?P. marshi, P. potens, and Zofiabaatar pulcher''. Triconodonts present included Aploconodon comoensis, Conodon gidleyi (AKA Phascolodon and Phascolotheridium), Priacodon ferox, P. fruitaensis, P. gradaevus, P. lulli, P. robustus, Triconolestes curvicuspis, and Trioracodon bisulcus.

Symmetrodonts were represented by Amphidon superstes, Eurylambia aequicrurius (probably Tinodon), and Tinodon bellus (including T. lepidus). Finally, two families of Dryolestoidea were present: Paurodontidae, including Araeodon intermissus, Archaeotrigon brevimaxillus, A. distgamus, Comotherium richi, Euthlastus cordiformis, Foxraptor atrox, Paurodon valens, Pelicopsis dubius, and Tathiodon agilis; and Dryolestidae, including Amblotherium gracilis, Dryolestes obtusus (common genus), D. priscus, D. vorax, Laolestes eminens, L. grandis, and Miccylotyrans minimus.

In 2009, a study by J. R. Foster was published which estimated the body masses of mammals from the Morrison Formation by using the ratio of dentary length to body mass of modern marsupials as a reference. Foster concludes that Docodon was the most massive mammal genus of the formation at 141g and Fruitafossor was the least massive at 6g. The average Morrison mammal had a mass of 48.5g. A graph of the body mass distribution of Morrison mammal genera produced a right-skewed curve, meaning that there were more low-mass genera.

Pterosaurs
Pterosaurs are very uncommon fossils in the Morrison, because the fragility of their thin walled bones often prevented their remains from being preserved. Despite being uncommon they are geographically widespread; indeterminate pterosaur remains have been found in stratigraphic zones 2 and 4-6. In addition to indeterminate remains, several species have been identified from both the rhamphorhynchoids (long-tailed pterosaurs) and pterodactyloids (short-tailed pterosaurs). Since the 1970s and 80s, pterosaur finds have become more common, but are still rare. Most Morrison pterosaurs have been found in marine and shoreline deposits. Pterosaur tracks have been found in both the Tidwell and Saltwash members. Morrison pterosaurs probably lived on fish, insects and scavenged dinosaur carcasses.

Turtles
Turtles (Testudines) are very common fossils in the Morrison, due to their bony shells. The most common were Glyptops plicatus (very common) and Dinochelys whitei (also common, but not as common as Glyptops). Also present were Dorsetochelys buzzops and Uluops uluops.