Amynodontidae: Amynodont rhinos are a group of large-bodied, hornless rhinos that were common in Asia and North America from the Middle part of the Eocene through the early Oligocene. Amynodonts are sometimes called swamp rhinos, in reference to an older idea that these animals spent much of their time wallowing in ponds, shallow lakes, and rivers, although there is very little evidence that this is the case. The last North American amynodont, Metamynodon, may have been semi-aquatic. It shows some hippo-like features, including shortened limbs, a long broad torso, and eyes that were positioned towards the top of its skull. Fossils of Metamynodon are also commonly found in sandstone river deposits, further suggesting a semi-aquatic lifestyle. However, there is little evidence that other amynodonts were semi-aquatic.
Amynodon, the animal which this family is named after, was an early member of the group from North America. Amynodon and other amynodont rhinos have a very large depression in the skull just in front of their eye socket, which is called an infraorbital fossa. That probably housed large muscles for an enlarged upper lip. Cadurcodon, an amynodont, from the middle Eocene of Mongolia had a retracted nasal bone, similar to that of modern tapirs, and probably had evolved a tapir-like proboscis.
You can download a copy of a 1936 scientific paper by AMNH president Henry Fairfield Osborn describing a skeleton of amynodont rhino from Mongolia by clicking here.
Amynodonts have no living relatives, and because they are so distantly related to living rhinos it is hard to say much about their behavior or diet, although the elaborate facial musculature suggested by their skull morphology suggests they were successful browsers, which used their enlarged upper lips or proboscis to pluck woody twigs and leaves as the main part of their diet. The higher-crowned teeth of later amynodontids, such as Metamynodon suggests their diets were changing to cope with more open habitats and coarser vegetation in the early Oligocene.
Left: Lateral view of AMNH 1496, a specimen of Metamynodon from the Big Badlands of South Dakota. Victoria Healy/AMNH. Right: ventral view of AMNH 1496, a specimen of Metamynodon from the Big Badlands of South Dakota. Victoria Healy/AMNH.