Continuing to clean up a recalcitrant basal Theria…
further reexamination finds overlooked similarities and another taxon shift. Here, the similarities are obvious as a former basal carnivorous placental moves next to a basal carnivorous marsupial.
Deltatherium fundaminis (Cope 1881, middle Paleocene 60 mya, 14 cm skull length, AMNH 16610, Figs. 1, 2) this former creodont and former tillodont now nests with the most basal carnivores of the Metatheria like Arctocyon (Fig. 1). Wikipedia reports, “its relatives are far from clear.” The large cranial crest anchored large jaw muscles for driving in those long saber teeth. These taxa are derived from the Virginia opossum, Didelphis (Fig. 1).
Figure 1. Arctocyon surrounded by related taxa to scale, including Deltatherium.
Arctocyon primaevus (Blainville 1841, Gould and Rose 2014; YPM VP 021233; Paleocene, 60 mya) was long and widely considered a primitive plantigrade ungulate condylarth procreodi placental. In the LRT Arctocyon nests with basal carnivorous/omnivorous marsupials. Essentially it is a giant opossum, like Didelphis, but with a few derived traits, more like Thylacinus, a taxon that reduces the epipubes and molar count, hence the earlier traditional confusion.
Figure 2. Deltatherium nests with Arctocyon as a basal carnivorous marsupial.
References
Blainville HM 1841. Osteographie et description iconographique des Mammiferes récentes et fossiles (Carnivores) 1, 2 Paris.
Cope ED 1881. Mammalia of the lower Eocene beds. American Naturalist 15:337-338.
Gould FDH and Rose KD 2014. Gnathic and postcranial skeleton of the largest known arctocyonid ‘condylarth’ Arctocyon mumak (Mammalia, Procreodi) and ecomorphological diversity in Procreodi. Journal of Vertebrate Paleontology 34(5):1180-1202.
The Pterosaur Heresies 