Gondwannan Metatherians Triplex

Biogeography of the Eocene Pontide mammal fauna of Turkey. Note an African origin for local Polydolopiformes.

So some time ago I ruminated on a rumination. This rumination became somewhat vindicated by a recent study on the Eocene fauna of Turkey.

Basically, Turkey at the time, as evidenced by the “Pontide mammal fauna” of the Lülük Member of the Uzunçarşıdere Formation, was an island continent. It lacked rodents, carnivorans, hyaenodonts and ungulates, but it did include pleuraspidotheriids (a lineage of herbivorous mammals that ocurred across Europe in the Paleocene but have since became extinct elsewhere outside of Turkey), embrithopod afrotherians as well as a massively diverse array of metatherian mammals.

This includes Anatoliadelphys maasae and the closely related Orhaniyeia nauta, both specialised carnivores and clear evidence that this Turkey island was operating on the South American school of metatherians at the top of the food chain and placentals being bush-burping, stem-smelling, garden-gorging, plant-popping, tree-tasting, dirt-devouring beasts!

Anatoliadelphys maasae by Peter Schouten.

But the curious thing about these carnivorous metatherians was their origin. Most Turkish metatherians are peradectids or herpetotheriids, both clades found in Laurasia, but these carnivores and a few related taxa are instead Polydolopiformes, a clade of metatherians otherwise best known from South America and possibly also the Eocene of Australia in the Murgon fossil site.

Jaws of the (putative) Polydolopiformes Epidolops ameghinoi (A) andKramadolops abanicoi (B), the argyrolagid Argyrolagus primus © and the modern shrew-opossum Lestoros inca (D).

Polydolopiformes themselves are a complicated bunch, sometimes recovered as stem-shrew-opossums, sometimes as part of a clade of gondwannan metatherians that also includes sparassodonts (which I mentioned previously in said rumination) and sometimes as entirely polyphyletic.

In this study, they are recovered as a fully monophyletic clade possibly nested inside of Marsupialia alongside several other south american and australian taxa. And, of course, these anatolian species.

Metatheria cladogram in this study. Notice that Pucadelphys, a taxon typically included in the sparassodont clade, is now a marsupial related to shrew-opossums.

The paper remains cautious and still refers to these taxa as metatherians rather than marsupials, but at any rate the idea of an unified sparassodont + Polydolopiformes seems to have gone murky. Note that the supposed Cretaceous North American sparassodont Varalphodon may not actually be a sparassodont.

If Polydolopiformes are marsupials, its interesting to note that so is Glasbius, a Cretaceous north american genus. This indicates multiple colonisations of South America by crown-group marsupials(!). Whereas Varalphodon is a sparassodont or not, the Mongolian Gurlin Tsav skull still appears to be related to them, and the Eocene metatherian Eobrasilia coutoi appears to be a stagodontid, indicating that there were in fact multiple colonisation events on the part of metatherians towards Gondwanna. Most surely seem to have come from North America, but I do still hope that sparassodonts may have taken the Indian route.

Because the anatolian taxa are deeply nested among the gondwannan Polydolopiformes, its been suggested that they arrived not from Laurasia but from Africa, alongside the contemporary embrithopods. Currently there are no fossils of metatherians in Africa from the early Paleogene — the earliest are Oligocene herpetotheriids, which clearly got there by rafting from Europe — but the Paleogene african fossil record is noted as notoriously poor anyways, and there are no Polydolopiformes in the better preserved fossil records of Europe and Asia. Still, possible Polydolopiformes are known from North America, so while definitely more romantic a Gondwannan origin is not fully in the cards yet; for all we know Polydolopiformes could have been in the Paleocene of Europe, just briefly enough to become isolated in Turkey.

A thing definitely worth mentioning is the ecology of these animals. Polydolopiformes are typically constructed as seed specialists and frugivores, many genera even bearing plaugiaulacoid teeth similar to those of multituberculates and still living rat-kangaroos. Yet, these anatolian taxa are specialised carnivores with bone-crushing teeth, as is the possibly related Archaeonothos henkgodthelpi from the Murgon fossil site. Assuming that these speciations towards durophagy didn’t evolve independently, there was apparently a guild of specialised bone-crushing metatherians roaming Australia, Antarctica, Africa and Turkey, eventually displaced by other metatherians and placentals.

Lastly, this paper throws the idea that eastern Europe was an ecological corridor between western Europe, Africa and India during the Paleogene completely out the window, since Turkey is right in the middle of these places and it clearly harboured an unique island fauna unlike that of anywhere else in the epoch. Groups like hyaenodonts, caecilians, pleurodire turtles, frogs and possibly ostriches must have either gotten around between western Europe and India either through Africa or through Asia.

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