Mosasaurus is a mosasaurid squamate reptile from the Late Cretaceous of the Netherlands, France, the United States of America, Morocco, Syria, and possibly New Zealand and Japan. It was named in 1829 by Gideon Mantell. It was one of the first marine reptiles (nicknamed antediluvian reptiles at the time) known to science, and was confused for being either a fish, a crocodile, or sperm whale, right before people discovered that it was a marine reptile. Like the earliest discovered Mesozoic animals, it was a wastebasket taxon.
Physiology
Mosasaurus resembled a flippered crocodile. It had an elongated, streamlined body, 4 flippers, (likely) a long tail with a shark-like fin on its end, and a long, conical head with a mouth containing 2 rows of massive, conical teeth. Its body would have been covered in scaly skin.
Diet
Mosasaurus was a predator, preying on fish, cephalopods, turtles, diving birds, plesiosaurs, pterosaurs, and smaller mosasaurs. Its teeth had knife-like serrations from front to back, and were used to cut into the hides of large prey; as well as this, behind its huge jaws, it had a second set of teeth, mainly for capturing prey and ensuring it did not escape.
Ecology
Mosasaurus almost never scavenged carcasses, due to poor olfactory senses. As well as this, it swam near the surface, waiting for marine reptiles or diving birds to come up to take a breath of fresh air, right before launching a surprise ramming attack on them. This attack was able to either kill prey, or maim them so the mosasaur could stalk them until they exhausted. Due to being an epipelagic apex predator, it did not dive down into deeper waters.
In popular culture
Mosasaurus was one of the 1st Mesozoic animals discovered, so it was featured in Crystal Palace Park (a pleasure ground in London) as a giant, iguana-like squamate, alongside Iguanodon (depicted as a horned dragon), Megalosaurus (also depicted as a dragon), Pterodactylus, Plesiosaurus, and Ichthyosaurus. However, for most of its publicity, it was overshadowed by another large marine lizard, Tylosaurus. So far, Mosasaurus's biggest public appearance was in the 2015 science fiction movie, Jurassic World, where it was featured as a major anti-hero, responsible for killing the deranged hybrid dinosaur, Indominus rex; as well as this, it was oversized (albeit not as much as the Liopleurodon in Walking with Dinosaurs).