Arsinoitherium and Pholidocercus compared for the first time

Early Oligocene Arsinoitherium
(Fig 1) was traditionally allied with elephants, hyraxes and sirenians, because it did not fit in well elsewhere. By convergence, Arsinoitherium had a graviportal stance with short elephantine toes – and it was neither a rhino nor a brontothere. Until recently (see below), Arsinoitherium stood alone in its own order, the Embrithopoda.

Which leads us to wonder, phylogenetically where does Arsinoitherium fit in best?

By adding taxa (= minimizing taxon exclusion)
in the large reptile tree (LRT, 2324 taxa), only elephants and sirenians remain closely related to one another. In earlier LRT studies Arsinoitherium nested close to Gobiatherium, with which it still shares a long list of traits, despite its flattened post-nasal skull. Hyraxes moved over to chalicotheres.

That changes today as Pholidocercus (Figs 2, 3) is reviewed with new insight and data.

A smaller (16cm), earlier taxon now enters the ancestry of Arsinoitherium.
Early Eocene narrow-skulled Pholidocercus (Fig 2) now nests closer to Arsinoitherium (Fig 1) for the first time here in the LRT. Horns were not present on guinea pig-sized Pholidocercus. Horns must have evolved later, as presently unknown transitional taxa grew to rhinoceros size and proportions.

Note the similarly-shaped skulls, 
(Figs 2, 3) the ascending process of the premaxilla, the compressed (in dorsal view) zygomatic arches and the ventrally directed acetabula. The full set of 48 teeth in Pholidocercus was reduced by only one molar and one incisor in Arsinoitherium, which also lacks a diastema between its teeth.

Pholidocercus is a traditional hedgehog ancestor, but hedgehog skulls and teeth are distinctly different.

This LRT pairing of small and large taxa came as a bit of a surprise,
but now seems to be a natural fit given early Cenozoic evolutionary trends among little scurrying herbivores evolving to become big, lumbering herbivores.

By convergence,
the gopher Ceratogalus (Fig 4) was another placental herbivore that developed twin rostral horns resembling those in Arsinoitherium.

No one knows what nasal horns were used for in Ceratogalus or Arsinoitherium.

Arsinoitherium zitteli
(Beadnell 1902, Figs. 1, 2; late Eocene, early Oligocene, 36-30 mya) was derived from a sister to a much smaller Pholidocercus. Wikipedia describes Arsinoitherium as, “an extinct genus of paenungulate mammal belonging to the extinct order Embrithopoda. It is related to elephants, sirenians, hyraxes and the extinct desmostylians.” None of these candidates have any hint of the huge nasal and tiny frontal horns that make Arsinoitherium distinct.

Another set of traits
that makes Arsinoitherium distinct from other placentals, but similar to Pholidocercus, is its complete arcade of teeth (lacking a diastema), none of them tusks, in which the canine is indistinct and the medial incisor is missing.

Figure 4. Namatherium palate.