![]() Osteology and functional morphology of the axial postcranium of the marine sloth Thalassocnus (Mammalia, Tardigrada) with paleobiological implications. Evolutionary adaptation to aquatic lifestyle in extinct sloths can lead to systemic alteration of bone structure. No other group left a similarly detailed fossil record of ever-increasingly aquatic lifestyle in a succession, but it’s expected that they must have underwent a similar phase in history.įor now, we have to be content that the bones Thalassocnus left behind gave us the chance to observe their evolution in such great detail.Ī life reconstruction of Thalassocnus in its natural habitat by Oliver DemuthĪmson, E., Billet, G., & Muizon, C. They actively regulate their buoyancy,” Amson said.Īfter their extinction, the sloths’ rich fossil record is what sets them apart from other swimming mammals. Extant cetaceans (whales and dolphins), for instance, have osteoporotic (less compact) bones. “Deep-divers, and more general active swimmers, can actually show the opposite trend. Such reduction has also been observed in the evolutionary history of bears and whales, but it’s not yet fully understood how it would’ve affected the animals’ lifestyles.Īs with the whales, it seems like they took their own path to live underwater. This trend is also followed by the reduction of air-filled spaces surrounding the nasal cavity, or sinuses. carolomartini (5 Ma), it was clear that the sloths became more and more aquatic before they finally went extinct around 2.5 million years ago. Interestingly, the different species of Thalassocnus he observed showed a constant trend of BMI throughout their history. “In shallow diving 'slow swimmers', such as manatees and aquatic sloths, the skeleton is more compact and acts like the weight-belt of human divers, to allow them to be neutrally buoyant passively,” Amson said in an email to Earth Archives. Amson’s study shows, at least in the sloths’ case, that BMI isn’t restricted to the animals’ limbs. The skull thickening is particularly notable, since every study on BMI so far has mostly looked into body parts related to locomotion. By examining slices of the bones, known as thin sections, he noticed the thickening of the sloths’ bones including the roof of the skull. In a study published in May 2018, postdoctoral researcher Eli Amson detailed his observation of their bone thickness. In short, aquatic mammals like otters, sea cows, and even platypus tend to have thickened, hardened bones compared to their terrestrial relatives.Ī similar case is reflected in the bones of the marine sloths. Such an evolutionary trend is called bone mass increase (BMI), which involves the increase of the bone’s robustness (pachyostosis) or density (osteosclerosis). The bones of flying creatures become filled with air sacs to reduce their weight, while shallow-diving species went the opposite direction. When a land-dwelling animal adapts to life in the sky or sea, their bone density tends to change. On top of it all, their bones yield another clue to their aquatic lifestyles. Features in their jaws suggest that they grazed on seaweed and sea grasses from shallow coastal waters, likely less than one meter (3.3 ft) deep. These sea-dwelling sloths all had features in their bones that resemble other aquatic animals like otters and beavers. Since then, four other species of Thalassocnus have been unearthed from various coastal environments of Peru and Chile. This particular group of ground sloths, named Thalassocnus natans, seemed to live alongside whales and crocodiles. Fossils of aquatic animals dominated the scene and no other land-dwelling mammal were found nearby. ![]() They were found in what was a coastal desert back in the Pliocene Epoch, around 2.5-5 Ma. In 1995, a scientific paper announced the discovery of a big number of sloth fossils in an unlikely environment in Peru. But some of Megatherium’s close cousins seemed to be even more comfortable in the water than Planet Earth’s sloth. The iconic Megatherium, also appropriately known as the giant ground sloth, even grew as large as today’s elephants. ![]() Though it broke our expectation of sloths as lethargic, tree-clinging animals, there are more surprises to discover about sloths when we look at their family tree.īefore extinction left us with the six species of tree sloths we have today, ground sloths the size of bears used to roam the Americas since 35 million years ago (Ma). When BBC teased its then upcoming series Planet Earth II with a footage of a swimming sloth, the internet exploded with awe and glee.
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