Sundance Sea

The Sundance Sea was an inland or epeiric sea covering much of western North America in the Jurassic period. It was an arm of what is now the Arctic Ocean, and extended through what is now western Canada into the central western United States. The sea receded (moved back) when highlands to the west began to rise.
Stratigraphy.
The Sundance Sea did not occur at a single time; evidence suggests that it was a series of five successive marine transgressions. Each was separated by a period of erosion.The sea advanced and receded from the Middle Jurassic onwards. The terrestrial sediments of the Morrison Formation eroded from rising highlands to the west were deposited on top of the calcareous Sundance Formation as the sea retreated for the last time, late in the Jurassic.
The sedimentary rocks which formed in and around the Sundance Sea are often rich in fossils.
Fauna.
The Sundance Sea had many types of animals. The oyster "Gryphaea" was very common, and shark teeth have been found. Fish, belemnites and to an extent ammonites swarmed in shoals. Crinoids and bivalvia dotted the seafloor. "Ophthalmosaurus", a large ichthyosaur, swam in the seas using its large, long jaws to catch belemnite 'squid'. "Pantosaurus", a plesiosaur the size of a seal, went after the easier-to-catch fish. The largest marine reptile in the Sundance Sea was "Megalneusaurus", a large pliosaur similar to "Liopleurodon". Its fossils have been found in Alaska and Wyoming, which were both covered by the Sundance Sea when it was alive.
During the periods of recession, dinosaurs and other Jurassic animals walked on the shores, leaving fossil footprints.