Mosasaur

Mosasaurs (from Latin Mosa meaning the 'Meuse river', and Greek σαύρος sauros meaning 'lizard') comprise a group of extinct, large marine reptiles containing 38 genera in total. Their first fossil remains were discovered in a limestone quarry at Maastricht on the Meuse in 1764. Mosasaurs probably evolved from an extinct group of aquatic lizards known as aigialosaurs in the Early Cretaceous. During the last 20 million years of the Cretaceous period (Turonian-Maastrichtian ages), with the extinction of the ichthyosaurs and pliosaurs, mosasaurs became the dominant marine predators. They became extinct as a result of the K-Pg event at the end of the Cretaceous period, about 66 million years ago. Mosasaurs breathed air, were powerful swimmers, and were well-adapted to living in the warm, shallow inland seas prevalent during the Late Cretaceous period. Mosasaurs were so well adapted to this environment that they gave birth to live young, rather than returning to the shore to lay eggs as sea turtles do. The smallest-known mosasaur was Dallasaurus turneri, which was less than 1 m (3.3 ft) long. Larger mosasaurs were more typical, with many species growing longer than 4 m (13 ft). Mosasaurus hoffmannii, the largest known species, may have reached up to 17 m (56 ft) in length. Currently, the largest publicly exhibited mosasaur skeleton in the world is on display at the Canadian Fossil Discovery Centre in Morden, Manitoba. The specimen, nicknamed 'Bruce', is just over 13 m (43 ft) long. Mosasaurs had a body shape similar to that of modern-day monitor lizards (varanids), but were more elongated and streamlined for swimming. Their limb bones were reduced in length and their paddles were formed by webbing between their long finger and toe bones. Their tails were broad, and supplied their locomotive power. Until recently, mosasaurs were assumed to have swum in a method similar to the one used today by conger eels and sea snakes, undulating their entire bodies from side to side. However, new evidence suggests that many advanced mosasaurs had large, crescent-shaped flukes on the ends of their tails, similar to those of sharks and some ichthyosaurs. Rather than use snake-like undulations, their bodies probably remained stiff to reduce drag through the water, while their tails provided strong propulsion. These animals may have lurked and pounced rapidly and powerfully on passing prey, rather than chasing after it. Early reconstructions showed mosasaurs with dorsal crests running the length of their bodies, which were based on misidentified remains of tracheal cartilage. By the time this error was discovered, depicting mosasaurs with such crests in artwork had already become a trend. Mosasaurs had double-hinged jaws and flexible skulls (much like those of snakes), which enabled them to gulp down their prey almost whole. A skeleton of Tylosaurus proriger from South Dakota included remains of the diving seabird Hesperornis, a marine bony fish, a possible shark, and another, smaller mosasaur (Clidastes). Mosasaur bones have also been found with shark teeth embedded in them. One of the food items of mosasaurs were ammonites, molluscs with shells similar to those of Nautilus, which were abundant in the Cretaceous seas. Holes have been found in fossil shells of some ammonites, mainly Pachydiscus and Placenticeras. These were once interpreted as a result of limpets attaching themselves to the ammonites, but the triangular shape of the holes, their size, and their presence on both sides of the shells, corresponding to upper and lower jaws, is evidence of the bite of medium-sized mosasaurs. Whether this behaviour was common across all size classes of mosasaurs is not clear. Virtually all forms were active predators of fish and ammonites; a few, such as Globidens, had blunt, spherical teeth, specialized for crushing mollusk shells. The smaller genera, such as Platecarpus and Dallasaurus, which were about 1–6 m (3.3–19.7 ft) long, probably fed on fish and other small prey. The smaller mosasaurs may have spent some time in fresh water, hunting for food. The larger mosasaurs, such as Tylosaurus, Hainosaurus and Mosasaurus, reached sizes of 10–15 m (33–49 ft) long and were apex predators of the Late Cretaceous oceans, attacking other marine reptiles, as well as preying on large fish and ammonites.