Prehistoric mammals are groups of mammals that live before humans developed writing. 164 million years ago, in the Jurassic period, Castorocauda lutrasimilis, a mammal-like (mammaliaform) animal weighing about 500 grams (1.1 lb), had a full mammalian pelt, with guard hairs and under fur, webbed feet, and scales on the tail like a modern beaver, as well as teeth specialized for catching fish.