Merycoidodontidae is an extint family from the Artiodactyla order. This diverse group of stocky prehistoric mammals grazed amid the grasslands, prairies or savannas of North and Central America throughout much of the Cenozoic era. First appearing 48 million years ago during the warm Eocene epoch of the Paleogene period, the oreodonts dominated the American landscape 34 to 23 m.y.a. during the dry Oligocene epoch. But they mysteriously disappeared 4 m.y.a. during the colder Pliocene epoch of the late Neogene period.