Physical characteristics

The skulls of heptaxodontids are massive, resembling those of the nutria (Myocastoridae). The rostrum (snout) was massive, the bullae (bones of the inner ear) were small, and many of the skulls bear sagittal crests (bony ridges on the top of the skull to provide added anchorage for strong jaw mucles).

The cheek teeth, or molars, were strongly hypsodont ("high-crowned"), each having four to seven laminae (layers), set more or less parallel while arranged at angles to the long axis of the skull. Quemisia and Elasmodontomys had teeth and jaw adaptations for propalinal (side-to-side) chewing.

The skulls and postcranial bones suggest that the He-pataxodontidae had heavy-set bodies. All species were considerably larger than the general run of rodents today. Amblyrhiza was nearly the size of a black bear (Ursus americanus).

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