Sea novelties

Cetaceans, like many other marine tetrapods, completely disappeared from Antarctica after the END. They likely went extinct elsewhere or became confined to small refuges, unable to expand their range. Their ecological niche was eventually taken over by aquashifts, a group of marine reptiles that thrived for tens of millions of years, only to decline as global temperatures began to fall. Around the same time, reefsurfers, a group of almost viviparous seabirds, partially filled the niche once occupied by these reptiles.

The extinction or rarefaction of cetaceans and aquashifts is further suggested by the emergence of another bizarre tetrapod group with cetacean-like body plans: the gulpingshrews (Clade: Cetodesmatidae). These relatively small, fully aquatic mammals are distant relatives of the Holocene desman, a shrew-like creature once adapted to a semiaquatic lifestyle. Some sea-dwelling desman species likely already existed during the Biancocene, but it was only with the onset of the Incertocene that their diversification and expansion accelerated.
One of the most common gulpingshrews found in Antarctica today is the browny gulpingshrew (Desmhydra piscivora), which can reach up to 1.5 meters in length. It is characterized by a very long snout, small eyes, and large frontal nostrils that allow it to breathe without fully resurfacing.
Although its body is fully adapted for marine life, several features still hint at its terrestrial ancestry. These include vestigial external ears and fins that retain functional, clawed fingers, especially in the hindlimbs. These “handy” fins likely help grip partners during frontal mating. The large single claw on the forelimbs also serves as a defensive weapon against sea predators and during intraspecific fights over mates.

Most gulpingshrews, including the browny gulpingshrew, are bottom-feeders that consume crustaceans and benthic fish, which are located through their long, sensitive whiskers. Some species can dive as deep as 100 meters and hold their breath for up to 15 minutes.
To support these "performance", they exhibit a mix of physiological and behavioral adaptations:

• Physiologically, gulpingshrews have proportionally large spleens, about three times larger than the expected size for animals of their size. These spleens store oxygen-rich blood, allowing longer dives without risking hypoxia. Similar adaptations are seen in Holocene marine tetrapods and even in sea-dwelling human populations like the Sama-Bajau.

• Behaviorally, gulpingshrews hyperventilate before diving and submerge with semi-empty lungs. This strategy increases the proportion of oxygen in the blood while reducing buoyancy and hydrostatic pressure, therefore the energetic effort to dive.

With the disappearance of strong competitors, aside from the reefsurfers, will these giant marine shrews reclaim mammalian dominance in the oceans after more than 40 million years? Only time will tell.