Didelphis marsupialis (extant, Linnaeus 1758) is the extant opossum. Despite its ancient pedigree, what we know of Didelphis is the extant species, which has not changed much, but no doubt has changed in subtle ways since the Triassic. Note the reduction of the cranium making room for enlarged jaw muscles. The cervicals are tall and unyielding dorsoventrally.

Periptychus carinidens (Cope 1881; Shelley et al. 2018) was originally and later unable to determine closest relatives. Here it nests with tiny Monodelphis. The manus and pes are primitive in having no reduced fingers or toes, yet the unguals are reduced to stub-like round hooves.

Orthaspidotherium edwardsi (Ladeveze et al. 2010; Late Paleocene, 58 mya, 7.5cm skull length) nests with Periptychius in the LRT. The skull is twice as wide as tall on this weasel-like terrestrial mammal. The jawline is straight. The zygomatic arch is robust.

Cope ED 1881. The Condylarthra (Continued). American Naturalist 84;18: 892–906.

Ladevèze S, Missiaen P and Smith T 2010. First Skull Of Orthaspidotherium Edwardsi (Mammalia, "Condylarthra") From The Late Paleocene Of Berru (France) And Phylogenetic Affinities Of The Enigmatic European Family Pleuraspidotheriidae. Journal of Vertebrate Paleontology 30(5): 1559–1578.

Linnaeus C 1758. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.

Mohamed R 2018. Anatomical and radiographic study on the skull and mandible of the common opossum (Didelphis marsupialis Linneaus, 1758 (in the Caribbean). Veterinary Sciences 5(44) 10 pp. doi:10.3390/vetsci5020044

Novacek MJ, Rogier GW, Wible JR, McKenna MC, Dashzev g D and Horovitz I 1997. Epipubic bones in eutherian mammals from the Late Cretaceous of Mongolia Nature 389: 483-486.

Shelley SL, Williamson TE and Brusatte SL 2018. The osteology of Periptychus carinidens: A robust, ungulate-like placental mammal (Mammalia: Periptychidae) from the Paleocene of North America. PlosOne July 18, 2018. https://doi.org/10.1371/journal.pone.0200132

wiki/Didelphis