Prehistoric mammals are groups of mammals that lived 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. The lineages of many varieties continued through the Tertiary period where some reached very large sizes. Most of the very large mammals became extinct in the last ice age, but have smaller descendants.