Symmetrodonta (subclass Theria, infraclass Pantotheria) An extinct order of mammals which lived during the late Jurassic and early Cretaceous and which may have appeared toward the end of the Triassic, making them among the earliest mammals known. They had molar teeth with three cusps arranged in symmetrical triangles. They were very small and probably were predators. They are believed to be ancestral to the Metatheria and Eutheria.

Symmetrodonta (infraclass Pantotheria) Extinct order of mammals which lived during the late Jurassic and early Cretaceous; they may have appeared toward the end of the Triassic, making them among the earliest mammals known. They had molar teeth with three cusps arranged in symmetrical triangles. They were very small and probably were predators. They are believed to be ancestral to the marsupial and placental mammals. See also MARSUPIALIA; and EUTHERIA.