Western Interior Seaway

inside; late Cretaceous Pierre Shale near Ekalaka, Montana. , located 25 miles south of Oakley. By the late Cretaceous, Eurasia and the Americas had separated along the south Atlantic, and subduction on the west coast of the Americas had commenced, resulting in the Laramide orogeny, the early phase of growth of the modern Rocky Mountains. The Western Interior Seaway may be seen as a downwarping of the continental crust ahead of the growing Laramide/Rockies mountain chain. each time rejoining the separated, divergent land populations, allowing a temporary mixing of newer species before again separating the populations. At its largest, the Western Interior Seaway stretched from the Rockies east to the Appalachian Mountains, some wide. At its deepest, it may have been only deep, shallow in terms of seas. 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. The western shore was thus highly variable, depending on variations in sea level and sediment supply. Widespread carbonate deposition suggests that the seaway was warm and tropical, with abundant calcareous planktonic algae. Remnants of these deposits are found in northwest Kansas. A prominent example is Monument Rocks, an exposed chalk formation towering over the surrounding range land. The Western Interior Seaway is believed to have behaved similarly to a giant estuary in terms of water mass transport. Riverine inputs exited the seaway as coastal jets, while correspondingly drawing in water from the Tethys in the south and Boreal waters from the north. During the late Cretaceous, the Western Interior Seaway went through multiple periods of anoxia, when the bottom water was devoid of oxygen and the water column was stratified. At the end of the Cretaceous, continued Laramide uplift hoisted the sandbanks (sandstone) and muddy brackish lagoons (shale), 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. This shrunken and final regressive phase is sometimes called the Pierre Seaway. Well-preserved fossil otoliths of the marine catfish Vorhisia suggest that around this time, following a major cooling trend from the highs of the Cretaceous Thermal Maximum, average water temperatures in the Western Interior Seaway were about . Ammonite-based biostratigraphy suggests that the lowest average temperatures reached by the Western Interior Seaway were during the early Maastrichtian, about , with a slight warming towards the end of the Maastrichtian to . ==Phases==

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 periods of transgression over time have been given different names relative to their cyclothem. The Niobrara sea was formed by the first sea level rise expanding the seaway westward, which then regressed through the Aquilian land-vertebrate age. Following this, the sea level rose again in the "Claggett transgression", named for the Claggett Shale, before regressing again in the Judithian. The sea level then rose again to form the shallow Bearpaw Sea of the "Bearpaw transgression" (named after the Bearpaw Shale), which expanded over much of Wyoming, Montana, and the prairie provinces, before regressing in the Edmontonian. Following this was a localized "Lewis" sea separated from the Bearpaw Sea to the north, before more regression of the Western Interior Seaway. ==Fauna==

The Western Interior Seaway was a shallow sea with abundant marine life. Vertebrate denizens included predatory marine reptiles such as plesiosaurs, mosasaurs and Deinosuchus. Other marine life included sharks such as Squalicorax, Cretoxyrhina and the giant durophagous Ptychodus mortoni (believed to be long); and advanced ray-finned fish including Pachyrhizodus, Enchodus and the massive long Xiphactinus, larger than any modern bony fish. Other invertebrate sea life included bivalves, ammonites, squid-like belemnites and plankton including coccolithophores (which secreted the chalky platelets that give the Cretaceous its name), foraminiferans and radiolarians. The seaway was home to early birds, including the flightless Hesperornis that had stout legs for swimming through water and tiny wings used for marine steering rather than flight; and the tern-like Ichthyornis, an early avian with a toothy beak. Ichthyornis shared the sky with large pterosaurs, such as Nyctosaurus and Pteranodon. Pteranodon fossils are very common; it was probably a major participant in the surface ecosystem, though it was found in only the southern reaches of the seaway. Inoceramids (oyster-like bivalve molluscs) were well-adapted to life in the oxygen-poor bottom mud of the seaway. These left abundant fossils in the Kiowa, Greenhorn, Niobrara, Mancos, and Pierre formations. There is great variety in the shells and the many distinct species have been dated and can be used to identify specific beds in those rock formations of the seaway. Many species can easily fit in the palm of the hand, while some, like Inoceramus (Haploscapha) grandis, could be well over a meter in diameter. Entire schools of fish sometimes sought shelter within the shell of the giant Platyceramus. 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. Image:Kansas sea2DB.jpg|Artist's impression of a Cretoxyrhina and two Squalicorax circling a dead Claosaurus in the Western Interior Seaway File:Elasomosaurus Face Clean.png|Elasmosaurus platyurus in the Rocky Mountain Dinosaur Resource Center in Woodland Park, Colorado Image:InoceramusCretaceousSouthDakota.jpg|Inoceramus, an ancient bivalve from the Cretaceous of South Dakota. File:Nodosaur.jpg|Borealopelta's holotype specimen, an exceptionally well-preserved specimen that was fossilized in the Western Interior Seaway. == See also ==