Monodelphis domestica (Burnett 1830, Pine et al. 2013) is the extant gray short-tailed opossum. Monodelphis is traditionally considered a member of Didelphidae, a group of exclusively New World opossums thought to be basal among the living marsupials.

Despite having prepubes, Monodelphis does not have a pouch and skeletal traits nest it at the base of the Eutheria, the placental therians. Monodelphis is approximately 50 times smaller in body size than Didelphis, The dental formula is: 5.1.3.4 and 4.1.3.4 as in other didelphids. Note (below) that even smaller species of Monodelphis, like M. kunis, are extant and known. These have three molars per side, as in many basal placentals.

Kraus and Fadem 1987 report: litter size ranged from 2 to 13 with a m:f gender ratio of 1:1. Females typically have 13 teats that can be retracted into her body. After a 14-day gestation, 1cm young emerge and attach to a teat. The young grow hair after 3 weeks, open their eyes a week later, and are weaned at eight weeks. Sexual maturity occurs after 5 months of age and monodelphids live to 4 years old.

Images from Digimorph.org used with permission.

Amphicyon major (Blainville 1841, MNHN F Sa844, Middle Miocene, 13mya) is a traditional large 'bear-dog". Here this specimen nests among the herbivorous condylarths, derived from Monodelphis domestica, close to Hyopsodus.

Hyopsodus lepticus (H. paulus type, Leidy 1870; Late Eocene, 56-45 mya; ~20 cm in length; AMNH 143783) was originally considered a condylarth or a perissodactyl, an odd-toed ungulate, despite having a five-clawed manus and a four-clawed pes. Three molars are present.

Blainville H 1841. Osteographie et description iconographique comparée du squelette et du systéme dentaire des cinq classes d’animaux vertébrés récents et fossiles. Vol. 6, Carnassiers. Paris: Arthus Bertrand.

Burnett GT 1830. Illustrations of the Quadrupeda, or Quadrupeds, being the arrangement of the true four-footed Beasts indicated in outline. Quarterly Journal of Science, Literature and Art, July to December, 1829, 336–353.

Carvalho, B de A, et al (5 co-authors) 2011. Phylogenetic relationships and phylogeographic patterns in Monodelphis (Didelphimorphia: Didelphidae). Journal of Mammalogy. 92(1): 121-133.

Kraus DB and Fadem BH 1987. Reproduction, development and physiology of the gray short-tailed opossum (Monodelphis domestica). Lab Anim Sci 37(4):478–482.

Leidy J 1870. Remarks on a collection of fossils from the western territories. Proceedings of the Academy of Natural Sciences of Philadelphia 22: 109–110.

wiki/Monodelphis
wiki/Hyopsodus
wiki/Amphicyon