Mosasaurus, a formidable marine reptile, dominated the ancient seas during the Late Cretaceous period. These powerful predators, known for their streamlined bodies and strong jaws lined with sharp teeth, were at the top of the food chain. Some species, such as Mosasaurus hoffmannii, could reach lengths of up to 12 meters (39 feet), making them among the largest of their kind.
Their Global Range
Mosasaurus fossils have been discovered on every continent, indicating a widespread presence during the Late Cretaceous period, approximately 82 to 66 million years ago. This broad distribution encompassed diverse oceanic climates, ranging from tropical to subpolar regions. Their fossil record includes finds in North and South America, Europe, Africa, Western Asia, and Antarctica.
During the Late Cretaceous, Earth’s geography differed significantly from today, with higher sea levels creating extensive inland seas. A prominent example is the Western Interior Seaway, a large, shallow inland sea that bisected North America, stretching from the Gulf of Mexico northward to the Arctic Ocean. This seaway was a significant habitat for various mosasaur species, with fossils found in areas like Texas, Kansas, and the Dakotas. Another important region was the Tethys Ocean, which covered parts of North Africa and Europe, providing an extensive range for Mosasaurus. Fossil evidence from places like Morocco’s phosphate deposits highlights their abundance in these Tethyan waters.
Life in Ancient Seas
Mosasaurus primarily inhabited warm, shallow, coastal, or epicontinental (inland) sea environments. These ancient seas were characterized by higher temperatures and often experienced periods of reduced oxygen in deeper waters. The Late Cretaceous climate was globally warmer than today, fostering productive marine ecosystems that supported the large prey base mosasaurs relied upon.
While these reptiles thrived in shallower waters, evidence suggests some species ventured into deeper, more open ocean environments. Certain mosasaur genera, like Tylosaurus, showed a wider range of habitat preferences, indicating their adaptability. However, other genera, including Mosasaurus itself, appeared to prefer more restricted habitats based on fossil analysis. The presence of juvenile mosasaurs in open ocean sediments suggests they gave birth to live young in these offshore areas, rather than in sheltered shallow nurseries.
How We Know Their Habitat
Scientists determine the ancient habitats of Mosasaurus through fossil discoveries and geological evidence. Fossil locations directly indicate their past distribution, with specimens recovered from marine sedimentary rock layers. For example, Mosasaurus remains in the Pierre Shale in Badlands National Park confirm their existence in the Western Interior Seaway.
Further insights come from associated fossils found alongside Mosasaurus. Discoveries of other marine organisms, such as ammonites, fish, sea turtles, and marine birds, within the same rock layers provide a detailed picture of the broader ecosystem these predators inhabited. Geological evidence, including rock layer composition, also helps reconstruct ancient environments. Paleogeographic reconstructions, which map ancient continents and oceans millions of years ago, illustrate the extent of ancient seas like the Western Interior Seaway and the Tethys Ocean. Analysis of rare earth elements within mosasaur fossils can provide clues about their habitat’s water depth, distinguishing between shallower and deeper preferences.