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.

Plants

The Morrison Formation is devoid of flowering plants; during the Jurassic, the Earth was dominated by ferns, seed ferns, and a wealth of gymnosperms—such as cycads, ginkgos, and conifers. Vegetation varied from river-lining forests of conifers, tree ferns, and ferns, to fern savannas with occasional trees such as the Araucaria-like conifer Brachyphyllum.

Invertebrates

Arthropods

Trace fossils reported from the Morrison Formation are indicative of greater diversity of arthropods than recorded by body fossils. Bone modifications likely produced by dermestid beetles were reported in vertebrate fossils from Wyoming and Colorado.

Name	Species	Locality	Material	Notes
Cambaridae	Indeterminate			A crayfish
Coleoptera	Indeterminate	Utah	Isolated elytra.
Decapoda		Utah	A specimen preserved with structures similar to eyes on stalks, antennae and a cheliped.	Possibly a freshwater hermit crab.
Eopolis	E. ekdalei	Utah, Brushy Basin member	Nests produced by social insects, possibly termites.
Morrisonnepa	M. jurassica	Utah		A hemipteran belonging to the group Nepomorpha.
Parapleurites	P. morrisonensis	Colorado	a forewing.	A Locustopsid orthopteran
Spinicaudata		Utah	At least 10 specimens preserved as shells.
Tektonargus	T. kollaspilus	Colorado, Brushy Basin member	Five specimens were reported in the original description of the ichnogenus.

Vertebrates

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. Single tooth fossil of pycnodont fish is represented from Dinosaur National Monument in Utah, found in stratigraphic zone 4. 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.

Name	Species	Locality	Material	Notes	Images
Ceratodus	C. fossanovum			A lungfish genus whose members ranged from 1 to 2 m in length and weights of up to 79 pounds, with most Morrison lungfish being on the smaller end of that range. These species are believed to have had similar diets to extant lungfish like the physically similar modern genus Neoceratodus.
Ceratodus	C. ?frazieri			A lungfish genus whose members ranged from 1 to 2 m in length and weights of up to 79 pounds, with most Morrison lungfish being on the smaller end of that range. These species are believed to have had similar diets to extant lungfish like the physically similar modern genus Neoceratodus.
Ceratodus	C. guentheri			A lungfish genus whose members ranged from 1 to 2 m in length and weights of up to 79 pounds, with most Morrison lungfish being on the smaller end of that range. These species are believed to have had similar diets to extant lungfish like the physically similar modern genus Neoceratodus.
Ceratodus	C. robustus			A lungfish genus whose members ranged from 1 to 2 m in length and weights of up to 79 pounds, with most Morrison lungfish being on the smaller end of that range. These species are believed to have had similar diets to extant lungfish like the physically similar modern genus Neoceratodus.
Ceratodus	Indeterminate.	Brushy Basin and Saltwash members		Represented by tooth plates.
"Hulettia"	"H. hawesi"	Colorado		A small fish, originally considered as a member of Halecostomi about 7.6 cm in length and 5g of live mass which probably preferred quiet water. Its fossils prominently preserve its thick interlocking scales. Later study question its assignment to genus Hulettia, and suggested that it would be a semionotid instead.
cf. Leptolepis	N/A	Colorado	Known only from a single nearly complete skeleton found at Rabbit Valley. Found in stratigraphic zone 5.	A 13 cm fish that was deeper bodied than its co-occurring contemporaries Morrolepis and "Hulettia". The Morrison cf. Leptolepis probably had a live mass of about 37g. It is the only teleost fish known from the formation and was morphologically more highly derived than other Morrison fish. It is believed to have fed on contemporary fish and small invertebrates.
Morrolepis	M. schaefferi	Colorado		A coccolepidid "palaeoniscoid" with forward-set eyes positioned past the front end of the lower jaw. It had a tall dorsal fin set far back on the body and an asymmetrical caudal fin. Adult specimens would reach about 20 cm in length and 113 g in mass.
Potamoceratodus	P. guentheri	Colorado		Once thought to be a species of Ceratodus.
Purgotybaris	P. fosteri	Utah		A regurgitalite containing frog and salamander bones, probably produced by an amioid fish.
A hybodont?	Indeterminate	Utah		Known from a dorsal spine.	-

Amphibians

Frogs are known from several sites in the Morrison Formation, but are not particularly well represented. 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. 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.

