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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. [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.
The Western Interior Seaway was gone from Nebraska by the early portion of the Cenozoic. It was replaced by a terrestrial environment dotted with lakes and rivers. The contemporary local wildlife were similar to modern forms. On occasion during the Cenozoic, volcanic activity in the Rocky Mountains covered regions of the state in ash. [2]
Ammonites were cephalopods, that thrived in tropical seas until becoming extinct along with the dinosaurs at the end of the Mesozoic era. The ammonites that form ammolite inhabited a prehistoric, inland subtropical sea that bordered the Rocky Mountains—this area is known today as the Cretaceous or Western Interior Seaway .
The Late Jurassic fossiliferous Morrison Formation is exposed in the western part of the state and has produced extensive remains of sauropod dinosaurs. [101] [102] Most of Oklahoma was submerged under the Western Interior Seaway during the Cretaceous. [103] Early Cretaceous life included "immense" ammonites, echinoids, and pelecypods.
This sea, the Western Interior Seaway, [3]: 5 was home to many invertebrates, aquatic birds, and marine reptiles. [2] The Cretaceous life of South Dakota was similar to that found in what is now Wisconsin. [1]: 256 A Late Cretaceous ammonite found in the Fox Hills Formation. Some of South Dakota's ammonites were very unusual for the group.
The presence of young mosasaurs in the formation suggests that mosasaurs were viviparous and gave birth hundreds of miles out to sea, as Niobrara was in the middle of the Western Interior Seaway at the time. [28] Juveniles would likely have been vulnerable to predation by the many large mid-ocean predators present in the ecosystem.
Bearpaw shale being excavated to recover ammonites for ammolite production. The formation was deposited in the Bearpaw Sea, which was part of the Western Interior Seaway that advanced and then retreated across the region during Campanian time. [ 4 ]
Its location at the changing conjunction of the eastern coast of Laramidia and the adjacent western shallows of the Western Interior Seaway led to the preservation of fossils of both marine and terrestrial creatures. [13] Vertebrates include dinosaurs, pterosaurs, crocodiles, champsosaurs, lizards, snakes, turtles, frogs and salamanders.