One of a number of sketches for another worldbuilding project; a world dominated by glaciers and tundra and rich in fauna from across Earth's history. This species (the largest of its clade) is descended from a species quite similar to Tyrannosaurus, but has clearly evolved a number of adapations for a lifestyle best described as a mix between that of a leopard seal and a polar bear. The legs are mostly embedded within the body which limits it to an awkward scooting motion when out of the water. In the water, however, it is highly maneuverable and can catch aquatic birds, fish, squid without difficulty. Particularly large individuals can even kill ichtyosaurs, plesiosaurs, and trunkoiforms (marine probiscideans similar to the cryptid Trunko). They are oviparous, swimming to more temperature regions and creating nests on islands free of permafrost. The eggs (usually 3-6) are closely monitored by the female (the male plays no role in parenting). Young are precocious and take to the water with the mother. She will remain in temperate waters until the offspring are about three old and a third of their full size. They are then abandoned and the mother returns to colder, icy waters.
These dinosaurs have little to fear from predators for most of their range. Only in a few locations where the water is free of pack ice are they taken by large sharks and pliosaurs. Outside of breeding season they are solitary and territorial, engaging in brilliant underwater duels that leaves both combatants with nasty bites to the face and limbs. Fortunately the thick blubber of this species means that fatalities are uncommon.
Future depictions of this species will have more extensive feathering and all around better anatomy. This was just a quick sketch at work. I have lots more to upload so hopefully you'll be seeing them soon!
Inspired by Velociraptor-King
's Polar Tyrannosaur