Borax, winds of change

Who're these aliens? Who's Sirocco? Where they come from? Why they arrived in Antarctica? There're a lot of questions, and you're in the right place for some answers.
Sirocco and its kind are something new for Antarctica, more or less: they are marsupials. But not just an ordinary marsupial, a very derivate one: the member of its species are commonly known as borax* (Glacithylacus alienus) and their appearance would resemble more a bear than any marsupial that you holocenic people are used to see.  It's ancestrally a didelphid, an american opossum, which have reached the top of the trophic chain.
It can attain up to 200 kg for adult males, however their average weight is usually around 100 kg or even less for females. The colour of its fur is perfect for camouflaging in the white environment where it live, resembling a very polar bear. However, unlike it, its fur is not transparent, but mostly white with some brown and grey areas on the feet. They are fiercy generalist predators that feed on anything they can find, both on land and seas, yet because of their environment they usually hunt marine faunas, like aquatic birds and fish, both on land and sea. 

Borax abduct their prey by using their long hind claws and then using their long carnassials for shearing the skin of the unfortunate prey. Teeth arrangement is similar to the one of a dog, but the jaw muscle are even 40% bigger, resulting in a higher bite force, which is useful to pierce the fatty skin of marine vertebrates. Canines are relatively long and, interestingly, moderately serrated, which help borax to subdue their prey even faster.
Like Sirocco shows, borax are mainly solitary mammals that possess their territory which is aggressively defended from intruders, including predators of other species. It's seems that females share part of their home range with the larger males, but their presence in Antarctica is still limited in the continent, being found only in the Ponti Archipelago. However, the breeding population of borax is slowly spreading outside of the Archipelago, with a trend towards west, where larger populations of reefsurfers can be found.

We're used to see American opossums as small, second-rate mammals, but they are more versatile than they seem. Didelphids are generalists capable of adapting to a wide variety of habitats and, during the Holocene, they made up about one-third of all marsupial species (about 100 confirmed species). Some species even took evolutionary paths toward becoming apex predators, with certain lineages developing hypercarnivory. Among the largest there was Thylophorops, a carnivorous Pliocene species that could reach nearly 10 kg. 

The arrival of placental carnivorans in South America seems to have coincided with the decline of many carnivorous marsupials. However, this decline was more likely driven primarily by climate change rather than direct placental competition. Evidence shows that carnivorous marsupials coexisted with placental carnivorans for millions of years and the latters started to become more common only after the disappearance of carnivorous marsupials. Some hypercarnivorous didelphids, such as species in the genus Lutreolina, persist even today and bear a strong resemblance to mustelids.
The false idea that marsupials are less competitive than placentals also doesn't match with the fact that didelphids expanded their range into Central and North America instead of becoming rarer after the connection ot the two Americas.
We don't know much about the current tectonic plate activity, but the presence of borax suggests that didelphids may have flourished on other continents during the Biancocene, Cambiocene, and Lentocene epochs, with at least one lineage that recently brought to apex predator species. These epochs likely saw the emergence of new hypercarnivorous species, some of the largest ever evolved within the didelphid group. A small mass extinction event, possibly linked to the Antarctic Traps, might have benefited these marsupials.: their generalist diet and adaptability would have given them an advantage over more specialized placental species during times of environmental changes.

Where

Sirocco has arrived in Antarctica from an undetermined continent by crossing the Ponti Archipelago, a large icy area that connects Antarctica to other continent since at least 400.000 years ago. With the worsening of the climate, this small strip of ice has become a true ice bridge at least 2 km wide. An ice bridge between two worlds, that not only Sirocco have crossed: some other animals, like ottofoxes, banchisaraptors managed to cross the border of Antarctica, leaving by foot the land that created and shaped them for millions of years.
Technically speaking, the project should end now: I've fulfilled my mission of narrating these Antarctic Chronicles until the continent'll reunite with other continents. Practically, Antarctica still remain geologically separated from the other continent: the only thing that connect it to other landmasses is a sea ice, most of it a seasonal one, which is impossible to cross for almost any animal and plant. The chronicles of the icy continent will continue, waiting patiently for the greatest biotic interchange that Antarctica has ever seen. Now, our interest is all focused on Sirocco and its future, together with the future of its species in Antarctica. And together with Sirocco, another species has recently has set foot on the icy continent.
How the future of this continent will change after the arrival of Sirocco and the others? It's something that we will know only in the next millennia.

*Borax plural is still borax