Masrasector: not a placental… a killer marsupial

Of course, by that I mean,
skunk-sized Masrasector nananubis (Fig. 1; Simons and Gingerich 1974; Borths and Seiffert 2017), like other hyaenodonts in the LRT (Fig. 2), is also a marsupial… a descendant of a sister to the quiet little, nocturnal Virginia opossum (Didelphis, Fig. 2), but closer to Borhyaena.

[Note: everyone agrees that hyaenodonts are NOT related to hyaenas.]

Taxon exclusion,
based on tradition, evidently keeps marsupials out of current phylogenetic analyses of hyaenodonts. Hopefully that will be corrected, or at least tested, in the future.

like Masrasector, are traditionally considered placental (eutherian) carnivorous creodonts. They have not one, but three carnassial teeth.

According to the LRT,
that was by convergence. Certain bats also have carnassial teeth. So it can happen.

By contrast
the large reptile tree (LRT, 1066) nests Masrasector and all other hyaenodonts as marsupial carnivores. Borths and Seiffert (2017) did not expand their taxon list to include marsupials and thus, taxon exclusion implanted a deadly flaw in their otherwise brilliant and technical paper.

Figure 3. Arctocyon is no longer an ungulate placental, but a carnivorous marsupial, close to Thylacinus.

Figure 2. Arctocyon is no longer an ungulate placental, but a carnivorous marsupial.

Borths and Seiffert note:
“Body mass in hyaenodonts is difficult to estimate because there are no living taxa analogous to these large-headed placental carnivores with multiple carnassials.”

“The most surprising part of this study was that Masrasector and its kin are part of a massive radiation of hyaenodonts that originated in Africa.” 

“Hyaenodonts have been a little neglected as a group.”

Wikipedia reports on the history of the Creodonta:
“Creodonta” was coined by Edward Drinker Cope in 1875. Cope included the oxyaenids and the viverravid Didymictis but omitted the hyaenodontids. In 1880. he expanded the term to include MiacidaeArctocyonidaeLeptictidae (now Pseudorhyncocyonidae), OxyaenidaeAmbloctonidae and Mesonychidae.[12] Cope originally placed creodonts within the Insectivora. In 1884, however, he regarded them as a basal group from which both carnivorans and insectivorans arose. Hyaenodontidae was not included among the creodonts until 1909. Over time, various groups were removed, and by 1969 it contained, as it does today, only the oxyaenids and the hyaenodontids.”

Figure 2. Masrasector nests with Borhyaena in the marsupial clade.

Figure 3. Masrasector nests with Borhyaena in the marsupial clade.

Earlier we looked at members at the base of this clade, like Amphicyon and Arctocyon (Fig. 2), giant closer descendants of Didelphis. Hyaenodonts and other creodonts are different from members of the Carnivora because they’re not placentals, something that has escaped the notice of paleontologists. Creodonts / hyaenodonts appear earlier in the fossil record because marsupials appear before placentals.

Borths MR, Seiffert ER 2017. Craniodental and humeral morphology of a new species of Masrasector (Teratodontinae, Hyaenodonta, Placentalia) from the late Eocene of Egypt and locomotor diversity in hyaenodonts. PLoS ONE 12(4): e0173527.
Simons EL, Gingerich PD 1974. New carnivorous mammals from the Oligocene of Egypt. Annals of the Geological Survey of Egypt. 1974; 4: 157–166.
Author interview online here. podcast interview here.

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