The Sundance Formation is a western

North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...

n sequence of

Middle Jurassic The Middle Jurassic is the second Epoch (geology), epoch of the Jurassic Period (geology), Period. It lasted from about 174.1 to 161.5 million years ago. Fossils of land-dwelling animals, such as dinosaurs, from the Middle Jurassic are relativel ...

Upper Jurassic The Late Jurassic is the third epoch of the Jurassic Period, and it spans the geologic time from 161.5 ± 1.0 to 143.1 ± 0.8 million years ago (Ma), which is preserved in Upper Jurassic strata.Owen 1987. In European lithostratigraphy, the name ...

age Dating from the

Bathonian In the geologic timescale the Bathonian is an age (geology), age and stage (stratigraphy), stage of the Middle Jurassic. It lasted from approximately 168.2 ±1.2 annum, Ma to around 165.3 ±1.1 Ma (million years ago). The Bathonian Age succeeds ...

to the Oxfordian, around 168-157 Ma, It is up to 100 metres thick and consists of marine

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...

, sandy shale,

sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

, and

limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...

deposited in the Sundance Sea, an

inland sea An inland sea (also known as an epeiric sea or an epicontinental sea) is a continental body of water which is very large in area and is either completely surrounded by dry land (landlocked), or connected to an ocean by a river, strait or " arm of ...

that covered large parts of western North America during the Middle and early Late Jurassic.

Geology

The Sundance Formation underlies the western North American

Morrison Formation The Morrison Formation is a distinctive sequence of Upper Jurassic sedimentary rock 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, siltston ...

, the most fertile source of

dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the #Evolutio ...

fossils in the Americas, and is separated by a

disconformity An unconformity is a buried erosion surface, erosional or non-depositional surface separating two Rock (geology), rock masses or Stratum, strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer ...

from the underlying

Gypsum Springs Formation.

Fossils

The Sundance Formation is known for fossils of an extinct species of marine

cephalopod A cephalopod is any member of the molluscan Taxonomic rank, class Cephalopoda (Greek language, Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral symm ...

, the

belemnite Belemnitida (or belemnites) is an extinct order (biology), order of squid-like cephalopods that existed from the Late Triassic to Late Cretaceous (And possibly the Eocene). Unlike squid, belemnites had an internal skeleton that made up the cone ...

'' Pachyteuthis densus'', as well as several extinct species of

oyster Oyster is the common name for a number of different families of salt-water bivalve molluscs that live in marine or brackish habitats. In some species, the valves are highly calcified, and many are somewhat irregular in shape. Many, but no ...

, including ''Deltoideum'', '' Liostrea'', and '' Gryphaea nebrascensis''. Other common invertebrates include crinoids, echinoids, gastropods, insects, ostracods, and foraminifera. Fossil dinosaur 'footprints' on an ancient ocean shoreline are preserved in the formation and protected at the Red Gulch Dinosaur Tracksite, located in the

Bureau of Land Management The Bureau of Land Management (BLM) is an agency within the United States Department of the Interior responsible for administering federal lands, U.S. federal lands. Headquartered in Washington, D.C., the BLM oversees more than of land, or one ...

Red Gulch/Alkali National Back Country Byway, near Shell in

Big Horn County, Wyoming Big Horn County is a county in the U.S. state of Wyoming. As of the 2020 United States census, the population was 11,521. The county seat is Basin. Its north boundary abuts the south boundary of Montana. History Big Horn County was created ...

.BLM−Bureau of Land Management, Wyoming Office: "Red Gulch Dinosaur Tracksite" website
info, maps, photo gallery, accessed 8.21.2015

Paleobiota

Vertebrates

Invertebrates

Fish

References

Jurassic geology of South Dakota Jurassic geology of Wyoming Jurassic System of North America Upper Jurassic Series Geology of the Rocky Mountains Geologic formations of Montana Geologic formations of Wyoming Paleontology in Wyoming {{Wyoming-geologic-formation-stub Middle Jurassic Series Bathonian Stage Callovian Stage Oxfordian Stage Shale formations of the United States Limestone formations of the United States Sandstone formations of the United States