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Mesonychidae (superorder Protoungulata, order Condylarthra) An extinct family of condylarths, formerly classified as Creodonta, which are believed to show that modern ungulates and carnivores are descended from a common stock. They appeared first in the Palaeocene, became more common in the Eocene, and had disappeared by the middle of the Oligocene. They had upper cheek teeth that were triangular, each with three blunt cusps and lower molars apparently adapted for shearing. The feet probably were hoofed. Some later forms were large. Mesonyx was about 1.5 m long, but judging by the comparative sizes of their skulls Andrewsarchus, which lived in Mongolia, may have attained a length of 4.5 m. The animals may have fed on tough vegetable material or carrion.