Hadrocodium (Greek for "big head"); pronounced HAD-roe-CODE-ee-um
Woodlands of Asia
Early Jurassic (195 million years ago)
Size and Weight:
About one inch long and two grams
Tiny size; relatively large brain
Whereas other Mesozoic mammals are known by their teeth, Hadrocodium is represented in the fossil record by a single, tiny skull--which has revealed plenty about how this tiny (less than one ounce) creature lived. Hadrocodium had a relatively large brain for its small size, and its ear bones were well separated from the bones of its lower jaw, an important mammalian trait; clearly its senses of hearing and sight were well developed. Beyond that, however, it's not certain if the early Jurassic Hadrocodium was a true mammal, or an intermediate form more properly classified as a therapsid ("mammal-like reptile").