Rusty river euplerrel

This is not a real animal, but a hypothetical concept of something that could evolve

Terrormyines (Theriomyidae) are a successful clade of carnivorous squirrels that inhabit a future New Zealand. Although many have lost their characteristic patagia, they are, in fact, descended from flying squirrels, particularly the Southern Flying Squirrel. But those squirrels live in North America, so how could they get all the way to New Zealand? 

Well, the southern flying squirrels became very successful in this future, their range slowly expanding southward. Eventually, they had split into several new species, now living all the way down to Peru! These flying squirrels were very social, often nesting in large colonies in hollow trees. Soon, a few of these tree-dwelling colonies got washed out to sea, potentially in a large storm. They then rafted on the current, and after a few months, ended up in New Zealand. This may seem like an unsurvivable long journey, however the Oplurid lizards have made a similarly lengthy journey from South America to Madagascar. The squirrels were able to survive the long trip thanks to torpor, another new adaptation in this (until now) Peruvian species. Those squirrels likely also had some food reserves in their home tree, helping them survive the trip.

Once there, the flying squirrels diversified dramatically. There were no other mammals there besides bats, so they faced almost no competition. Soon, some started occupying the mostly vacant carnivore niche, becoming the first terrormyines. One clade of terrormyines lost the ability to glide, now being able to feed on mostly exhaustible food sources on the ground. Most terrormyines are a part of this clade, and are thus flightless, no longer gliding.

(Side note: This entire concept happens in the future, in a timeline where humans never developed agriculture or civilization. So, that’s why there are no invasive cats, stoats, or possums for them to compete with in New Zealand.)

The Euplerrels (Viverrisciurinae) are one such group of terrormyine. These creatures have almost fully lost their patagium, with just a few vestigial remnants remaining. The wrist spurs that once supported their patagia have now been repurposed into sharp pseudo-claws that they use in territorial disputes. These claws are much more pronounced in males, as they are the more territorial gender in terrormyines. The young of euplerrels are mostly precocial, unlike their relatives, the canopy-clingers. Euplerrels are also fully carnivorous, and can reach much larger sizes.

Not all euplerrels are so large and imposing, though. Some, such as the Rusty River Euplerrel (Fluviovulpes vulgaris), are smaller and elusive. Riverine euplerrels, such as the species just mentioned, are a diverse genus of euplerrels that, like the name suggests, primarily live in rivers. However, they don’t live exclusively in rivers. The rusty river euplerrel can often be found in estuaries (as seen above) and even sometimes in the ocean! This species can tolerate a range of salinities. Riverine euplerrels are piscivorous, but will still on occasion eat crustaceans or other smaller creatures. The rusty river euplerrel, named for the rust-like coloration of its fur, is rather small, only growing to about 2 feet (~60 centimeters) in length, much smaller than its close relatives. Riverine euplerrels have shorter legs, ears, and a paddle-like tail, helping them swim more efficiently.

A unique feature of riverine euplerrels are their venomous wrist spurs. The wrist spurs used for territorial disputes in other euplerrels were modified to contain venom channels, very similarly to the toxic spurs on platypuses. These spurs both evolved independently of each other, a case of convergent evolution. Riverine euplerrels still use their spurs for territorial disputes, the venom aiding in injuring their rivals. They can use their spurs to defend against predators too, though.