Western Interior Seaway

Western Interior Seaway as it appears in the second segment of Oceans

### Temporal Range

100 - 66 million years ago, Cenomanian - Maastrichtian Stage, Middle - Late Cretaceous Period

### Episodes

* Coasts (Segments 1 and 5)
* Oceans (Segment 2)
* North America (Segments 1 and 2)

### Filmed In

Dunsborough, Western Australia (Oceans, Segment 2)

## Characteristics

### Environment

Inland Sea

* **Maximum Length** of 3,200 kilometers (2,000 miles)
* **Maximum Width** of 1,000 kilometers (600 miles)
* **Maximum Depth** of 800 - 900 meters (2,600 - 3,000 feet)

***“ By day, the prehistoric ocean provides opportunities for a very different type of hunter. In the warm, shallow seas of North America, fish numbers can almost match the nocturnal lantern fish shoal. ”***

― **David Attenborough**, *Oceans*
The ***Western Interior Seaway*** (also called the Cretaceous Seaway, the Niobraran Sea, the North American Inland Sea, and the Western Interior Sea) was a large but shallow epeiric (inland) sea that split the continent of North America into two landmasses.

## Description

The Western Interior Seaway begun to form sometime during the Cenomanian Stage of the Middle Cretaceous Period, when part of the Arctic Ocean begun extending down North America, forming what is known as the Mowry Sea. Eventually, as this arm of the Arctic Ocean reached the Gulf of Mexico, the Western Interior Seaway was completely formed, separating North America into two landmasses, Laramidia in the west, and the more enigmatic Appalachia in the east. This marine transgression (sea level rise) is part of a geological event known as the "Zuñi Sequence".

With the splitting of North America, species and entire ecosystems on both sides were isolated, allowing them to diversify into forms and populations distinct from one another. On occasion, as sea levels rise and fall, land bridges would briefly form and allow animals from both Laurasia and Appalachia to cross over to the other side. As for the sea itself, it became home to a great variety of marine life, from several ammonite species to predatory fish like *Xiphactinus* and the mackarel sharks *Squalicorax* and *Cretoxyrhina* (also known as the "Ginsu Shark"), from plesiosaurs like elasmosaurs and polycotylids (short-necked plesiosaurs), to various pterosaurs and birds that adapted a fishing lifestyle. Eventually, the Western Interior Seaway, like all other oceans and seas across the world during the Late Cretaceous, ended up being dominated by the mosasaurs, a group of massive predatory marine lizards.

At its largest extent, the Western Interior Seaway was over 3,200 kilometers (2,000 miles) long, 1,000 kilometers (620 miles) wide (stretching from the Rocky Mountains to the Appalachians), and around 800 - 900 meters (2,600 - 3,000 feet) deep, shallow for a sea, fluctuating throughout the Cretaceous Period. By the end of the Cretaceous, as tectonic activity continued to form the Rocky Mountains, the sea was further shrunken into its final phase, sometimes known as the "Pierre Seaway". Though parts of the sea would remain for a short while afterwards, these remnants are instead associated with a different event known as the "Tejas Sequence" (the last known major marine transgression across North America), which was not as extensive as the Zuñi Sequence.

## Appearances

The Western Interior Seaway and its Laramidian shores are featured as the setting of the first and fifth segments of Coasts, the second segment of Oceans, and the first and second segments of North America.

## Fauna

*Globidens alabamaensis**Hesperornis regalis**Mosasaurus hoffmannii*ProtostegidScaphitid*Sphenodiscus sp.**Xiphactinus audax*
## References

1. ↑ I Know Dino: The Big Dinosaur Podcast, Episode 446: Dinosaur-era Oceans and Darren Naish from Prehistoric Planet 2
2. ↑ 2.0 2.1 Stanley, Steven M. (1999). *Earth System History*. New York: W.H. Freeman and Company. pp. 487–489.
3. ↑ Relation of unconformities, tectonics, and sea-level changes, Cretaceous of Western Interior, U.S.A.