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.

Image courtesy of Digimorph.org and used with permission. Click here for YouTube video..

Siamotherium pondaungensis (Suteethorn et al. 1988; Soe et al. 2017; Eocene) was originally considered a small anthracothere close to Hippopotamus, but here nests with the marsupial, Dromiciops (above).

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

Soe AN et al. (6 co-authors) 2017. New remains of Siamotherium pondaungensis (Cetartiodactyla, Hippopotamoidea) from the Eocene of Pondaung, Myanmar: Paleoecologic and phylogenetic implications. Journal of Vertebrate Paleontology 37(1):e1270290https://doi.org/10.1080/02724634.2017.1270290

Suteethorn V, Buffetaut E Helmcke-Ingava Rt JaegerJ-J and Jongkanjanasoontorn Y 1988. Oldest known Tertiary mammals from South-East Asia: Middle Eocene primate and anthracotheres from Thailand. Neues Jahrbuch f€ur Geologie und Palaontologie, Monatshefte 9:563–570.

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.

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