Name	Species	Locality	Material	Notes
Comobatrachus	C. aenigmaticus	Wyoming, Brushy Basin member	A partial right humerus.	A dubious genus of prehistoric frog erected by O. C. Marsh to house fragmentary remains recovered from Reed's Quarry 9 near Como Bluff Wyoming. Along with Eobatrachus it was among the earliest frog remains from the formation, although the two dubious genera were erected decades apart.
Comonecturoides	C. marshi	Wyoming, Brushy Basin member	Represented by a single femur.	Considered a nomen dubium because the name is based on non-distinctive remains which cannot be classified in detail.
Enneabatrachus	E. hechti	Utah and Wyoming		A small discoglossid frog whose live weight would have only been a few grams.
Eobatrachus	E. agilis	Wyoming, Brushy Basin member	A partial right humerus.	A dubious genus of prehistoric frog erected by O. C. Marsh to house fragmentary remains recovered from Reed's Quarry 9 near Como Bluff Wyoming. Along with Comobatrachus it was among the earliest frog remains from the formation, although the two dubious genera were erected decades apart.
Iridotriton	I. hechti	Utah, Brushy Basin	A partial skull and incomplete postcranial skeleton.	A basal salamandroid closely related to today's advanced salamanders.
Rhadinosteus	R. parvus	Utah	Known from several slabs of rock which contain multiple partial specimens in association.	A pipoid and possible rhinophrynid, Rhadinosteus parvus was only about 42 mm long in life.

Sphenodonts

Name	Species	Locality	Material	Notes	Images
Eilenodon	E. robustus	Colorado	Several fragmentary skull, mandible, and tooth elements.	A herbivorous eilenodontine of relatively large size.
Opisthias	O. rarus	Colorado, Utah and Wyoming, Brushy Basin member	Several skulls and partial skeletons.	An opisthodontian sphenodontian
Opisthiamimus	O. gregori	Northern Wyoming, possibly west-central Colorado. Brushy Basin Member	Most of the skull and postcranium	A small-bodied eusphenodontian, and one of the most complete rhynchocephalian taxa yet known from North America.
Theretairus	T. antiquus	Wyoming, Brushy Basin member	A mandible.	A small sphenodontian.
Sphenodontia'	Indeterminate	Dinosaur National Monument	A crushed partial skull	The specimen is undescribed. Although previously considered to have belonged to Opisthias or Theretairus, recent studies have doubted this referral, thus placing it as an indeterminate sphenodont.'

Squamates

Numerous squamate remains have been found in the sediments of the Morrison Formation, most commonly at sites in Utah, Colorado and Wyoming. A number of taxa have been described, coming from three different groups: anguimorph and scincomorph lizards and early snakes. The first squamates to be reported from the Morrison Formation were Paramacellodus and Dorsetisaurus, which were described from Wyoming's Quarry 9 by Don Prothero and Richard Estes. Later remains would include Diablophis, originally described as a species of Parviraptor by Susan Evans in 1996 but subsequently moved to the new genus Diablophis by Michael Caldwell et al. in 2015, with extra material also being reported from Utah's Cisco Mammal Quarry, and Schillerosaurus, originally described as "Schilleria" and reported from Dinosaur National Monument by Evans and Dan Chure in 1999. Two later additions to the Morrison's squamate assemblages are Eoscincus and Microteras, two scincomorph lizards found at Dinosaur National Monument and Como Bluff's Quarry 9, respectively. They were described by Chase Brownstein et al. in 2022. Indeterminate squamate remains have currently been described from Dinosaur National Monument.
The majority of modern-day scincomorph lizards are small insectivores that feed on a range of invertebrates. It is thought that their counterparts from the Morrison Formation would have occupied a similar niche due to their morphological similarities. Anguimorph lizards most likely hunted small vertebrates, and Diablophis is thought to have done so too. Prey items would have included the other squamates from the formation as well as its large diversity of small mammals. All squamates might have been prey for the larger predators of the Morrison Formation, including the abundant theropod dinosaurs and crocodilians.

Name	Species	Locality	Material	Notes
Diablophis	D. gilmorei	Fruita Palaeontological Area, Colorado and Cisco Mammal Quarry, Utah	Broken skull material including a right maxilla, mandible and dentary. Broken axis vertebrae, precloacal vertebrae, one caudal vertebra and a possible sacral vertebra have also been found.	A basal snake. Originally described as a species of Parviraptor, it was subsequently moved to its own genus.
Dorsetisaurus	D. sp.	Quarry 9 at Como Bluff, Wyoming, Utah and Colorado	Multiple dentaries.	An anguimorph lizard.
Eoscincus	E. ornatus	Site 412 at Dinosaur National Monument, Utah	A partial skull including a complete rostrum and palate, a partial skull roof, and both mandibles.	A scincomorph lizard.
Helioscopos	H. dickersonae	Site 317 at Dinosaur National Monument, Utah	A nearly complete skull consisting of a partial cranium and mandibles, lacking the anterior snout.	A stem-gecko.
Microteras	M. borealis	Quarry 9 at Como Bluff, Wyoming	A partial skull consisting of an associated maxilla and braincase.	A scincomorph lizard.
Paramacellodus	P. sp.	Fruita Palaeontological Area & Rainbow Park, Colorado and Quarry 9 at Como Bluff, Wyoming	Numerous specimens including jaws, skulls, osteoscutes, trunk vertebrae and hindlimbs. Possibly a left prefrontal and postcranial material consisting of a scapulocoracoid, trunk vertebra and multiple articulated bones consisting of a partial pelvis, seven caudal vertebrae and a nearly complete left hind limb.	A small scincomorph lizard with blunt teeth.
Saurillodon	S. sp	Fruita Palaeontological Area, Colorado		A scincomorph lizard whose remains have been found in Middle Jurassic strata in England and Scotland as well as Late Jurassic strata in Portugal in addition to the Morrison formation remains.
Schillerosaurus	S. utahensis	Dinosaur National Monument, Utah	Part and counterpart of a partial dissociated skeleton and a few limb bone fragments.	A small scincomorph lizard of otherwise uncertain evolutionary affinities.
Squamata	Indeterminate	Dinosaur National Monument, Utah	Multiple specimens including fragmentary and disarticulated skeletons.	Squamates of uncertain affinities. Noted as distinct from Paramacellodus and Dorsetisaurus, of which remains have been found in the same quarry.

