Two worlds, two rats

With no other mammals to compete with, it didn't take long for rodents to reach gigantic proportions. Today, these mammals dominate the terrestrial ecosystems of the Antarctic continent, alongside a multitude of ground birds.
Many fairly large species have recently vanished due to the progressive global cooling, resulting in the disappearance of subtropical habitats in Antarctica. However, mountain species, adapted to cold temperatures, have experienced a significant evolutionary radiation in flat environments.
One notable radiation is that of the heavy tramplerat (Ronnia fortipes), a one-ton cricetid descended from a branch of hogsters that now dominates and sustains the productivity of the tramplesteppe. Without its presence, this biome would be significantly impoverished and unable to sustain its relatively high biomass. It is hypothesized that the tramplesteppe can locally support even 3-4 tons of herbivores per square kilometer, an exceptionally high value for a recently formed biome. As this biome expands, these values could potentially increase further.

Heavy tramplerats are gregarious species that form herds consisting of hundreds of individuals. Their ancestors, which evolved in the Half Plateau, experienced a tremendous demographic boom, leading the species to colonize even the low-altitude environments. They possess a plantigrade limb posture, enabling them to distribute weight better on the ground, aligning perfectly with their extensive migratory movements. The large and heavy head is one of the keys to their evolutionary success: large and continuously growing molars and incisors facilitate greater masticatory efficiency of the graminoid plants that dominate the tramplesteppe. The species performs hindgut fermentation, similar to horses, elephants, and other rodents. Their large size accommodates a lengthy intestine used for food digestion, which is subsequently assimilated into the cecum.

Apart from the tramplesteppe, tramplerats can also be found in adjacent coldvannas during the summer period. However, their presence is limited by heavy snowfalls, preventing year-round habitation. Other species have adapted to this relatively species-poor but still productive environment. Some have developed a primarily folivorous diet, like brumbles, feeding above the snowy cover throughout the year, which can easily exceed one and a half meters, four times higher than the tramplesteppe. Others, instead, have evolved strong claws for digging and accessing turf; the thagomicer (Deinomus excavatoris), a large murid with an extremely elongated body, falls into this category.
In addition to possessing strong claws used for digging tubers and roots, the thagomicer is characterized by a large thorny club, serving as a deterrent against predators and for dominance fights. They have digitigrade locomotion, enhancing their running abilities. Despite its relatively short legs, the species can reach speeds of up to 45 km/h, equal to that of a rhinoceros.

The thagomicer is a descendant of the rattrees, a group of arboreal rodents from the Cambiocene epoch. With the onset of the first glaciations, a clade of these mammals transitioned to increasingly terrestrial behaviors, becoming one of the most distinctive species of coldvanna. Due to their arboreal lineage, thagomicers have a stomach poorly adapted to feed on herbaceous species and are now restricted to the coldvanna and the Reliquia Forest, where woody associations dominate. To cope with the significant temperature fluctuations between summer and winter, thagomicers undergo significant seasonal changes, developing thick fur in the cold period but almost shedding it entirely in warmer months.
Unlike heavy tramplerats, which possess elongated snouts to increase chewing surface at the expense of muscle strength, thagomicers have very short muzzles with powerful bites, adapted for felling trees and shrubs like beavers. This enables the large murid to feed on foliage, a less abrasive food source than herbaceous plants, thus obviating the need for optimized chewing capacity. The increase in dead woody material, facilitated by thagomicers, indirectly contributes to the presence of numerous xylophagous insects and microvertebrates, including woodclimbers.