A declining aquashift world

Aquashifts, a group of marine reptiles that emerged between the Lentocene and the Cambiocene from a group of marine-dwelling iguanas, once stood as the most abundant and characteristic marine tetrapod group in Antarctica after the apparent (and plausible) extinction of cetaceans. They diversified into numerous species, both carnivorous and herbivorous, reigning unchallenged throughout the entire Cambiocene. However, like any king, their dominance eventually waned, dwindling in diversity to just two local species at the present time.
The decline of aquashifts could be attributed to countless factors, first among them being the rapid cooling trend of oceans and competition from other marine animals such as sharks and reefsurfers, which now comprise the majority of the medium-sized marine tetrapod assemblage.

The whitemoon aquashift (Yangiguana ramphocephala), the larger of the two remaining species, is likely one of the biggest herbivorous marine vertebrates to have ever existed, reaching lengths approaching 8 meters (26 feet). It possesses a sharp, toothless beak used to grasp and ingest the kelp and seagrass found along the coastal regions of the entire continent. However, this food source is now inconsistently available compared to the past: during interglacials, sunken continental platforms create enormous submerged gardens where these aquashifts feast, but during glacial periods, this paradise nearly disappears due to the drainage of these plateaus, often leading to the local extinction of this reptile. The largest antarctic kelp forest in fact forms in the interior Weddel Sea, which completely disappear during glacials, living place to the Great Depreasseaon.

Due to their clumsy bodies and slow movements, whitemoon aquashifts are highly vulnerable to predators. Perhaps for this reason, this species evolved large osteoderms on the lateral part of the body to deter carnivorous animals, like the blackmoon aquashift (Yiniguana predatoria), the second and last species of aquashift. Though half the size of their herbivorous relative (rarely exceeding 4 meters in length) but are much more widespread and abundant. As mentioned, blackmoon aquashifts are carnivorous, feeding mostly on marine vertebrates such as fish, reefsurfers, and young whitemoon aquashifts. They possess a more dynamic body, with a well-developed caudal and a negligible dorsal fin to propel themselves towards their prey. The species is an ambush specialist, hiding in the depths to swiftly pierce and kill prey that ventures near the water surface.
Because of this hunting behavior, blackmoon aquashifts are often seen breaching, breaking through the water surface, along with a few species of reefsurfers that have independently evolved a similar hunting strategy. Breaching is not the only surface behavior observed in blackmoons, which exhibit many analogies with cetaceans, such as slapping, spyhopping, logging, and even porpoising. Like cetaceans, the purposes of these behaviors are still far from being understood: some may aid in communication, others in hunting, and perhaps some serve both functions.

Aquashifts, despite their wide distribution, remain enigmatic creatures like many other marine species, possessing a multitude of anatomical and behavioral characteristics that prove challenging to comprehend and study. Our hopes?  That their mysteries will not remain entombed in their uncertain future...