Metamynodon planifrons (Scott and Osborn 1867; Early Eocene; 4m long) This hippo with anteriorly migrated orbits (as in rhinos) nests with aquatic hipposin the LRT. Note the large self-sharpening canines and short cervicals. The calcaneum is long.

Promerycochoerus macrostegus (Leidy 1858; Early Miocene 22mya; 1m) is a small traditional oreodont from North America. It nests with hippos in the LRT. Note the bowl-shaped cheekbones, perhaps misaligned in a too vertical plane, leaving less room for jaw muscles, as in other related taxa.

Merycopotamus dissimilis (Falconer & Cautley 1847; Middle Miocene to Late Pliocene) was considered an Asian anthracothere, but here nests with Hippopotamus and kin. Note the migration of the orbit posteriorly, the re-appearance of the postfrontal and prefrontal, the massive dentary with massive retroarticular process and the massive dentary canine, as in hippos.

Hippopotamus amphibius (Linnaeus 1758) is the living hippo, not an artiodactyl ungulate. Once thought to be related to whales, like Maiacetus, that relationship with toothed whales was not supported by the large reptile tree. However, hippos are basal to baleen whales like Balaeonoptera by way of transitional taxa that include Behemotops. Hippopotamus is derived from Mesonyx and the Oreodonta.

Compared to Merycopotamus., Hippopoatamus was larger and more bulky overall with shorter, stouter limbs and a shorter tail. The teeth became tusks and the retroarticular process of the dentary curved anteriorly, unlike any other mammal. The diet is completely herbivorous as Hippopotamus inhabits freshwater rivers in warm climates.

Hippopotamus madagascariensis (aka Choeropsis, aka Hexaprotodon; Solweig Stuenes 1989; recently extinct) This taxon first arrived in Madagascar as cargo on a ship arriving from East Africa around the year 1. By the year 1000 to 1500 they all had been butchered to extinction. Selection favored smaller forms. The orbit is not elevated and the ascending process of the dentary is very sharp. Note the straight, rather than convex lower mandible.