Today’s paleontology lesson

A new paper reports on an incredible collection of aquatic fossils from Cerro Ballena, Chile. Spanish speakers who can translate Cerro Ballena to Whale Hill might hazard a guess at what fossils were found.

The paper, available to all in “Proceedings B of the Royal Society“, describes a series of whale strandings that occurred 9-6.5 million years ago. Science has a lay summary of the work.

Such a fine collection of fossils is called a lagerstätte and can reveal a lot about the environment at the time.

Under a deadline due to the impending highway construction that revealed the lagerstätte, the research team relied more on 3-D virtualization than on old-fashioned digging.

Because these fossils are relatively recent, they don’t reveal much about whale evolution, but research did teach me one thing I never knew before: the phrase “aquatic sloths”.

I went to the fantastic Web site the team put together. Not only can you download the 3-D models of some of the specimens, but you can cruise a list of all of them. Near the bottom, you can find Thalassocnus natans, the aquatic sloth.

I knew sloths could swim – there are a few videos of that – but that’s not the same as being aquatic. Carl Zimmer wrote about these things ten years ago so perhaps I need to pay better attention.

Aquatic sloths were not small animals. I could not find a published estimate of their size, but I did find an unofficial estimate: six feet long and up to 500 pounds. Here’s a photo of the skeleton (click to enlarge):

And a couple of artists’ depictions are available here, although the beast probably didn’t have that much hair.

This is more like it, but the face needs some of that fine coat, too.

Note the snout elongation from the first depiction of an older species to the second, more derived species.

The driving force that led to sloths’ adopting an aquatic lifestyle (although they almost certainly came back to land after feeding, sort of the opposite of hippopotamuses) was the terrestrial environment. This area has been arid for a long time, which makes it tough for a dedicated herbivore to find enough to eat on land. Tooth wear tells us that the early aquatic sloths fed on or near shore where sand would abrade their tooth enamel. The later species show no evidence of sand abrasion and probably fed in deep water, where the sea bed is calm.

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