Too slow for quick changes

Cold-blooded animals suffered a large range contraction with the worsening of the environmental conditions in Antarctica.
Some, such as lipidragons, have managed to maintain an almost capillary diffusion thanks to their preadaptations to the rigid climate. Others, such as tortoises, were much less fortunate: unable to withstand prolonged winters, the Testudinae family is now represented by just a single species, which has survived only in the Reliquia Forest and some northern portions of coldvanna: the bunker slowcust (Relictotestudo sauropodomimus).
It is undoubtedly the largest terrestrial reptile of the continent, reaching 800 kg of weight in males. Their giant size was surely a great adaptation to withstand cold temperatures, but it wasn't the only one: the species also has large dermal ossicles all over the skin, which reduce heat loss. This dermal covering gives the bunker slowcust a very wrinkled appearance, apparently similar to an armadillo. It is a discrete method of thermoregulation, not as efficient as hairs and feathers but still more effective than a simple scaly cover. Evolution works with what it has, and the fact that this adaptation took place relatively quickly is already surprising on its own. However, the strange appearance of the bunker slowcust is not a novelty in the tortoises' evolution: even Hesperotestudo, a species of giant tortoise living in North America during the Pleistocene, presented these ossicles to its increase tolerance to the cold. 

It is not uncommon to see slowcust herds grazing in late autumn, with nights that can drop for some hours under thermal zero (32 °F). However, when temperatures become pungent and the first snowfalls begin, the species is forced to retreat inside caves for wintering. These cool and dry places are very widespread in the Antarctic Peninsula due to its mountainous and hilly morphology, unlike other northern areas such as the Belgrano plateau, where the species is practically absent, with the exception of a small population in the northernmost mountainous areas.

Despite its great survival capabilities, the bunker slowcust is destined to be an evolutionary dead end: the hardening of temperatures is progressively reducing the number of suitable caves for wintering, which is causing an increase in interspecific competition. Prolonged winters also drive this species to remain in their caves for longer, which rise its mortality by starvation and predation during lethargy. The recent spread of a large bone-splitting predator who frequently uses caves as shelters was probably a further severe blow to the species. 

Once again, the reptile era has come to an end in Antarctica...