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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.
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 Sundance Sea was rich in 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.
- Kuehn, Steve. "Geology of the Mesozoic Era: 245 to 66 million years ago" (PDF). Department of Physics, Physical Sciences, and Geology at California State University, Stanislaus. Archived from the original on 2006-09-14. http://web.archive.org/web/20060914124516/http://geology.csustan.edu/kuehn/Geol2200/Mesozoic2_4.pdf. Retrieved 2007-02-06.
- "Mesozoic Stratigraphy in the Thermopolis Area". Big Horn Basin Foundation. http://server1.thermopwy.net/bhbf/messtrat.html. Retrieved 2007-02-06.