Dromiciops gliroides, (Thomas 1894, Marshall 1978) is the extant, monito del monte or colocolo opossum, a tiny living basal marsupial from South America. The middle ear (endotympanic + alisphenoid (epiterygoid) is greatly inflated. Note the canine-like premolar 1. The canine is absent in this clade. Siamotherium (below) is a larger, extinct relative.

Hadrocodium wui (Luo, Crompton and Sun 2001; Early Jurassic; skull length: 1.2cm), known only from a skull. Hadrocodium was originally considered a juvenile basal mammal, due to the emerging teeth and low molar number. The first premolar replaces the lost canine. A close relative is the living Dromiciops..

Gracilinanus agilis (Gardner and Creighton 1989 This South American gracile mouse opossum nests at the base of the Placentalia 1 clade, so is basal to Nasua and humans. It was separated from the genus Marmosa in 1989. Diet that includes insects, spiders, fruits, termites, beetles and ants, A litter size of up to 15, but usually 7. Females lack a pouch. The young are only attached to the nipples for a short time period and are left in their nests early on in their life.

Gardner AL and Creighton GK 1989. A new generic name for Tate's microtarsus group of South American mouse opossums (Marsupialia: Didelphidae). Proceedings of the Biological Society of Washington 102:3–7.

Gray JE 1821. On the natural arrangement of vertebrose animals. London Medical Repository 15(1):296–310.

Marshall LG 1978. Mammalian species, Dromiciops australis, American Society of Mammalogists, pp. 1-5. pdf

Thomas O 1894. On Micoureus griseus, Desm., with the description of a new genus and species of Didelphyidae. Ann. Mag. Natural History Ser 9, 3:199-212.

wiki/Dromiciops
wiki/Gracilinanus