The Western Interior Seaway, also called the Cretaceous Seaway, the Niobraran Sea, 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 when the Pacific and North American tectonic plates collided, causing the uplifting of the Rocky Mountains. Numerous North American fossil sites owe there existence to the Western Interior Seaway.
It was a shallow sea with diverse marine life including predatory marine reptiles, such as mosasaurs growing up to 18 meters long, ichthyosaurs, and plesiosaurs. There were abundant sharks, such as Squalicorax, and advanced bony fish including Pachyrhizodus, Enchodus, and the huge 18 foot long Xiphactinus, a fish larger than any extant bony fish, and another monster, Ichthyodectes. Other sea life included invertebrates such as mollusks, ammonites, squid-like belemnites, and plankton including coccolithophores that secreted the chalky platelets that give the Cretaceous its name, foraminiferans and radiolarians. The sea was probably less than 600 feet deep in most areas, and had a relatively flat and soft, oxygen-depleted mud bottom fostering fossilzation.
One of the more famous fossil sites is the so-called Kansas Chalk (Niobrara chalk formation). Near the middle of the sea in what is now Kansas sediments were deposited at a fast rate creating about an inch of compacted chalk each 700 years. Some of the worlds finest fossils have been discovered here. The crinoid Uintacrinus and fish Ichthyodectes and two examples.
Fossils of early birds had there originds in the Western Interior Seaway, including the flightless Hesperornis that 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 bird with teeth. There giant clam Inoceramus is found in the Pierre Shale. Paleontologists suggest that the giant size was an adaptation for life in murky bottom waters, where a correspondingly large gill area would have allowed the animal to cope with anoxic conditions.