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an order of extinct primitive terrestrial mammals that lived in the Mesozoic. Triconodonts were small animals, no larger than rats. In the structure of the skull and lower jaw they preserved features characteristic of therapsid reptiles of the group Cynodontia. Each molar tooth had three conelike cusps along the axis (hence the name Triconodonta). The dental formula was primitive. Remains of the Triconodonta have been found in deposits in North America, Europe, and Asia.