Prehistoric mammals are groups of mammals that became extinct before humans developed writing. 164 million years ago, in the Jurassic period, Castorocauda lutrasimilis, a mammaliaform (mammal-shaped) animal weighing about 500 grams (1.1 lb), had a full mammalian pelt, with guard hairs and underfur, webbed feet, and scales on the tail like a modern beaver, as well as teeth specialized for catching fish.
Later, about 130 million years ago in the Cretaceous, there existed larger mammals; a fossil of Repenomamus giganticus indicates that the animal was about 1 meter (3¼ ft) long. In the stomach of a smaller cousin, Repenomamus robustus at 52 cm (20½ in), the remains of a juvenile dinosaur have been preserved.
List of prehistoric mammals
Prehistoric mammals include: