this appears to be just one more example of paleontologists wearing blinders, evidently waiting for an amateur without access to the fossil itself to reveal the answer to this long-standing enigma/paradox. It just took a little phylogenetic analysis.
According to Wikipedia
Paleoparadoxia tabatai (Reinhart 1959, 2.2m long; Miocene, 20-10 mya) “is a genus of large, herbivorous aquatic mammals that inhabited the northern Pacific coastal region. Originally interpreted as amphibious, Paleoparadoxia is now thought to have been a fully marine mammal like their living relatives, the sirenians, spending most of their lives walking across the sea bottom like marine hippos.”
(“ancient paradox”) is considered a member of the clade Desmostylia. Wikipedia reports, “The relationship between Desmostylia and the other orders within Tethytheria (elephants, hyraxes, sirenians) has been disputed. That assignment has been seriously undermined by a 2014 cladistic analysis that places anthracobunids and desmostylians, two major groups of putative non-African afrotheres, close to each other within the laurasiatherian order Perissodactyla” (odd-toed ungulates). That’s strange because Paleoparadoxia had four fingers and toes (Fig. 1), like a hippo (Fig. 2).
Carroll (1988) considered the hands and feet specialized as paddles and seal-like in their abilities, but the jaws were similar to those of primitive elephants with the exception that the canines were also tusk-like. He considered desmostylians close to proboscidians, even though hippos have a similar set of tusks that include the canines.
in the large reptile tree Paleoparadoxia nests strongly with Hippopotamus (Fig. 2) and Mesonyx members of the Artiodactylia (even-toed ungulates). I don’t think any prior studies have recovered this pretty obvious relationship, but several prior papers have noted the convergent size, body type and lifestyle. The nares have moved to the dorsal skull on Paleoparadoxia and a premaxillary ascending process reappears. Note the vertical orientation of the pelvis. The elevation of the ankles likely signaled the placement of hippo-like pads beneath the feet, rather than spreading toes creating paddles (contra Carroll 1988).
This cladistic nesting
keeps the topology of the large reptile tree simpler than traditional trees.
Carroll RL 1988. Vertebrate Paleontology and Evolution. W. H. Freeman and Co. New York.
Reinhart RH 1959. A review of the Sirenia and Desmostylia. University of California Publications in Geological Sciences 36(1):1–146.