COMMON DESCENT

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Let’s take a trip back to the Cretaceous of North America, when the whole middle of the continent was flooded by an inland sea unlike anything in the world today. It split the continent in two and hosted a unique ecosystem now preserved across fossil sites from Canada to the Gulf of Mexico. It was the Western Interior Seaway.

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Flooded continent

Starting in the Jurassic Period, two major changes altered the face of North America. First, intense tectonic activity in the west began the development of the modern Rocky Mountains, in turn creating a low-elevation foreland basin to the east. Second, changes in seafloor activity led to a global sea level rise of hundreds of meters. The result: the continent flooded.

An inland sea or epeiric sea is a shallow sea that covers part of a continent. We have these today – the Baltic Sea and Hudson Bay for example – but none compare to the Western Interior Seaway. At its greatest extent, it ran from the Arctic to the Gulf of Mexico, spanning 45 degrees of latitude and mixing polar with subtropical waters; it was around 2,000 km (1,200 mi) wide, completely submerging entire modern-day states and provinces; and it was hundreds of meters deep, home to unique and fascinating aquatic ecosystems.

Under the Sea

Many of North America’s most significant fossils come from the Western Interior Seaway, including the largest sea turtles in Earth history, famous fish like the huge Xiphactinus, enormous inoceramid clams, and many of the world’s best-known mosasaurs. The oxygen-poor waters of the seafloor were great for preserving fossils.

The sea also preserved remains of land organisms, including lots of tree trunks and dinosaur carcasses that washed out to sea. Incredibly, the seaway is also the site of some of the most significant discoveries of Cretaceous birds and pterosaurs, animals that likely spent time flying above the ocean and occasionally met their end in its depths.

A land divided

Oceans are barriers for land animals, and with the Western Interior Seaway in the middle of North America, the continent was split into two halves so distinct they’ve earned their own names: Laramidia to the west and Appalachia to the east.

Laramidia is extremely well-known from fossil sites all up and down western North America. Many of the continent’s most famous dinosaurs are Laramidian: monster-mouthed tyrannosaurs, horned ceratopsids, dome-headed pachycephalosaurs, armored ankylosaurs, and more, including big names like Sue and Leonardo. With all the tectonic activity going on, this was an active landscape, host to a diversity of environments and with lots of eroding sediment to bury and fossilize their inhabitants.

Appalachia is less forthcoming with its secrets. There aren’t many good fossils from the more tectonically-stable eastern side of the continent, so most of what we know comes from fragmentary remains. Appalachia seems to have been missing some of the more famous Laramidian dinos, but was home to plenty of armored nodosaurs, little leptoceratopsians, and lots and lots of herbivorous hadrosaurs. In the southeast, Appalachia was also the domain of giant crocodilians like Deinosuchus.

At the end of the Cretaceous, Laramidia and Appalachia began to reconnect as the Western Interior Seaway receded. The sea level dropped, the interior continent uplifted, and the continent gradually drained, leaving behind its signature in the form of an incredibly varied Cretaceous fossil record.

Oh, and also this strange quirk of United States politics.

Learn More
Check out Oceans of Kansas for lots of Seaway info. (Non-technical)
Lots of information and paleo-maps in this USGS publication from 1995. (Technical)
Explore how Appalachia differed from Laramida in this paper. (Technical)

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If you enjoyed this topic and want more like it, check out these related episodes:

• Episode 72 – Plesiosaurs
• Episode 51 – Mosasaurs
• Episode 79 – Pterosaurs
• Episode 5 – The K-Pg Extinction (the “Dinosaur Extinction”)

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