Turtles

Turtles are very common fossils in the Morrison, due to their bony shells.
The most common were Glyptops plicatus and Dinochelys whitei. Also present were Dorsetochelys buzzops and Uluops uluops.

Name	Species	Locality	Material	Notes
Chelonipus		Colorado and Utah, Salt Wash member
Compsemys	C. plicatulus	Colorado
Dinochelys	D. whitei	Colorado, Utah and Wyoming, Brushy Basin member	Several shells and some postcranial material.
Dorsetochelys	D. buzzops
Glyptops	G. plicatulus	Colorado and Wyoming, Brushy Basin member	Several shells, skulls, and partial skeletons.
Glyptops	G. ornatus	Wyoming, Brushy Basin member	A skull.	Synonym of Glyptops plicatulus.
Glyptops	G. utahensis	Utah, Brushy Basin member	A complete shell.	Synonym of Glyptops plicatulus.
Uluops	U. uluops	Wyoming, Brushy Basin member	A cranium.	Most basal Pleurosternid from the Morrison Formation.

Choristoderes

Name	Species	Locality	Material	Notes
Cteniogenys	C. antiquus	Colorado, Oklahoma, South Dakota, Utah and Wyoming		A choristoderan about 25 to 50 cm in length.

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.

Name	Species	Locality	Material	Notes	Images
Amphicotylus	A. gilmorei	Wyoming
Amphicotylus	A. lucasii	Colorado
Amphicotylus	A. milesi	Wyoming, Brushy Basin member	A nearly complete skeleton.
Amphicotylus	A. stovalli	Oklahoma
Diplosaurus	D. felix	Colorado
Eutretauranosuchus	E. delfsi	Colorado and Wyoming
Fruitachampsa	F. callisoni	Colorado, Brushy Basin and Saltwash members
Hallopus	H. victor	Colorado, Brushy Basin member	A partial skeleton including a fragmentary skull roof.
Hatcherichnus	H. sanjuanensis	Colorado and Utah
Hatcherichnus	H. sp.	Utah and Wyoming, Salt Wash member
Hoplosuchus	H. kayi	Arizona and Utah
Macelognathus	M. vagans	Colorado and Wyoming, Brushy Basin member	A partial left mandible and several fragmentary referred fossils.
Theriosuchus	T. morrisonensis	Wyoming	A nearly complete left mandible missing teeth.

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 and pterodactyloids. 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.

Name	Species	Locality	Material	Notes	Images
Comodactylus	C. ostromi	Wyoming, Brushy Basin member	A metacarpal.	Nomen dubium
Dermodactylus	D. montanus	Wyoming, Brushy Basin member	A metacarpal.	Nomen dubium
Harpactognathus	H. gentryii	Wyoming, Brushy Basin member	A partial snout.	A large rhamphorhynchid with a wingspan of about 2.5 m and live mass of about 1.5 kg. Harpactognathus was related to the Solnhofen genus Scaphognathus.
Kepodactylus	K. insperatus	Colorado, Brushy Basin member	A partial postcranial skeleton.	A large pterodactyloid with a 2.5 m wingspan and a live weight of about 1.5 kg. Kepodactylus may be related to the Asian dsungaripteroid pterosaurs.
Laopteryx	L. priscus	Wyoming, Brushy Basin member	A braincase.	Nomen dubium initially misidentified as a bird.
Mesadactylus	M. ornithosphyos	Colorado, Brushy Basin member	A synsacrum.	Several specimens have been incorrectly referred to Mesadactylus.
Pteraichnus	P. saltwashensis*	Arizona and Oklahoma, Saltwash member
Utahdactylus	U. kateae	Utah, Tidwell member	A fragmentary skeleton.	Previously thought to be an indeterminite diapsid, newer material suggests an affinity with ctenochasmatids.

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 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, Comodon gidleyi, Priacodon ferox, P. fruitaensis, P. gradaevus, P. lulli, P. robustus, Triconolestes curvicuspis, and Trioracodon bisulcus.
Tinodontids were represented by Eurylambia aequicrurius, and Tinodon bellus. 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, 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.