Western Interior Seaway in the context of Transgression (geology)

A marine transgression is a geologic event where sea level rises relative to the land and the shoreline moves toward higher ground, resulting in flooding. Transgressions can be caused by the land sinking or by the ocean basins filling with water or decreasing in capacity. Transgressions and regressions may be caused by tectonic events like orogenies, severe climate change such as ice ages or isostatic adjustments following removal of ice or sediment load.

During the Cretaceous, seafloor spreading created a relatively shallow Atlantic basin at the expense of a deeper Pacific basin. That reduced the world's ocean basin capacity and caused a rise in sea level worldwide. As a result of the sea level rise, the oceans transgressed completely across the central portion of North America and created the Western Interior Seaway from the Gulf of Mexico to the Arctic Ocean.

The Niobrara Formation /ˌnaɪ.əˈbrærə/, also called the Niobrara Chalk, is a geologic formation in North America that was deposited between 87 and 82 million years ago during the Coniacian, Santonian, and Campanian stages of the Late Cretaceous. It is composed of two structural units, the Smoky Hill Chalk Member overlying the Fort Hays Limestone Member. The chalk formed from the accumulation of coccoliths from microorganisms living in what was once the Western Interior Seaway, an inland sea that divided the continent of North America during much of the Cretaceous. It underlies much of the Great Plains of the US and Canada. Evidence of vertebrate life is common throughout the formation and includes specimens of plesiosaurs, mosasaurs, pterosaurs, and several primitive aquatic birds. The type locality for the Niobrara Chalk is the Niobrara River in Knox County in northeastern Nebraska. The formation gives its name to the Niobrara cycle of the Western Interior Seaway.

Ichthyornis (meaning "fish bird", after its fish-like vertebrae) is an extinct genus of toothy seabird-like ornithuran from the late Cretaceous period of North America. Its fossil remains are known from the chalks of Alberta, Alabama, Kansas (Greenhorn Limestone), New Mexico, Saskatchewan, and Texas, in strata that were laid down in the Western Interior Seaway during the Turonian through Campanian ages, about 95–83.5 million years ago. Ichthyornis is a common component of the Niobrara Formation fauna, and numerous specimens have been found.

Ichthyornis has been historically important in shedding light on bird evolution. It was the first known prehistoric bird relative preserved with teeth, and Charles Darwin noted its significance during the early years of the theory of evolution. Ichthyornis remains important today as it is one of the few Mesozoic era ornithurans known from more than a few specimens.

Laramidia was an island continent that existed during the Late Cretaceous period (99.6–66 Ma), when the Western Interior Seaway split the continent of North America in two. In the Mesozoic era, Laramidia was an island land mass separated from Appalachia to the east by the Western Interior Seaway. It was home to many dinosaurs including ankylosaurs, ceratopsians, and tyrannosaurs. The seaway eventually shrank, split across the Dakotas, and retreated toward the Gulf of Mexico and the Hudson Bay. The masses joined, forming the continent of North America.

Laramidia is named after the Laramide orogeny. The name was coined by J. David Archibald in 1996.

Nyctosaurus (meaning "night lizard") is a genus of nyctosaurid pterosaur from the Late Cretaceous period of what is now the Niobrara Formation of the mid-western United States, which, during the time Nyctosaurus was alive, was covered in an extensive shallow sea. Some remains belonging to a possible Nyctosaurus species called "N." lamegoi have been found in Brazil, though it likely belongs to a different genus Simurghia. The genus Nyctosaurus has had numerous species referred to it, though how many of these may actually be valid requires further study. At least one species possessed an extraordinarily large antler-like cranial crest.

Nyctosaurus was a mid-sized pterosaur that lived along the shores of the Niobrara Formation of the United States, which back then was within a large inland sea called the Western Interior Seaway. It has been suggested that it would have flown similar to modern-day soaring birds such as albatrosses, which consisted of flying very long distances and rarely flapping. The species N. gracilis and N. nanus have previously been considered as species of the closely related Pteranodon, back then known as P. gracilis and P. nanus, due to their similarities.

Pteranodon (/təˈrænədɒn/; from Ancient Greek: πτερόν, romanized: pteron 'wing' and ἀνόδων, anodon 'toothless') is a genus of pterosaur that included some of the largest known flying reptiles, with P. longiceps having a wingspan of over 6 m (20 ft). They lived during the late Cretaceous geological period of North America in present-day Kansas, Nebraska, Wyoming, South Dakota and Alabama. More fossil specimens of Pteranodon have been found than any other pterosaur, with about 1,200 specimens known to science, many of them well preserved with nearly complete skulls and articulated skeletons. It was an important part of the animal community in the Western Interior Seaway.

When the first fossils of Pteranodon were found, they were assigned to toothed pterosaur genera, such as Ornithocheirus and Pterodactylus. In 1876, Othniel Charles Marsh recognised it as a genus of its own, making particular note of its complete lack of teeth, which at the time was unique among pterosaurs. Over the decades, multiple species would be assigned to Pteranodon, though today, only two are recognised: P. longiceps, the type species, and P. sternbergi. A third species, P. maiseyi, may also exist. Some researchers have suggested the latter two as a genus of their own, Geosternbergia, though this is the subject of some debate. Another genus split from Pteranodon, Dawndraco, may be synonymous with Geosternbergia if that genus is valid, or with Pteranodon if it is not.