Slow and steady like a tortoise

Tortoises were the first livestock introduced by Aves ex Machina in Antarctica. They rapidly adapted to the alien and depleted continent, colonizing the warmest regions of the northern forests. Thanks to the ongoing global warming, these reptiles were able to occupy the entire Antarctic Peninsula and the Belgrano lowland. However, due to their high sensitivity to cold temperatures, no tortoise species was capable of moving in the cold temperate forests of southern Antarctica. Things changed about 1 million years ago, with the rise of the first truly giant tortoise of Antarctica, called black slowcust (Antarcochelys sulcata). Weighing exceptionally 200 kg (880 lbs) in adulthood, this peaceful giant can better tolerate the relatively cold climate of the central austral forest thanks to a partial gigantothermy and the black color pattern of its scales of the carapax, which also act as a countershading. Dark colors can better absorb light and therefore heat from the sun, keeping the animal sufficiently warm enough to maintain its metabolism even at 8°C: at this temperature, other tortoises would enter a torpidity state. 
Of all large vertebrates, black slowcusts are the ones with the highest biomass density: in optimal conditions, they can reach locally 50 ind./km2 (31 ind/mi)!
When night temperatures start to fall under the freezing point, black slowcust prepares a giant burrow where they'll hibernate; some hibernated individuals were recorded also inside lava and karstic caves.