Paleoparadoxia tabatai (Reinhart 1959, 2.2m long; Miocene, 20-10 mya) was a desmostylian derived from a sister to Anthracobune and Ocepeia and was basal to Behemotops and Balaeonoptera. No prior studies have recovered the hippo and desmostylian relationship, but several prior papers have noted the convergent size, body type and lifestyle with hippos. Rather most studies ally desmostylian with sirenians and elephants.
The nares continue to move to the dorsal skull on Paleoparadoxia, ideal for breathing while otherwise underwater. Note the vertical orientation of the pelvis and procumbent incisors. Only four fingers were present here, as in whale paddles and mesonychid hooves.
The origin of baleen might be with such genera (see below).