A tweeting ocean
The reefsurfer revolution

After the apparent disappearance of aquashifts during the Biancocene, the seas were left without any large marine tetrapods. Reefsurfers, a marine group of aquatic eggpouchers, quickly took advantage of this absence and filled the vacant niche once held by these sea reptiles. However, the sudden warming of Antarctic waters caused the collapse of the rich cold marine ecosystems, leading to a catastrophic extinction that wiped out most reefsurfers. With the start of the Incertocene, only a single lineage of these birds managed to survive, the one with the most derivate adaptations to live  completely in the sea. We're talking about the descendants of reverse reefsurfers and relatives, which thanks to their better manoeuvrability and the use of not one but two pairs of limbs for more optimal propulsion helped these birds to adapt to new food source after the sudden global warming of the early Incertocene.

Within only ten million years of the Incertocene, reefsurfers diversified again into many forms, though they remained confined to the sea, losing any kind of connection with the coast. This ecological revolution mirrors earlier marine tetrapod radiations, such as that of cetaceans, where only the most marine-adapted forms survived into the late Cenozoic.
Reefsurfers are the only fully aquatic birds known to have evolved in the entire earth history, unique in that they no longer return to land to lay eggs. This is thanks to ovoviviparity, inherited from their eggpoucher ancestors. In their case, ovoviviparity reaches an extreme: uncalcified eggs are retained within the oviduct, without placental blood exchange, though nutrient and gas transfer occurs through osmotic contact with maternal blood vessels and the egg. Depending on the species, after three to seven months the chick embryo is expelled inside a soft-shelled egg. Normally, avian eggshells would calcify by passing trough the oviduct, but special hormones and the rapid expulsion prevent the formation of a thick layer of calcium.

The hatching process requires the mother’s assistance. If the egg would hatch underwater, the chick would drown almost immediately: to avoid this, the mother rolls onto her back, exposing her belly at the water’s surface, and expels the egg from the cloaca ((which is slightly concave, in order to not make the egg roll away). The chick can then easily break the soft part of the egg on the mother's belly, takes its first breath, and then slips into the water. Reefsurfers can give birth from one to five chicks in a row, making the entire process last from few minutes to half an hour depending on the number of chicks (for comparison, a dolphin can need more than a hour to give birth to a single calf, also because it's born porpotionally larger than young reefsurfers). The chicks are superprecocial and can follow the mother immediately after birth and usually spend a year with her before becoming fully independant. A comparable forced ovoviviparity may have existed in mosasaurs, though it is unclear if they birthed at the surface or underwater.
Aside from their interesting method of giving birth, these new reefsurfers are characterized also by their interesting way of swimming, which is convergent to the extinct plesiosaur and sea turtles: rather than using a tail fin, they employ four large flippers for moving the water mass. These flippers can move alternately for pursuit hunting, or synchronously for ambush maneuvers. While not as fast as Cenozoic cetaceans, current reefsurfers are built for maneuverability rather than sheer speed, excelling at hunting schooling fish. All four limbs are laterally flattened and lack movable digits, but the hind flippers bear a strong flattened claw used both for propulsion and in defense or combat. Unlike their basal ancestors, which retained plumage, Incertocenic reefsurfers have lost most of their feathers as adults. Hatchlings are born downy but they soon replace feathers with blubber, which provides insulation while reducing drag and increasing swimming efficiency.

One of the most widespread species is the school reefsurfer (Pelagicuculus pinguinus), with an estimated population of about 200,000 individuals around Antarctic seas. Reaching up to four meters in length, it is one of the largest and most important predators of the Antarctic marine ecosystems, alongside some species of sharks. As its name suggests, it specializes in hunting schools of fish, which it captures with a long, narrow beak lined with small false teeth. Usually solitary, it often joins others during hunts, where cooperative behavior is observed.
School reefsurfers are highly intelligent and they often approach curiously other organisms with no hunting interest. Tool use is well established in the culture of these solitarily animals, which however comunicate a lot with other wandering individuals and can apprehend a lot from other reefsurfers.  They have two ways of communication: one is physical, through beak clattering, where vibrations pass along the mandibles during face-to-face contact, even above water. The other is acoustic, producing high-pitched clicks that serve for communication and rudimentary echolocation. These calls can reach a maximum of 8.000 kHz, which means that their calls would be easily heard by a human, which can hear sound as high as 20.000 kHz, yet a human cannot vocally produce sounds over 1300 kHz.
They also use a bizzare method for food detection: smell. By expelling and inhaling bubbles, they trap dissolved odors in water, allowing them to track prey without sight. This strategy is remarkably similar to methods used by otters and the star-nosed mole when hunting underwater.

Other reefsurfers show even more stranger adaptations, like the marbled bottomtrailer (Plesiosauropteryx phoenicopterimimus), a six-meter-long bird with a neck making up half its body. It possesses a beak strikingly similar to that of a flamingo, adapted for filter feeding. Moving close to the seabed, it scoops up detritus, small fish, and crustaceans. Water and debris are expelled by the tongue, while dense false teeth act like baleen to trap food. Despite its specialized feeding on small prey, its great size makes it the largest known reefsurfer. They are not a mere giant exception, but part of a broader trend toward gigantism in the group, likely to continue as marine ecosystems expand and diversify further.
The already astonishing diversity of reefsurfers has not yet reached its true potential...