Paleobiota of the Morrison Formation

The Morrison Formation is a special layer of rock from the Late Jurassic period found in the western United States. It is made up of different types of rock like mudstone, sandstone, siltstone, and limestone, and it can be light grey, greenish gray, or red in color. This formation is very important because it contains many fossils, including dinosaur fossils, which help scientists learn about life in North America long ago.

Most of the fossils are found in the green siltstone beds and lower sandstones. These rocks are leftovers from the rivers and floodplains that existed during the Jurassic period. The many different kinds of plants and animals found here give us a rich picture of what the world was like millions of years ago. The Morrison Formation is one of the best places in the world to study the ancient paleobiota of this time, offering clues about the environment and the creatures that lived there.

Plants

The Morrison Formation does not contain flowering plants. During the Jurassic period, the Earth had many plants like ferns, seed ferns, and gymnosperms. These included cycads, ginkgos, and conifers. The plants grew in different places, from forests along rivers made of conifers, tree ferns, and ferns, to open areas with ferns and some trees like the Araucaria-like conifer Brachyphyllum.

Conifers

Ginkgos

Cycadophyta and relatives

Gnetales

Invertebrates

Arthropods

The Morrison Formation has clues that show there were many more arthropods, like insects, than we know from fossils. Scientists found marks on bones that were probably made by special beetles in places like Wyoming and Colorado.

Vertebrates

Fish

Even though the climate of the Morrison Formation was dry with only occasional rain, there were enough water bodies to support many kinds of fish. Fish fossils are mainly found in certain areas, especially in Wyoming. In these places, scientists find many fish bones. Some fish, like ray-finned fish, have been found in places such as Ninemile Hill and the Black Hills. These fish don’t have many modern relatives. In Colorado, other fish fossils have been found, including a single tooth from a pycnodont fish in Dinosaur National Monument in Utah. These fish fossils are often well preserved.

Amphibians

Frogs have been found in several places in the Morrison Formation, but they are not very common. The earliest frog fossils were found near Como Bluff, Wyoming. Over time, scientists have named two main frog species from this area: Enneabatrachus hechti and Rhadinosteus parvus.

Other frog fossils have been found in Colorado, Wyoming, and Utah, with the best specimens in Dinosaur National Monument and Quarry 9. Some salamander fossils have also been found in different layers of rock.

Sphenodonts

Squamates

Many lizard and early snake fossils have been found in the Morrison Formation, mainly in Utah, Colorado, and Wyoming. Some of the first lizards found here were Paramacellodus and Dorsetisaurus, discovered in Wyoming. Later finds included Diablophis, originally thought to be a different species but later named its own, and Schillerosaurus. These lizards likely ate small animals, and may have been eaten by larger predators like dinosaurs.

Turtles

Turtles are common fossils in the Morrison Formation because of their bony shells. The most common turtles were Glyptops plicatus and Dinochelys whitei. Other turtles found include Dorsetochelys buzzops and Uluops uluops.

Choristoderes

Crurotarsans

Crocodiles of different sizes and habitats lived in the Morrison Formation. Small, fast crocs included Hallopus victor and Fruitachampsa callisoni. Larger crocs included Diplosaurus ferox, Amphicotylus, Hoplosuchus kayi, and Macelognathus vagans.

Pterosaurs

Pterosaurs are rare fossils in the Morrison Formation because their thin bones often didn’t preserve well. However, they have been found in different areas. Some pterosaurs were long-tailed, while others were short-tailed. They likely ate fish, insects, and sometimes scavenged from dinosaur remains.

Dinosaurs

Main article: Dinosaurs of the Morrison Formation

Mammaliaforms

Many early mammal-like animals lived in the Morrison Formation. These included small, rodent-like animals, carnivorous animals, and even some that looked like anteaters. Common genera included Docodon, Ctenacodon, and Priacodon, among others. These mammals varied greatly in size, from very small to slightly larger.

Name	Species	Locality	Material	Images
Ceratodus	C. fossanovum
	C. ?frazieri
	C. guentheri
	C. robustus
	Indeterminate.	Brushy Basin and Saltwash members
"Hulettia"	"H. hawesi"	Colorado
cf. Leptolepis	N/A	Colorado	Known only from a single nearly complete skeleton found at Rabbit Valley. Found in stratigraphic zone 5.
Morrolepis	M. schaefferi	Colorado
Potamoceratodus	P. guentheri	Colorado
Purgotybaris	P. fosteri	Utah
A hybodont?	Indeterminate	Utah

Name	Species	Locality	Material	Images
Eilenodon	E. robustus	Colorado	Several fragmentary skull, mandible, and tooth elements.
Opisthias	O. rarus	Colorado, Utah and Wyoming, Brushy Basin member	Several skulls and partial skeletons.
Opisthiamimus	O. gregori	Northern Wyoming, possibly west-central Colorado. Brushy Basin Member	Most of the skull and postcranium
Theretairus	T. antiquus	Wyoming, Brushy Basin member	A mandible.
Sphenodontia	Indeterminate	Dinosaur National Monument	A crushed partial skull (DINO 16454)

Name	Species	Locality	Material	Images
Amphicotylus	A. gilmorei	Wyoming
	A. lucasii	Colorado
	A. milesi	Wyoming, Brushy Basin member	A nearly complete skeleton.
	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
	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 (type) and several fragmentary referred fossils.
Theriosuchus	T. morrisonensis	Wyoming	A nearly complete left mandible missing teeth.

Name	Species	Locality	Images
Cifellilestes	C. ciscoensis	Utah, Brushy Basin Member
Docodon	D. victor
Fruitafossor	F. windscheffeli	Colorado