Made as part of an ongoing project of mine, called "Pterraforming", featuring an alternate Earth (dubbed "Pterearth") where the K-T extinction event was less severe. Not all that original, I know, but really it's an excuse to work on a group of Pteranodon descendants that survived to become the dominant pterosaurs, and, in the case of the water, the dominant tetrapods; The Archopterans
This is a rework of my Pteroviathans, the largest of the "seawing" pterosaurs ever to exist, and the largest animal ever to live on Pterearth. While (on average) they tend to be shorter than blue whales, they are much heavier, and are built like literal submarines. They fill a similar niche to the blue whale, but are a bit more ornery and able to eat things a blue whale could never dream of. Pteroviathans are part of the "crown" seawings (pterocetaceans), which are the most advanced forms that have evolved. They all share a melon formed from modified salt glands, highly specialized flippers and bones, and a unique respiratory system that allows them to change their buoyancy similar to the chambers within the hull of a sub. They also possess an unusual tissue, derived from skeletal muscle tissue, that essentially acts like fat, but for storing oxygen. This allows them to so saturate their body with oxygen that they can stay submerged for hours!
Another interesting adaptation is the transformation of their pycnofibers that actually mirrors the evolution of teeth. The body pycnofibers are formed into "psuedodenticals", which are convergent and function to the dermal denticles of sharks. In the pterocetaceans, these pycnofibers have actually worked their way into the mouth, forming new "teeth" made of keratin. In "baleen" forms these "teeth" are similar to the flight feathers of birds, whereas in "toothed" forms they mimic both cetacean and shark teeth, depending on the species.
The hearing is especially advanced in pterocetaceans, being extremely hypertropied to the point some species have gone completely blind! Multiple gel-like structures on the jaw and on the forehead (the "melon" mentioned earlier) help both amplify outgoing sound while also helping to focus sound towards the ear, which like whales are internal. The entire jaw is formed into the auditory system, having special sound amplifying properties and having a radar-dish like shape in the back that helps funnel sound. This allows them to hunt for food and watch for predators in a near 360 degree range, even in the dark, while also not needing to put extra resources into reinforcing the eyes for underwater life.
Seawings, as a group, have near total dominion over aquatic tetrapod niches on Pterearth, the only other fully marine animals they share the water with being turtles and snakes. No other group has fully returned to the water. They simply can't. There's no room for them. Seawings were the first animals to return to the sea after the K-T mass extinction event, being able to revert to a more pelagic lifestyle by reactivating genes that laid dormant in the ancestral protoarchopteran. This allowed them to fully return to the sea when mammals were still only becoming semiaquatic on Earth, giving them a head start. This, combined with the young's ability to fly (which allows them to spread far and wide, into bodies of water not otherwise accessible by swimming or walking alone) and the ability to give live birth, meant that they took the waters by storm, and once they were in, they blocked anything else from following the same path. So on Pterearth, the pterosaurs rule the sea completely.
So the only competition they had was from each other.
Today, the pterocetaceans are the most widespread and speciated group of seawings, their advanced features allowing them an edge over their competing relatives. Over 200 species are known, ranging from passive filter feeders, to lunge feeders, to grazers, to carnivores that hunt fish, other seawings, and even pelagic birds and pterosaurs. In the north and far south, there are even psuedocrocodiles that pick up where their cold blooded contemporaries cannot survive. They are, in essence, kings among kings. And the mighty pteroviathan is the highest of all.
