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.

Figure 1. Arsinoitherium in several views. DGS colors added here. ” data-image-caption=”

Figure 1. Arsinoitherium in several views. DGS colors added here.

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/09/arsinoitherium588.jpg?w=126″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/09/arsinoitherium588.jpg?w=431″ tabindex=”0″ role=”button” class=”size-full wp-image-88930″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/09/arsinoitherium588.jpg” alt=”Figure 1. Arsinoitherium in several views. DGS colors added here.” width=”584″ height=”1387″ />

Figure 1. Early Oligocene Arsinoitherium in several views. DGS colors added here.

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.

Figure 2. Skull of Early Eocene Pholidocercus in three views. ” data-image-caption=”

Figure 2. Skull of Early Eocene Pholidocercus in three views.

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/09/pholidocercus-skull588-1.jpg?w=180″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/09/pholidocercus-skull588-1.jpg?w=584″ tabindex=”0″ role=”button” class=”size-full wp-image-88931″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/09/pholidocercus-skull588-1.jpg” alt=”Figure 2. Skull of Early Eocene Pholidocercus in three views.” width=”584″ height=”972″ />

Figure 2. Skull of Early Eocene Pholidocercus in three views. Compare to Arsinoitherium in figure 1. Transitional taxa are not known. Note the tiny premaxillary ascending process in dorsal view.

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.

Figure 3. Only one of the several Messel Pit Pholidocercus specimens. This one has a truncated tail and a halo of soft tissue (pre-spines). ” data-image-caption=”

Figure 3. Only one of the several Messel Pit Pholidocercus specimens. This one has a truncated tail and a halo of soft tissue (pre-spines).

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/09/pholidocercus-insitu588.jpg?w=300″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/09/pholidocercus-insitu588.jpg?w=584″ tabindex=”0″ role=”button” class=”size-full wp-image-88934″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/09/pholidocercus-insitu588.jpg” alt=”Figure 3. Only one of the several Messel Pit Pholidocercus specimens. This one has a truncated tail and a halo of soft tissue (pre-spines). ” width=”584″ height=”374″ />

Figure 3. Only one of the several Messel Pit Pholidocercus specimens. This one has a truncated tail and a halo of soft tissue (pre-spines). The premaxillary ascending process is visible here, too.

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.

Figure 3. Ceratogaulus, the extinct horned gopher ” data-image-caption=”

Figure 3. Ceratogaulus, the extinct horned gopher

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2018/07/ceratogaulus_hatcheri588.jpg?w=153″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2018/07/ceratogaulus_hatcheri588.jpg?w=524″ tabindex=”0″ role=”button” class=”size-full wp-image-32244″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2018/07/ceratogaulus_hatcheri588.jpg” alt=”Figure 3. Ceratogaulus, the extinct horned gopher” width=”584″ height=”1142″ />

Figure 3. Ceratogaulus, the extinct horned gopher – by convergence.

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.

And, as Steve Jobs used to say, “one more thing…”
the premaxilla redevelops an ascending process, a trait not seen in mammals since their origin in the Triassic. Gobiatherium and Pholidocercus share this trait. Phylogenetic bracketing suggests Orthaspidotherium may have had one, too, but it is taphonomically missing. They are all related to each other in the LRT.

Arsinoitherium: No longer alone in the traditional order Embrithopoda
According to ucmp.berkeley.edu: “Embrithopods looked something like rhinos but were more closely related to elephants. Arsinoitherium had two great bony horns on its head, unlike living rhinos, in which the horns are not bony but formed from modified hair papillae. It is now known that not all embrithopods had horns. Until recently, Arsinoitherium, from the Oligocene of Fayum, Egypt, was the only embrithopod known. In the late 1970s, a few more fossil embrithopod species were described from Mongolia, Turkey, and Romania.”

These are scrappy taxa
known from teeth or, at best and dubiously, a palate in Middle Eocene Namatherium (Fig 4, Pickford et al 2008). That description reports, “Arsinoithere in which the zygomatic arches flare strongly laterally” which is the oppsite of Arsinoitherium and Pholidocercus. Pickford et al wrote, “The Black Crow arsinoithere diverges greatly from other genera of embrithopods, both in its cranial festures and it dental ones.”

IMHO Namatherium is a doubtful relative since tiny Pholidocercus is so similar to giant Arsinoitherium. They don’t flare their cheekbones = zygomatic arches.

Figure 4. Namatherium palate. ” data-image-caption=”

Figure 4. Namatherium palate.

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/09/namatherium588.jpg?w=300″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/09/namatherium588.jpg?w=584″ tabindex=”0″ role=”button” class=”size-full wp-image-88964″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/09/namatherium588.jpg” alt=”Figure 4. Namatherium palate. ” width=”584″ height=”469″ />

Figure 4. Namatherium palate.

Pickford et al had an outdated phylogeny in mind
when they wrote, “It is thus likely that arsinoitheres are phylogeneticlly closer to proboscideans than they are to hyracoids, a conclusion already reached by Court 1992.”

Let’s keep looking for a close relative for Namatherium, perhaps similar to Numidotherium (Fig 5), an elephant ancestor in the LRT, which has flared cheekbones.

Figure 5. Numidotherium, an elephant ancestor in the LRT, seen here in several views. DGS colors added here. ” data-image-caption=”

Figure 5. Numidotherium, an elephant ancestor in the LRT, seen here in several views. DGS colors added here. Compare the palate to Namatherium in figure 4.

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/09/numidotherium.skull588.gif?w=144″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/09/numidotherium.skull588.gif?w=492″ tabindex=”0″ role=”button” class=”size-full wp-image-88966″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/09/numidotherium.skull588.gif” alt=”Figure 5. Numidotherium, an elephant ancestor in the LRT, seen here in several views. DGS colors added here.” width=”584″ height=”1216″ />

Figure 5. Numidotherium, an elephant ancestor in the LRT, seen here in several views. DGS colors added here. Compare the palate to Namatherium in figure 4.

Say goodbye to Paenungulata
According to Wikipedia, “In 1945, George Gaylord Simpson used traditional taxonomic techniques to group these spectacularly diverse mammals in the superorder he named Paenungulata (“almost ungulates“), but there were many loose threads in unravelling their genealogy. For example, hyraxes in his Paenungulata had some characteristics suggesting they might be connected to the Perissodactyla (odd-toed ungulates, such as horses and rhinos). Indeed, early taxonomists placed the Hyracoidea closest to the rhinoceroses because of their dentition.”

In the LRT hyraxes are indeed basal members of the Perissodactyla. Arsinoitherium is a much more basal placental herbivore without tusks and maxillary diastema, but with that rare premaxillary ascending process.

This appears to be a novel hypothesis of interrelationships.
If not, please provide a citation so I can promote it here.

References
Beadnell HGC 1902. A preliminary note on Arsinoitherium zitteli, Beadnell, from the Upper Eocene strata of Egypt. Public Works Ministry, National Printing Department. Cairo: 1–4.
Pickford M et al (4 co-authors) 2008. Mammalia from the Lutetian of Namibia. Memoir Geol. Surv. Namibia, 20, 465-514.

wiki/Arsinoitherium
wiki/Embrithopoda
ucmp.berkeley.edu/mammal/mesaxonia/embrithopoda
wiki/Paenungulata