The last antarctic mammal

Paleontological findings have become increasingly important in Antarctica in the last millennia, due to the ongoing deglaciation. Lots of new fossil formations have been found, which have helped a lot at understanding the paleobiogeography of the continent. The large fossil gap of Antarctica between Eocene and Holocene is now slowly disappearing with the discovery of new terrestrial biotas. 

Tetrapod's fossils are still uncommon and often incomplete, but the few of them are very informative: one of the most important finds is probably a fragmentary fossil of a mammal from the late pliocenic Meyer Desert Formation (Discovery date: 25th January 4237). This formation was already important in the past for the paleobotanical associations, which included even woody species (Nothofagus shrubs and podocarps). The proposed paleoenvironment is a shrubby coastal tundra that would resemble a lot modern-day Hermite Islands of Cabo de Hornos.
The mammal fossil, excavated with special intraplanetary rovers, was instantly shipped overseas for scientific purposes and it was later described by the south american paleontologist Mattias Papera.
The remain was the youngest terrestrial mammal ever found in Antarctica, dated to just 3 million years old. It was classified, with some difficulties, as a very specialized and bizarre marsupial of the Microbiotheria order, now represented by a single species, the monito del monte (Dromiciops gliroides). The fossil was named Criobiotherium relictus (Papera, 4237), but it became informally known as Monito del hielo ("little monkey of the ice") in the scientific community.

Compared to its modern relative, the monito del hielo was exceptionally large: while the first one doesn't exceed 42 grams, the inferred mass of Criobriotherium relictus is 10-15 kg! It's also interesting the presence of a large patella, which is absent in Dromiciops. Some of the teeth found suggest that the monito del hielo was mainly a carnivore, but this assumption is in contrast to the lack of other terrestrial vertebrates in the Meyer Desert Formation. It was initially speculated that the carnivorous diet of C. relictus foreshadows the presence of some terrestrial mammals, but this hypothesis was lately contested since the left hind limb morphology of the fossil apparently suggests a small degree of semiaquatic lifestyle: it's probable that the monito del hielo was primarily a sea-dependent predator that mostly fed on sea wastes, breeding seabirds (including penguins) and young seals. The tundra environment was probably extremely fragmented and didn't allow the persistence of large herbivores, which rapidly went extinct at the start of the Pliocene due to glacial expansion.
In the following centuries, other remains of Criobiotherium relictus were found in other fossil deposits as north as the antarctic peninsula. At that time, West Antarctica formed a tundra archipelago analogous to the Canadian Arctic Archipelago: it's probable that seasonal sea ice corridors allowed this marsupial to move from an island to another. This behaviour is already documented in some arctic carnivores, which often migrate from a continent to another by using the sea ice: for example, in 2018, an arctic fox walked from Norway (Svalbard) to Canada (Ellesmere Island) in just 76 days.