The great return of rodent biodiversity

While surviving with as few as 8 species around 5 million years ago, rodents have rapidly rebounded in the Incertocene, with more than 150 species now present, 90% of them being microrodents (smaller than 5 kg/11 lbs). Although small, these rodents are far from similar, already showing a great diversification even after 5 million years.
Apart from standard ground vole-like forms, the new microrodent fauna includes a group that has become increasingly arboreal, called possumus (Simiarattus ssp.). They vary in size from a squirrel to a marmot, all being skilful climbers, with the smaller ones being more arboreal than the larger, which are more scansorial. They are characterized by elongated fingers and limbs, and a robust, long tail used for balance while running and jumping on branches. Some smaller species can even minimally use the tail as an extra hand to partially support their weight. If useful, this feature will surely be maintained and optimized by this rising group of rodents. At least, by some of them.

All possumus possess a compressed skull with simpler teeth compared to their ancestors, an adaptation to softer foods like leaves, fruits, and nectar. Since the return of forests was extremely fast, few species were able to rapidly exploit this new habitat, and possumus have almost no competition for food thanks to their arboreal lifestyle, except for some species of ragos, a group of scansorial and arboreal otters that survived the end of the Biancocene.
Some species of possumus have evolved to live in semi-open environments, like the striped possumus (Simiarattus frugivorus) found in the Great Depresseaon. This species, while retaining several arboreal adaptations, has sturdier feet for walking in open spaces to reach nearby trees. Like many possumus species, it has adapted to feed on the toxic leaves of doorpeas, which dominate the landscape of the Great Depresseaon. It can do so thanks to its very efficient gut, which rapidly digests this food before toxic accumulations become too high.

The arboreal lifestyle of possumus probably evolved due to empty niches, but also likely from some degree of predatory pressure to escape from the ground. While most Antarctic rodents are mostly herbivorous, a separate lineage started to develop a more carnivorous diet: these animals are called topungers, and they rapidly diversified with the rise of temperature shortly after the Biancocene, also benefiting from the lack of fully carnivorous micromammals, which became extinct at the start of the Incertocene. This group began to adapt to fill the niche of their former predators, becoming practically the new enemy itself. It's a rat-eat-rat world out there now!
They are characterized by a long and narrow muzzle, with sharp incisors and molars similar to the carnassials of a carnivoran. Due to the lack of canines, topungers use their incisors for the role of both incisors and canines. These adaptations for carnivory are not new in rodent evolution: several Holocene rodents developed carnivory, like some rats from Indonesia, grasshopper mice from North America, and rakalis from Australia.

Most topunger species are mainly invertivorous (invertebrate-based diet) due to their small size, no heavier than 50 grams, like the shrew topunger (Rattiosrex fossorium), a widespread species that dwells in the closed environments of heat forests. They mostly feed on insects and snails, which they ambush on the ground.
Shrew topungers have poor scansorial adaptations but they're occasionally seen climbing trees to feast on bird eggs and young possumus. They pose no threat to adults possumus, which can potentially kill these small vertebrates. This can't be said for the woodland topunger (Carnymys giganteum), the largest of the topungers, which can weigh up to 500 grams (1lbs). They are fierce predators of small vertebrates, which account for more than half of their diet; because of their size, they can't climb trees, limiting their hunting ground to the undergrowth of the woodlands where they live.
Some populations can be found in the Great Depresseaon, where they have adapted to ambush prey even in tall grass. They possess large eyes, which they use as much as smell for hunting, a unique adaptation among rodents. Thanks to their size, woodland topungers can take down prey even twice their weight. While being ferocious predators, they're still only half a kilo in size and often fall prey to larger vertebrates, like the several species of sheardogs that radiated since the end of the Biancocene.

While rodents have developed great ecological diversity, continental species are still not as numerous as expected for the available space, due to insufficient time for speciation. Of the cited 150 species, more than half are found on islands, which were once part of the continent at the end of the Biancocene. When the sea level started to rise, some continental fauna remained isolated, giving rise to unique species. 

The Sparso Archipelago hosts the majority of insular rodent species, but the most peculiar species is actually found in the less productive insular environment now present in Antarctica: Boitomb. This island, in the process of glacial rebounding, became almost completely submerged, killing off its terrestrial fauna with few exceptions. Marine vertebrates and a few small terrestrial one survived, including a rodent species that gave rise to the present-day clade of survivoles, a group of ground-dwelling rodents with a predominantly herbivorous diet. 

Thanks to glacial rebound, land began to reemerge from the ocean, reforming Boitomb island. Thanks to this, survivoles spread throughout the island. Nowadays, the island is still covered with large ice caps in its southernmost part, but the large tundra in the north sustains these small mammals. Some species have diverged more than others, increasing their size twentyfold compared to their ancestors, like the horned survivole (Boitombia unica), which can weigh up to 1 kg in large males.
Size, however, is not the really interesting feature of this species: both sexes possess a keratinous horn on their head, used for sexual display and defense. It is, to our knowledge, the second-known rodent to develop a horn, after the horned gopher, an extinct group that lived in America during the Miocene and Pliocene. However, unlike horned gophers, the horned survivole’s horn lacks a bony part and is composed only of keratin, like that of a rhino.
Due to its size and the difficulty of digging in Boitomb’s frosty environment, the horned survivole likely evolved this feature to fend off aerial predators, which are the island’s only predators (except for a primitive species of borax, which has however a more seafood-based diet). These rodents do not hibernate but remain active through the entire polar winter, often cuddling in social groups to keep warm. They are tough creatures, adapted to what was once the norm on the entire continent: a white desert punctuated by sparse grasses. But fortune favors these little guys, as glaciers continue to thin even on the island they call home. Time will only tell what lies ahead for them.