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 003 the higher-level classifications will vary as new finds are made.

Plants

Gnetales

Arthropods

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.Foster, J. (2007). "Pycnodontoidea." ''Jurassic West: The Dinosaurs of the Morrison Formation and Their World''. Indiana University Press. p. 135. 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.Foster, J. (2007). "Appendix." ''Jurassic West: The Dinosaurs of the Morrison Formation and Their World''. Indiana University Press. pp. 327-329. Found at a fossil site not far from Cañon City, Colorado. Remains usually in a state of rather complete preservation.

Amphibians

Frogs are known from several sites in the Morrison Formation, but are not particularly well represented.Foster, J. (2007). "Anura (Frogs)." pp. 135-136. The history of Morrison anuran discoveries began with the recovery of remains from 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 based on additional fragmentary remains. Despite the erection of multiple new names, only two frog species are currently recognised from the Morrison: ''Enneabatrachus hechti'' and ''Rhadinosteus parvus.''

In addition to formally named taxa, indeterminate anuran remains have been recovered from Morrison strata in Colorado, Wyoming, and Utah, with the best specimens found in Dinosaur National Monument and Quarry 9. Stratigraphically speaking, indeterminate anurans have been found in stratigraphic zones 2 and 4. Indeterminate anurans with remains diagnostic down to the family level have also been reported from the Morrison, with pelobatids being represented by the ilium of an unnamed and indeterminate species, which was recovered from Quarry 9.Foster, J. (2007). "Pelobatidae indet." p. 137. Pelobatids are present in stratigraphic zones 5 and 6.

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

Sphenodonts

Lizards and snakes

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''.

Choristoderes

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''.

Pterosaurs

Pterosaurs are very uncommon fossils in the Morrison, because the fragility of their thin walled bones often prevented their remains from being preserved.Foster, J. (2007). "Soaring Overhead: The Pterosaurs." pp. 157-158. Despite being uncommon they are geographically widespread;Foster, J. (2007). "''Laopteryx priscus''." p. 160. 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, or even foraged for prey, and actively hunted; they are fairly ecologically diverse, ranging from small hawking insectivore '' Mesadactylus'' to the raptorial '' Harpactognathus''. While relatively few pterosaur genera are named from the Morrison Formation, fragmentary material that is not referrable to the genus level suggests the presence of dsungaripteroids, ctenochasmatids, dimorphodontids, and more tentatively wukongopterids and pteranodontians.Jensen, J. A., and Padian, K. (1989) Small pterosaurs and dinosaurs from the Uncompahgre fauna (Brushy Basin Member, Morrison Formation: ?Tithonian), late Jurassic, western Colorado. ''Journal of Paleontology'' 63:363–374.Sprague, M. & McLain, M. A. (2018). Resolving the ''Mesadactylus'' Complex of Dry Mesa Quarry, Morrison Formation, Colorado. ''Journal of Vertebrate Paleontology'', Program and Abstracts, 2018, p. 220.Stephen Czerkas; Tracy Ford (2018). "Pterosaur or diapsid? The search for the true ''Utahdactylus''". ''Flugsaurier 2018: The 6th International Symposium on Pterosaurs. Los Angeles, USA. Abstracts'': 35–36.

Dinosaurs

Mammaliaforms

Many types of mammaliaform cynodonts, mostly early mammals, are known from the Morrison; almost all of them were small sized animals, though occupying a very large variety of ecological niches, from the more rodent-like multituberculates to the carnivorous eutriconodonts (including the possibly volant '' Triconolestes'') to the anteater-like ''Fruitafossor''. 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'', ''Glirodon grandis'', ''Morrisonodon brentbaatar'', '' Psalodon fortis'', ''?P. marshi'', ''P. potens'', and '' Zofiabaatar pulcher''. Triconodonts present included '' Amphidon superstes'', ''Aploconodon comoensis'', '' Conodon gidleyi'', '' Priacodon ferox'', ''P. fruitaensis'', ''P. gradaevus'', ''P. lulli'', ''P. robustus'', '' Triconolestes curvicuspis'', and '' Trioracodon bisulcus''.

Tinodontids were represented by '' Eurylambia aequicrurius'' (probably ''Tinodon''), and '' Tinodon bellus'' (including ''T. lepidus''). Finally, two families of Dryolestoidea were present: Paurodontidae, including '' Comotherium richi'', '' Euthlastus cordiformis'', ''Paurodon valens'', 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 mammaliaform 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.Foster, J.R. 2009. Preliminary body mass estimates for mammalian genera of the Morrison Formation (Upper Jurassic, North America). ''PaleoBios'' 28(3):114-122.

Tinodontids

Eutriconodonts

Multituberculates

Others

Dryolestoids

See also

* List of dinosaur-bearing rock formations
* Pycnodontoidea

Molluscs

Synthesis

lacustrine carbonates

Footnotes

Bibliography

* Foster, J. (2007). ''Jurassic West: The Dinosaurs of the Morrison Formation and Their World.'' Indiana University Press. 389pp.
* Lockley, M.; Harris, J.D.; and Mitchell, L. 2008. "A global overview of pterosaur ichnology: tracksite distribution in space and time." ''Zitteliana''. B28. p. 187-198. {{ISSN, 1612-4138

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