Size and Weight:
The remains of Mosasaurus were discovered well before educated society knew anything about evolution, dinosaurs, or marine reptiles--in a mine in Holland in the late 18th century (hence this creature's name, after the nearby Meuse river). The unearthing of these fossils led early naturalists like Georges Cuvier to speculate, for the first time, about the possibility that currently extinct species had once lived on earth, which flew in the face of accepted religious dogma of the time.
The fearsome Mosasaurus was a giant member of the family of marine reptiles known as mosasaurs, which were characterized by their big heads, powerful jaws, streamlined bodies and front and rear flippers. Mosasaurs were only distantly related to the pliosaurs and plesiosaurs that preceded them (and which they largely supplanted from dominance of the world's oceans during the late Cretaceous period); today, evolutionary biologists believe they were most closely related to modern-day snakes and monitor lizards.