Chronicles of an alien

Over the last 5 million years, the surviving biancocenic species began to speciate, filling the empty habitats left by the retreating glaciers. This included borax and rompos, two alien species that reached Antarctica by traveling across the sea ice. 

Borax did not diversify extensively, with only about five species known today, yet their ecological role is fundamental. They are the top predators of the continent, the first ones capable of subduing geotters after millions of years of their dominance.
Some borax species have retained a size similar to their ancestors, while others have become relatively dwarfed, weighing “just” 50 kg. Their ecology has diversified far more than in the past: only one dwarf species remains adapted to marine life (which is restricted to Boitomb Island), while the other four have gradually become more terrestrial. All borax are fierce ambush predators, relying on stealth rather than speed, as their plantigrade locomotion limits fast pursuit. The rise of large lush forests make this adaptation very suitable in almost every area of the continent.
Among the largest species, the austral borax (Thylacoursus revolutus) is surely a fearsome sight. This mammal is one of the continent’s largest land carnivore and it's adapted to the dense undergrowth of Antarctica, despite some populations can also live in the shrub tundra in the southern part.
Though similar in mass to its ancestors, the austral borax has a less robust, more elongated build, an adaptation for terrestrial movement reminiscent of extinct bear dogs (Amphicyonidae). Limb muscles however are still enormous and they are fundamental weapons to take down prey. It's the most widespread species of borax and because of this they can vary their size a lot, depending on the latitude, following the Bergmann's rule: southern populations can be 40% larger, weighing on average 70 kg, while northern populations of the Great Depresseaon average around 50 kg.

Despite their size, they are elusive hunters, capable of hiding their presence from prey thanks to their striped coloration. Like all borax, the austral borax possesses serrated canines that allow for quicker kills, similarly to a sabretooth cat. They are not extremely long so they resemble more the one of dirk toothed cats, like Homotherium. The species use brutal force to take down its prey and only after the prey collapse to the ground the predator use its teeth to cut through the troath, causing a massive loss of blood in the prey.
Few animals can defend themselves against these solitarily predators; only some large species of groundlifters can fend them off with stomps and bites. This has led to notable coevolution: under pressure from borax predation, groundlifters have increased in size from the 40 kg triple stottmice of five million years ago to the peninsular groundlifter, with males reaching up to 400 kg. In the future, borax may no longer be able to overpower groundlifters, forcing them to evolve even further. 

The arms race has only just begun.