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The skull of Hadrocodium (Science Magazine)


Hadrocodium (Greek for "big head"); pronounced HAD-roe-CODE-ee-um


Woodlands of Asia

Historical Period:

Early Jurassic (195 million years ago)

Size and Weight:

About one inch long and two grams



Distinguishing Characteristics:

Tiny size; relatively large brain


About Hadrocodium:

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").

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