Thanks to their dense bones, sea cows and other aquatic herbivores were able to graze effortlessly on the bottom of the ocean. The tree-dwelling sloth friends, on the other hand, is not known for their seafaring prowess... but they were.
A series of five aquatic sloths over the course of four million years reveals the development of dense bones good for swimming down to the seafloor to nosh on marine grasses. All of these ancient sloth species (genus Thalassocnus) were found in the Pisco Formation in Peru in distinct, successive geological ages. That makes this the first series of fossils to precisely document the transition from terrestrial to aquatic lifestyles.
Between four and eight million years ago, the area where these fossils were found was mostly a coastal desert, much like it is today. Scientists believe that as food became more scarce, the sloths took to the sea more and more. To survive the arid environment, they probably started wading in the water to eat sea grasses during low tide. Eventually they ventured deeper down to the seafloor. (Sounds almost Lamarckian.) Well now there’s evidence for bone densification and swelling to back that story up.
Eli Amson and colleagues from Sorbonne Universités in Paris placed several sloth bones -- ribs, femurs, tibias -- on loan from multiple museums into a CT scanner to measure bone compactness. Since the bones had all been previously dated, the team was able to build a clear timeline.
As time passed from the Late Miocene to the Late Pliocene, the bones of the successive sloth species grew more dense: up to 20 percent in a period of just three million years. For example, T. carolomartini, who lived around five million years ago, had more compact bones than earlier species, like T. antiquus, who lived around eight million years ago. They were clearly getting less buoyant, and at a speedy rate too.
The bones of land animals are generally not very dense. It keeps our weight down so we use up less energy when we move around, and in water, our cavity-filled bones keeps us buoyant. Not so for aquatic sloths. “Think about a scuba diver who has a weight belt,” Amson tells National Geographic. “It allows them to sink.”
These ancient sloths had other adaptations too. They ranged between 6.5 to 8 feet long -- with about 3 feet of that being all tail. “The tail is actually reminiscent of a platypus tail or a beaver tail,” Amson explains, and it probably kept them stable as they dove.
So, our distant ancestors emerged from the sea to colonize the land. Then, some sloths found a way to go back -- like manatees, dugongs, and whales. Sadly, around four million years ago, the Isthmus of Panama closed and warm water from the Caribbean stopped flowing onto their Peruvian shores. The water grew colder, and they lost the plants they fed on, or maybe they were unable to adapt to the lower water temperatures. Either way, we lost all the aquatic sloths.
The work was published in Proceedings of the Royal Society B this week.
Images: bone compactness from E. Amson et al., Royal Society 2014