At the base of the Eutheria (placental mammals)
nests the small, extant, didelphid marsupial Caluromys. So it’s worthwhile to put the skulls of a few basal placentals next to Caluromys to see what the similarities and differences are.
In the large reptile tree (LRT, 1272 taxa) Caluromys nests with Vulpavus (Fig. 1, Eocene; Marsh 1871), a basalmost member of the placental clade, Carnivora. Two molars characterize this clade. Juvenile Caluromys (Fig. 2; Flores, Abdala and Giannini 2010) also have two molars.
(pen-tailed tree shrew, extant, Le Gros-Clark 1926) is best compared, both in size and morphology to the juvenile Caluromys (Fig. 2). Though not a permanent member of the LRT, a test nested the juvenile Caluromys with Ptilocercus.
Caluromys derbianus (Allen 1904; Flores, Abdala and Giannini N 2010, Fonseca and Astúa 2018. ) is the living ‘wooly opossum’, native to Central America. This taxon nests just inside of the first placental clade, Carnivora, despite retaining a marsupium (pouch). It is an omnivore, as in related basal Carnivora, like Nandinia.
Juvenile skulls have only two molars, the same as those found in Vulpavus (below) and other Carnivora, so this trait is neotonous in Carnivora.
Allen JA 1904. Mammals from southern Mexico and Central and South America. Bulletin American Museum of Natural History 20(4): 29-80.
Flores DA, Abdala F and Giannini N 2010. Cranial ontogeny of Caluromys philander (Didelphidae: Caluromyinae): a qualitative and quantitative approach. Journal of Mammalogy 91(3):539–550.
Fonseca R and Astúa D 2018. Geographic variation in Caluromys derbianus and Caluromys lanatus (Didelphimorphia: Didelphdiae) Zoologica 32(2):109–122.
Heinrich RE and Rose KD 1997. Postcranial morphology and locomotor behavior of two early Eocene miacoid carnivorans, Vulpavus and Didymictis. Palaeontology 40:279-305
Le Gros-Clark WE 1926. On the Anatomy of the Pen-tailed Tree-Shrew (Ptilocercus lowii.) Proceedings of the Zoological Society of London 96: 1179-1309.
DOI – 10.1111/j.1096-3642.1926.tb02241.x
Marsh 0C 1871. Notice of some new fossil mammals and birds from the Tertiary formations of the West. American Journal of Science, Series 3, 2: 120-127