Extinct order comprising seven families of primitive ungulates, including some resembling the original eutherian (see eutheria) stock so closely that their classification has presented many difficulties. Condylarths appear to be transitional between insectivores and true ungulates. They ranged from the late Cretaceous to the latter part of the Miocene and were a major part of the Palaeocene fauna. Some forms possessed claws and may have been arboreal (Hyopsodontidae), others became highly specialized, but the development of hoofs and ungulate dentition can be traced to families in the order.