Millions of years ago, otters were bigger than leopards

An artist’s rendition of the massive, prehistoric otters.
An artist’s rendition of the massive, prehistoric otters.
Image: Mauricio Antón
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One undeniable truth is that otters are adorable.

From rafts of sea otters holding hands while they sleep to romps of river otters scampering through forests, these creatures are universally beloved. But personally, I don’t know if I would feel the same way if they were larger than predatory cats.

On Jan. 23, researchers from the China and the US published a paper describing the remains of an otter with an 8 inch (21 cm) skull and teeth that tore mollusks from their shells. These otters, called Siamogale melilutra, lived roughly 6.2 million years ago in a swampy, humid climate over at least 870 miles (1,400 km) of territory in what is now the Yunnan province in China.

The team first discovered the crushed, fossilized remains in 2010 in southwestern China. “The bones are pretty fragile, so we couldn’t really reconstruct it physically,” Denise Su, the head of paleobotany and paleoecology at the Cleveland Museum of Natural History, told NPR. “So what we did is we took CT scans of the cranium, and then we digitally reconstructed it.” It took them years to piece together the 200 individual fragments of skull, teeth, and limb bones.

Based on the size of the remains, they concluded that S. melilutra was likely around 110 pounds (50 kg)—larger than a leopard today, which are just under 70 pounds, and about the size of a wolf. This is about twice as big as the largest species of otter living in South America today and a whopping ten times the size my favorite species, the small-clawed Asian otter.

By examining the fossilized teeth, the team concluded that the shape of the molars of  S. melilutra had actually evolved at least four separate times among other otter species. So rather than sharing a common ancestor, this prehistoric giant otter and modern otters evolved on their own, through a process called convergent evolution.

Like many scientific discoveries, this one raises even more questions for scientists. For one thing, it’s hard to say how these otters grew to be so big. Xiaoming Wang, a paleontologist at the Natural History Museum of California and co-author of the paper, speculates that this otter’s size could reflect the size of its prey. The research team is still figuring out how these animals got around on land and in the water, and, presumably, if they terrorized their fellow mammals like otters reportedly do today.