Mosasaurs 101 | National Geographic

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• [Narrator] During the Cretaceous period, Mosasaurs were among the oceans most fearsome and successful predators. Mosasaurs were marine reptiles that are thought to be closely related to snakes and monitor lizards. They were highly adaptable and many different species evolved and came to dominate ocean habitats worldwide. Some even took to fresh water rivers to hunt. Many prowled the open ocean, devouring fish, sharks, plesiosaurs, sea turtles, sea birds, and sometimes even smaller mosasaurs.

Mosasaur size varied greatly. Two of the largest mosasaurs were Mosasaurus and Tylosaurus. Each stretched 30 to 50 feet, longer than a T-Rex. Meanwhile, many smaller mosasaurs were no larger than a dolphin. Yet most species, no matter how large or small, could be characterized by a long serpentine body with a powerful tail which they moved side to side as they slithered through the water. They had paddles that were likely used for stability and large heads that had powerful, flexible jaws.

Their jaws contained two rows of conical teeth, designed to chomp and hold prey before swallowing it whole. The top marine reptiles since the Triassic had been ichthyosaurs and plesiosaurs, but they were in decline when mosasaurs came onto the scene during the Cretaceous, leaving room for a new apex predator. According to theory, mosasaurs evolved from terrestrial lizards that adapted to the oceans by the middle of the Cretaceous.

Then, during the 20 to 30 million years leading up to the extinction of the dinosaurs, mosasaurs rapidly adapted to ocean habitats the world over. We know this because fossils have been found on every continent, including Antarctica. When dinosaurs became extinct some 65 and a half million years ago, mosasaurs vanished from the fossil record as well. While it's not known whether they died off instantly or gradually, one thing is for certain: the ocean has never again seen marine reptiles as massive and as great as mosasaurs.

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