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Antarctic waters are cold, but not as cold as in the Holocene. This is only thanks to the break of the Antarctic Circumpolar Current, now partially admixed with the near warm sea currents Temperate sea ecosystems have slowly expanded south, like the kelp forest, now found across the entire coast of the continent. Seagrass has also made its appearance in the northern part of the region, which creates large submerged meadows in the most shallow waters of the Weddell Sea. While fish, seals, penguins and small cetaceans are the main vertebrate groups observed, two new clades were able to make their way into Antarctica for the first time.
If you ever try to swim inside the dense antarctic kelp forest, you would note some gregarious giants, busy at browsing algae and plants. They are anteseacows (Neosteller antarctica), giant mammals that are far relatives of manatees, that have slowly expanded in all of South America and have inevitably colonized Antarctica. These sirenians compete their size with the extinct Steller seacow, which can be easily distinguished by the morphology of its caudal fin and a somewhat thinner and elongated body.
Head structure is very primitive, nearly identical to its ancestors, but that wouldn't be a surprise, since sirenians have not changed much their skull since Eocene. This species can be found in every kelp forest and seagrass meadow of the continent, sometimes falling prey to sharks, large seals and large predatory cetaceans.
A rare antarctic visitor of antarctic coasts is the austral mariguana (Mariguana crassa), an omnivorous iguana that has made its way to the sea. It's the largest iguana ever known, being as long as 3 meters (9,8 feet). This species is usually found in warm waters, but adults can be seen wandering along the northernmost coasts during summer. Mariguanas' carrion are also common on the coast during the warm season. Their discrete cold tolerance is caused by a thick fat reserve along the body, that isolates the body from the environment; this species is mesothermic, capable of maintaining its body temperature relatively stable. They usually live near the coast, despite being capable of swimming in the open ocean, searching for jellyfish and plankton to eat. Seagrasses and crustaceans can also become a daily diet of this very generalist vertebrate. Despite their aquatic adaptations, mariguanas still need to deposit eggs on land, similar to turtles. At the moment, no egg colonies were ever found in Antarctica, probably due to cool summer temperatures that do not permit the proper growth of embryos. Life is harsh here for this reptile, but ongoing global warming could eventually enlarge their range in the future, if they manage to survive. Other evolutionary experiments were less lucky in the past, like the shark seal, which rapidly went extinct due to the contraction of kelp forests in the middle Lentocene.
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