Recovered by phylogenetic analysis
The large reptile tree (LRT 1040 taxa) nests desmostylians as proximal outgroups to the mysticetes. Mysticeti, like the gray whale Eschrichtius robustus (Fig. 2), have a large robust tail and no visible hind limbs. That’s just the opposite of what you find in what little we know of known desmostylians (Fig. 1). Noteworthy: we don’t know the tail length in several of the long-jawed desmostylians, the ones most like mysticetes. This is when you have to put away traditional biases and let the cladogram truly be your guide.
In similar fashion
pterosaur workers have been looking in vain for a long manus reptile as the proximal ancestor to the Pterosauria, overlooking taxa with small forelimbs like Cosesaurus, Sharovipteryx and Longisquama. All this time, they did not realize the elongation of the forelimb elements to various degrees was the LAST transformation that occurred in the known outgroups to the Pterosauria (Peters 2002). You, too, will make discoveries like this when you put away traditional biases, expand your taxon list and let the cladogram be your guide.
As you might remember
the origin of the Odontoceti (toothed whales) started with swimming tenrecs like Leptictidium (Fig. 3) and its close relative Pakicetus, both with a very long tail and large feet.
Barnes LG 2013. A new genus and species of Late Miocene Paleoparadoxiid (Mammalia, Desmostylia) from California. Contributions in Science 521:51-114.
Peters D 2002. A New Model for the Evolution of the Pterosaur Wing – with a twist. – Historical Biology 15: 277–301.