Rise of geotters
Geotters (Family: Geolutridae) is a recently evolved group of mammals that originated in Polarica, now found across all of Antarctica. They are descendants of the newotter, a semi-aquatic otter that was able to reach Antarctica by swimming. In a few million years, geotters have rapidly occupied different terrestrial niches, remaining in the shadows of predatory birds. They are mostly small-sized, but in just 7 million years of their appearance, they were able to radiate in different forms.
Marters: dwarfs and "giants"
Geotters' ancestors rapidly underwent a dwarfism trend before becoming truly terrestrial, probably to avoid competition with the most ferocious herdstalkers, giving rise to marters, the most primordial forms of terrestrial geotters.
Some of them retain ancestral adaptations, like the semiaquatic primal marter (Ancestrimustela planicola): they do not exceed 800 grams (1.7 lb) and possess a mink-like ecology, being both capable of hunting on land and in rivers. Common prey are fish, birds and invertebrates. While this species can be found in large wetlands of entire Antarctica (including major islands), the interior dry and colder tundra of Polarica is dominated by an even smaller geotter, the lesser marter (Lutramustela minor). They are ground tyrant-specialized predators, entering their den to kill juveniles or eat their eggs. When most of the ground tyrants overwinter under the snow, lesser marter are forced to find other food sources, like carrion.
Both species are obligate hypercarnivores, with no plant matter found in their diet.
The largest geotter species is the carrion marter (Osteophagus robustus), found across any ecosystem of Polarica and, recently, on Marie Byrd island. Retaining the huge bite force of their ancestor, this species is a very specialized scavenger, capable of even munching bones. They possess a large olfactory bulb, an adaptation to find carrion even under the snow cover. Due to their ecology, carrion marters live solitarily at very low density like wolverines (1 individual/ 100 km2 ).
They are plantigrade, incapable of climbing, and very aggressive towards other predators: sometimes they were observed while stealing prey from woodland herdstalkers, large predators that are twice their size. Its relatively bulky structure is the key of its success, being more tolerant to pecking birds.
Two derived lineage
Some marters have started an anatomical diversification. One group of marters has diverged from the ancestral body plan, becoming semi-arboreal, commonly known as wotters (Subfamily: Arborilutrinae). Several species are present, found across the coastal forests of Polarica. One species has recently settled the Weddell-Ellsworth macro-island while another one was able to reach Marie Byrd, both thanks to the synergic action of land bridges and floating woods. Wotters' claws are recurved, for better grasping on trees when climbing while stalking perched birds. Wotters are also the most divergent group from a feeding point of view: even if meat comprises most of their diet, they supplement it with highly energetic plant matter, like fruits and seeds. The most common wotter is the D.D. wotter (Arborilutra meridionalis), mainly found in the humid taiga-like forest of the D.D. area.
While wotters are managing to conquer the canopy realm, other geotters are refining their running abilities. The black biwolf (Geolutra cursoria), the only one of its kind (Subfamily: Lutracyoninae), has drastically reduced the leg-body length ratio, becoming relatively cursorial. It has a semidigitigrade locomotion, using the entire palm and only the foot digits to bear its weight, and it's comparable in size to a cuon.
These adaptations are perfect to chase medium-sized birds in woodlands, where this species only resides. Its brown-black coat is very helpful in the forest canopy, acting as a camouflage to hide from both predators and prey.
As marters, they are obligated carnivores with a very short intestine, incapable of digesting plants.
