Didelphis virginiana (extant, Kerr 1792) is the Virginia 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.

Didelphis marsupialis (extant, Linnaeus 1758) is the common opossum found from Mexico and the Caribbean to Bolivia. It has three large molars and a fourth vestige. It also has postfrontals (orange) and a relatively shorter skull.

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.

Kerr R 1792. The Animal Kingdom, or Zoological System, of the celebrated Sir Charles Linnæus; being a translation of that part of the Systema Naturæ

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.

Lucas SG and Kondrashov PE 2004. A new species of Deltatherium (Mammalia, Tillodntia) from the Paleocene of New Mexico. In Lucas SG, Zeigler KE. and Kondrashov PE, eds., 2004, Paleogene Mammals, New Mexico Museum of Natural History and Science Bulletin No. 26.

Matthew WD 1937. Paleocene faunas of the San Juan Basin, New Mexico: Transactions of the American Philosophical Society, new series 30: 1-510.

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

Williamson T et al (4 co-authors) 2022. Triangular beast: new fossils shed light on Deltatherium, an enigmatic early Paleocene mammal from New Mexico. SVP abstracts p366.

wiki/Common_opossum
wiki/Virginia_opossum