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.

Later, about 130 million years ago in the Cretaceous, there existed larger mammals, including Repenomamus giganticus and Repenomamus robustus. Fossils up to one meter (3¼ ft) long have been found, with dinosaur remains in their stomach contents.

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.

The oldest prehistoric mammal fossil was Adelobasileus. Adelobasileus lived during the Late Triassic, which was when the dinosaurs also appeared. Adelobasileus, being only 4 inches long, would have been a nocturnal mammal, hiding during the day and eating as much insects as possible before the Sun came up.

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