Didelphis marsupialis (extant, Linnaeus 1758) is the extant opossum. Galdai is basal to Didelphis and Ambolestes, which nest basal Monodelphis and other marsupials in the lineage of Placentalia.

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.

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.

wiki/Didelphis