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The Western Interior Seaway, also called the Cretaceous Seaway and the North American Inland Sea, was a huge inland sea that split the continent of North America into two halves during most of the early and mid-Cretaceous period.
The Seaway was created as the Pacific and North American tectonic plates collided, causing the Rocky Mountains to form in western North America. With high eustatic sea levels obtaining worldwide during the Cretaceous, cold water from the Arctic Ocean in the north and warm water from the Gulf of Mexico in the south flooded the central lowlands, forming a sea that transgressed (grew) and regressed (receded) over the roughly 100 million years of its existence. At its largest, it stretched from the Rockies to the Appalachians in the east, some 1000 km wide. At its deepest, it may have been only 800 or 900 meters deep, shallow as seas go. Two great continental watersheds drained into it from east and west, diluting its waters and bringing resources in eroded silt that formed shifting delta systems along its low-lying coasts. There is no modern parallel to compare it to, but wherever warm and cold waters mix in the contemporary world, and wherever fresh and salt waters mix in estuaries and deltas, there is an abundance of nutrient upwelling and photosynthetic activity that supports a rich planktonic base. Rudy Slingerland of Penn State University has computer-modelled a counter-clockwise gyre for the Cretaceous Seaway, with cooler waters flowing south along the eastern seacoasts of Wyoming and Colorado.
The Western Interior Seaway was a shallow sea, filled with abundant marine life. Interior Seaway denizens included predatory marine reptiles, the largest animals in the Cretaceous seas: the huge Mosasaur, growing up to 18 meters long; Ichthyosaur, and Plesiosaur, an inspiration for the Loch Ness Monster; sharks, such as the Squalicorax; and advanced bony fish including Pachyrhizodus, Enchodus, and the massive 5-meter long Xiphactinus, a fish larger than any modern bony fish. Other sea life included invertebrates such as (mollusks, ammonites, squid-like belemnites), plankton including coccolithophores that secreted the chalky platelets that give the Cretaceous its name, foraminiferans and radiolarians. The Western Interior Seaway was also home to early birds, including the flightless Hesperornis, which had stout legs for swimming through the water and small wing-like appendages used for marine-steering rather than flight; and the tern-like Ichthyornis, an early avian with a toothy beak.
On the bottom the giant clam Inoceramus has left common fossilized shells in the Pierre Shale. The clam had a thick shell paved with "prisms" of calcite deposited perpendicular to the surface, which gave it a pearly luster in life. Paleontologists suggest that the giant size was an adaptation for life in the murky bottom waters, where a correspondingly large gill area would have allowed the animal to cope with oxygen-depleted waters.
At the end of the Cretaceous continuing uplift in a mountain-building episode called the Laramide orogeny hoisted the sandbanks (sandstone) and muddy brackish lagoons (shale), the thick sequences of silt and sandstone still seen today as the Laramie Formation, while low-lying basins between them gradually subsided. The Western Interior Seaway divided across the Dakotas and retreated south towards the Gulf of Mexico.
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