"[7][4] In 1981, a large jawbone, 41723-5, from Brewster County, Texas was described as M. mckinneyi, named after one of the discoverers Billy Pat McKinney, and was found in an area dated to 43 mya in the Late Eocene.

[5] Like other aquatic mammals, Metamynodon had small neural spines projecting upwards from the thoracic vertebrae, indicating weak neck muscles, which probably was due to buoyancy and a lack of necessity to support the head while submerged.

The ribcage was broad and Metamynodon had a barrel-like chest similar to the hippo, which could either be an adaptation to an expanding digestive tract or to develop muscles necessary to prevent rolling over in the water.

Like other aquatic mammals, the leg muscles–popliteus in the knee; gastrocnemius, soleus, and peroneus tertius in the calf; and extensor digitorum longus in the foot–were all large and well-developed, probably to exert more force while walking and to better wade through muddy soil.

Metamynodon was probably abundant and restricted to the streams; common in the gallery woodlands were the horse Mesohippus and artiodactyl Merycoidodon; and in the savannas there were the lagomorph Palaeolagus, the deer-like Leptomeryx and Hypertragulus.

It also had a wide array of other animals including: the entelodonts Daeodon and Archaeotherium; the deer-like Leptomeryx and Protoceras; the rodents Ischyromys, Eumys, and Megalagus; the lizard Peltosaurus; and the small metatherian Herpetotherium.