extinct genus of early mammals known as fossils from Middle Jurassic deposits (of 176 million to 161 million years ago). Amphitherium is the earliest representative of the pantotheres, a group of early mammals that, it is believed, represents the stock that gave rise to all the higher mammals of later times. Amphitherium is known from a lower jaw found in Europe and is characterized by the large number and distinctive structure of its teeth.