Mammals on Spec generally small, timid things, kept firmly in their place by the mighty dinosaur since the Permian extinction, but one would be mistaken to assume that all hairy animals are tiny.  A few niches, in high mountains, on cold plains, and most importantly, in the water, have offered mammals an edge over the dinosaurs, and a few clades have made use of their advantage.

    Weighing up to 500 kg and often over six meters in length, the rhoan selkie (Selkis mysticum) is the largest mammal on the planet.  These vicious predators travel in closely-knit packs through the waters of the Arctic, hunting their favorite food, seaguins .  These large aquatic birds are formidable prey, and must be driven nearly to exhaustion by the pack before they can be safely killed and eaten.  Selkies are easily capable of keeping up with a fleeing seaguin, propelling themselves through the water with undulations of the spine that must be similar to the locomotion of RL's early whales.  Indeed, while the more primitive each-uisges of Europe retain large and functional limbs, selkies, like those ancient whales, have begun to loose their hind legs, which are nearly functionless in their aquatic lives.  

    Rhoans breed toward the end of the long arctic winter, using their short gestation period to swim southward and mark out a spot on a suitable breeding beach.  As soon as they have left the water, the females curl up to allow their tiny, marsupial pups to crawl out of their wombs.  For roughly four weeks, these tiny babies feed upon high-calorie milk, while their father makes daily fishing trips in search of nourishment for their landbound mother. Once the pups are weaned, they are transferred to the father, who begins to teach them to swim and hunt while the mother fishes for the family.  The rhoan parents may switch off as many as five times before the winds of winter drive them and their pups into the water, where they integrate themselves into a pack.