On April 27th, 2016 we learned about

Ancient giant sloths’ aquatic adaptations

You might assume that a land-based animal moving to the ocean would need to first worry about their nostrils, or webbing in their hands, or maybe even the shape of their tail if they wanted to be successful. After all, these are some of the obvious traits that whales changed when their ancestors moved from the land to the sea millions of years ago, making them look a lot more like many native fish who had never left the ocean. Members of the sloth genus Thalassocnus demonstrate how the long distance locomotion aren’t always the biggest concern when an animal is transforming itself to take advantage of a new environment. For aquatic sloths, the biggest changes were all focused on their mouths.

Snacking along the shore

The most primitive members of this sea sloth story were Thalassocnus antiquus, dating back to the Miocene epoch. At the time, the area in what is now Peru was fairly arid moving inland, giving plant-munching sloths few options for food. Instead, they seem to have discovered the joys of seaweed and sea grass, and began doing most of their foraging in the shallows along the beach. Aside from where we find their fossils, this feeding is indicated by the damage to the animals’ teeth, with telltale scratches from sand that they likely stirred up while scouring the sea floor for plants. Of course, nobody likes biting down on sand that’s ended up in their sandwich or seaweed, which may have kicked off the first big change in the sloths’ anatomy.

Follow the food

Well, irritating sand probably wasn’t as big a motivation as much as being more efficient with their feeding. Over generations, and various species, the Thalassocnus lineage didn’t evolve flippers or fins, but longer, more shovel-shaped mouths. The shape would have lent itself to foraging for soft sea plants, but eventually showed openings for sophisticated vasculature, reminiscent of a manatee. One possibility the is that while the slots where using their arms for motion, their lips grew larger and more dexterous, making them able to pluck food without a need for fingers.

We have well preserved specimens from at least five species of aquatic sloth, and they seem to have been moving farther from the confines of the coast over time. Some are found in fossil beds surrounded by bones from whales, dolphins, fish and crocodiles, indicating that an aquatic lifestyle suited them well (outside of the occasional red algae bloom that left such well articulated evidence behind.) If given more time, perhaps these sloths would have gotten around to evolving full-blown fins and shifting the location of their nostrils, but in the late Pliocene, the Isthmus of Panama closed, changing currents and water temperatures, thus disrupting the sloths’ ecosystem too quickly for these slow-moving swimmers to keep up.

Source: Doing the crawl? by Ross Barnett, TwilightBeasts

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