Western Interior Seaway

In time, the southern embayment merged with the Mowry Sea in the late Cretaceous, forming a completed seaway, creating isolated environments for land animals and plants.

At its largest, the Western Interior Seaway stretched from the Rockies east to the Appalachian Mountains, some 1,000 km (620 mi) wide.

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 was little sedimentation on the eastern shores of the seaway; the western boundary, however, consisted of a thick clastic wedge eroded eastward from the Sevier orogenic belt.

[1] During the early Paleocene, parts of the Western Interior Seaway still occupied areas of the Mississippi Embayment, submerging the site of present-day Memphis.

[8][9][10] The Western Interior Seaway experienced multiple sequences of transgression and regression as the sea level rose and lowered, respectively.

Over at least the last 20 million years of the Cretaceous, the seaway generally regressed, but period of transgression over time have been given different names relative to their cyclothem.

Other marine life included sharks such as Squalicorax, Cretoxyrhina, and the giant durophagous Ptychodus mortoni (believed to be 10 metres (33 ft) long);[12] and advanced bony fish including Pachyrhizodus,[13] Enchodus, and the massive 4-to-5-metre (13 to 16 ft) long Xiphactinus, larger than any modern bony fish.

[20] The shells of the genus are known for being composed of prismatic calcitic crystals that grew perpendicular to the surface, and fossils often retain a pearly luster.