Mosasaurus lived roughly 82 to 66 million years ago, during the final million years of the Cretaceous Period. It wasn't a dinosaur — it belonged to the mosasaurs, a group of marine reptiles more closely related to modern snakes and monitor lizards than to any dinosaur.
The last and largest of its group
Mosasaurus was the largest known member of the mosasaur family. Size estimates for the biggest individuals range up to around 17 to 18 meters (55–60 ft) — rivaling some of the largest predators the oceans have ever held, on par with the biggest estimates for megalodon. It had a long, powerful, flattened tail for swimming and two pairs of paddle-like flippers.
An apex predator with a broad diet
Mosasaurus fossils and bite marks suggest a wide-ranging diet that included fish, ammonites, other marine reptiles, and even other mosasaurs. Its jaws had a second set of teeth on the roof of the mouth, similar to modern snakes, likely helping to grip struggling prey and move it toward the throat.
Discovered before "dinosaur" was even a word
The first Mosasaurus skull was found in a chalk quarry near Maastricht, Netherlands, in 1764 — decades before the word "dinosaur" was even coined. It was initially misidentified, at various points, as either a crocodile or a whale, before scientists eventually recognized it as something entirely new.
Gone with the dinosaurs
Mosasaurus and the rest of the mosasaur family went extinct in the same mass extinction event, roughly 66 million years ago, that wiped out the non-avian dinosaurs.