- Species: Kentrosaurus aethiopicus
- Era: Late Cretaceous (70-66 Million years ago)
- Locale: Europe
Mosasaurus (/ˌmoʊzəˈsɔːrəs/; "lizard of the Meuse River") is the type genus of the mosasaurs, an extinct group of aquatic squamate reptiles. The genus lived from about 82 to 66 million years ago during the Campanian and Maastrichtian stages of the Late Cretaceous. The earliest fossils known to science were found as skulls in a chalk quarry near the Dutch city of Maastricht in the late 1700s, which were initially thought to have been the bones of crocodiles or whales. One particular skull discovered at around 1780, and which was seized during the French Revolutionary Wars for its scientific value and transported to Paris, was famously nicknamed the "great animal of Maastricht". In 1808, naturalist Georges Cuvier concluded that it belonged to a giant marine lizard with similarities to monitor lizards but otherwise unlike any animal known today. This concept was revolutionary at the time and helped support the then-developing ideas of extinction. However, Cuvier did not designate a scientific name for the new animal; this task was completed by William Daniel Conybeare in 1822 when he named it Mosasaurus in reference to its origin in fossil deposits near the Meuse River; the name is accordingly a portmanteau derived from the words Mosa (the Latin translation for the Meuse River that passed along Mount Saint Peter) and saurus (the romanization of the Ancient Greek σαῦρος, meaning "lizard"). The relationships between Mosasaurus and modern reptiles are controversial and scientists continue to debate whether its closest living relatives are monitor lizards or snakes.
Traditional interpretations have estimated the maximum length of Mosasaurus to be up to 17.6 meters (58 ft), making it one of the largest mosasaur genera. Its skull, which was either broad or slender depending on the species, was equipped with robust jaws capable of swinging back and forth and strong muscles capable of powerful bites using dozens of large teeth designed for cutting prey. Its four limbs were shaped into robust paddles to steer the animal underwater. Its tail was long and ended in a paddle-like fluke that bent downwards. Mosasaurus was a predator that had excellent vision to compensate for its poor sense of smell, and a high metabolic rate that suggests it was endothermic ("warm-blooded"), an adaptation only found in mosasaurs among squamates. The classification of Mosasaurus was historically problematic due to an unclear diagnosis of the type specimen. As a result, over fifty different species have been attributed to the genus in the past. A rediagnosis of the type specimen in 2017 helped resolve the taxonomy issue and confirmed at least five species to be within the genus and another five species still nominally classified within Mosasaurus are planned to be reassessed in a future study. Each species was variable with unique anatomical features differentiating them from the robustly-built M. hoffmannii to the slender and serpentine M. lemonnieri.
Fossil evidence suggests that Mosasaurus inhabited much of the Atlantic Ocean and the seaways adjacent to it. Continents that have recovered Mosasaurus fossils include North America, South America, Europe, Africa, Western Asia, and Antarctica. This distribution encompassed a wide range of oceanic climates including tropical, subtropical, temperate, and subpolar climates. Mosasaurus was a common large predator in these oceans and a dominant genus positioned at the top of the food chain. Scientists believe that its diet would have included virtually any animal; it likely preyed on bony fish, sharks, cephalopods, birds, and other marine reptiles including sea turtles and other mosasaurs. It likely preferred to hunt in open water near the surface. From an ecological standpoint, Mosasaurus probably had a profound impact on the structuring of marine ecosystems; its arrival in some locations such as the Western Interior Seaway in North America coincides with a complete turnover of faunal assemblages and diversity. Mosasaurus faced competition with other large predatory mosasaurs such as Prognathodon and Tylosaurus—which were known to feed on similar prey—though they were able to coexist in the same ecosystems through niche partitioning. There were conflicts among them, as an attack on Mosasaurus by Tylosaurus has been documented. Several discovered fossils illustrated deliberate attacks on Mosasaurus individuals by members of the same species. Infighting likely took place in the form of snout grappling, similarly seen in modern crocodiles today.