Mosasaurus lived in oceans across the globe during the Late Cretaceous period, roughly 98 to 65.5 million years ago. Fossils of the genus have been found on every continent except Australia, spanning North America, South America, Europe, Africa, Antarctica, and Asia. Far from being confined to one region, these marine reptiles thrived in a world where sea levels were dramatically higher than today, warm shallow seas flooded continental interiors, and no permanent ice caps existed at the poles.
The Western Interior Seaway of North America
The most famous mosasaur habitat in North America was the Western Interior Seaway, a vast body of water that split the continent in two, stretching from the Gulf of Mexico all the way to the Arctic Ocean. States like South Dakota, Kansas, and Manitoba in Canada were completely submerged. Many of the earliest North American mosasaur fossils were discovered in South Dakota, and it’s possible that Lewis and Clark made the first western mosasaur discovery in Gregory County during their expedition.
The seaway was likely more than 300 feet deep in places, creating a lush marine environment. As recently as June 2025, a 14-year-old from Texas unearthed a mosasaur specimen in southern Manitoba, with subsequent digs revealing teeth, jawbone, limb bones, vertebrae, and skull fragments from an animal that lived roughly 80 million years ago. Kansas, in particular, has produced so many mosasaur specimens that an entire book, “Oceans of Kansas,” documents the state’s rich marine fossil record from this period.
Europe and the Original Discovery
The very first recognized mosasaur fossil came from limestone quarries near Maastricht in the Netherlands, where it became known as “the great fossil animal of the quarries of Maestricht.” This specimen of Mosasaurus hoffmannii gave both the genus and the entire Maastrichtian geological stage their names. The Maastricht area, straddling the Netherlands and Belgium border, has been producing mosasaur fossils for over 250 years and remains one of the most important collection sites in the world. Additional fossils have been documented across France and other parts of Western Europe.
A Truly Global Range
While most of its diversity is known from deposits around the ancient Tethys Sea (a tropical ocean that once separated the northern and southern landmasses) and the North American Western Interior Seaway, Mosasaurus spread far beyond those core regions. Fossils from South America, Africa, and Asia show the genus colonized waters across the Southern Hemisphere and the western Pacific rim. Specimens have even been recovered from Antarctica, where ocean temperatures hovered around 7 to 8°C during the latest Cretaceous. That’s warmer than Antarctic waters today, consistent with a planet that had no polar ice sheets.
Different species adapted to different regional environments. In the Dukamaje Basin of what is now Nigeria, one related mosasaur (Goronyosaurus) developed a crocodile-like skull suited to coastal and estuarine waters. The genus Mosasaurus itself appears to have radiated outward from the Tethys and Western Interior Seaway, spreading north and west along the rim of the western Pacific.
Not Just Open Ocean
Mosasaurs are traditionally classified as marine reptiles, but growing fossil and chemical evidence shows they weren’t confined to saltwater. A mosasaur tooth recovered from the Hell Creek Formation in North Dakota, a site dominated by terrestrial and freshwater species with no other marine animals present, showed no signs of being transported from elsewhere. Chemical analysis of the tooth’s strontium isotope ratios confirmed a freshwater origin, meaning the animal likely lived in a river or lake environment rather than washing in from the sea.
This wasn’t a one-off occurrence. Geochemical evidence from Hungary also indicates mosasaurs inhabited freshwater environments there, and oxygen isotope analysis from specimens in the northern Western Interior Seaway suggests regular freshwater excursions. Isotopic studies of mosasaur teeth from Alabama, Kansas, and Manitoba reveal a striking pattern: these animals appear to have migrated between saltwater and freshwater on a semi-monthly cycle, possibly to regulate their internal salt balance, much like modern sea snakes do today.
During the final stage of the Cretaceous, as salinity levels in the Western Interior Seaway dropped and rainwater increasingly dominated the basin, some mosasaur lineages appear to have adapted fully to freshwater habitats like the rivers and floodplains preserved in the Hell Creek Formation.
What Their World Looked Like
The Late Cretaceous was one of the warmest periods in Earth’s history. Sea levels were high enough to flood continental interiors, creating the shallow seas where mosasaurs thrived for 32.5 million years. There were no permanent ice caps. Ocean water at roughly 45°S latitude (equivalent to modern Patagonia) measured around 13°C, and even waters near the Antarctic Peninsula sat near 7°C. For comparison, Antarctic surface waters today hover closer to freezing.
A global cooling trend began during the Campanian stage, and the final 200,000 years of the Cretaceous were relatively cool by the standards of the era, though still far warmer than the modern world. Mosasaurs persisted through these temperature shifts until the mass extinction event at the end of the Cretaceous, 65.5 million years ago, which wiped them out along with the non-avian dinosaurs. By that point, they had occupied virtually every marine environment on the planet, from tropical shallows to polar waters to inland rivers.