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.

Monodelphis sorex (Burnett 1830, Pine et al. 2013) is the extant southern red-sided opossum. It is found on the Atlantic coast of South America. No pouch is present. The young cling to their mother's nipples, then, when older, ride on her back. Postfrontals are retained on this smaller taxon, missing in Monodelphis domestica.

Ptolemaia grangeri (P. lyonsi, Osborn 1908; Bown and Simons 1987, Simons and Bown 1995, Miller et al 2015; Oligocene) is a traditional enigma taxon, but nests here with extant Monodelphis sorex. The image shows the extent of the premaxilla and tall postparietal.

Tetraclaenodon puercensis (Cope 1881, Scott 1892; Kondroshev and Lucas 2012, Paleocene) was originally considered a member of the Phenacodontidae, close to Phenacodus. Kondroshev and Lucas wrote, "The ungual phalanx is intermediate in morphology between a claw-like and a hoof-like phalanx".

Here in the LRT Tetraclaenodon is transitional between the smaller Monodelphis domestica and the larger Amphicyon major. Compare skulls to scale below. The nasals are here considered wider posteriorly than originally figured on this heavily cracked skull to match sister taxa.

Bown TM and Simos EL 1987. New Oligocene Ptolemaiidae (Mammalia:?Pantolesta) from the Jebel Qatrani Formation, Fayum Depression, Egypt. Journal of Vertebrate
Paleontology 7:311–324.

Burnett GT 1830. Illustrations of the Qwuadrupeda, 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.

Cope ED 1881. On some Mammalia of the lowest Eocene beds of New Mexico. Palaeontological Bulletin 33:484-495.

Kondrashov PE and Lucas SG 2012. Nearly complete skeleton of Tetraclaenodon (Mammalia, Phenacodontidae) from the Early Paleocene of New Mexico: Morpho-Functional Analysis. Journal of Paleontology 86(1):25–43.

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.

Miller ER et al (5 co-authors) 2015. Ptolemaia from West Turkana, Kenya. Bulletin of the Peabody Museum of Natural History DOI: 10.3374/014.056.0105

Osborn HF 1908. New fossil mammals from the Fayum Oligocene, Egypt. Bulletin of the American Museum of Natural History 24:265–272.

Scott WD 1892. A revision of North American Creodonta with notes on some genera which have been referred to that group. Proceedings of the Academy of Natural Sciences of Philadelphia, 4:291–323.

Simons EL and Bown TM 1995. Ptolemaiida, a new order of Mammalia–with description of the cranium of Ptolemaia grangeri. Proceedings of the National Academy of Sciences USA. 92 (8): 3269–73.

wiki/Ptolemaia
wiki/Monodelphis
wiki/Tetraclaenodon