Rare fossil specimen offers evidence of social behaviour among ancient snakes

A perfectly preserved burrow of fossilized snakes shows that the reptiles have been social creatures for almost 40 million years, says a new research paper co-authored by a University of Alberta paleontologist. 

It's well known that snakes today gather in burrows for all kinds of reasons, including breeding and surviving cold temperatures by piling together to maintain their body temperature. But new research on a fossil found almost 50 years ago provides a rare glimpse at the deep roots of that behaviour.

"We expect this out of mammals, of course, but we don't go looking for it in the fossil record for reptiles," said Michael Caldwell, a University of Alberta professor and co-author of a new paper published in the Zoological Journal of the Linnean Society. 

The fossil in question was a 1976 discovery of three "beautiful, articulated" specimens of boa-type snakes found together in a burrow in Wyoming. The fossil hasn't been properly researched until now.

Caldwell said the find was rare. Scientists are usually limited to studying isolated fossil snake vertebrae, "and we don't know what part of the snake they've come from."

In contrast, these fossilized snakes had almost completely preserved vertebral columns and skulls.

Researchers speculate the Wyoming burrow could have been flooded, filled with sediment and fossilized the three snakes in it together, said Jasmine Croghan, a clinical instructor for Oklahoma State University's Center for Health Sciences, and the paper's first author.

A fourth snake fossil was found nearby, Croghan said. 

"Whether they're hibernating or they're just running from a volcanic ash storm because they're terrified, which is possible, we really don't know," Caldwell said. 

"But we do have evidence that they were at least willing to hang out with each other when they died." 

While this paper's authors don't suggest they were the first to discover the first fossil evidence of social behaviour in snakes, Caldwell says the find is still extraordinary. 

"That's another really cool part of this for me, is just the idea that we have a nearly 40 million year old piece of evidence that shows social behaviour in reptiles."

In Alberta, rattlesnakes, bull snakes and garter snakes are known to often overwinter in groups.

"Most snakes will take shelter in an easy-to-access place underground — like a burrow usually dug out by something else. So they are pretty dependent on mammals usually, or maybe some turtles or tortoises, so that they can actually have burrows to access," Croghan said. 

James Gardner, a curator at the Royal Tyrrell Museum in Drumheller, Alta., who specializes in fossil amphibians and reptiles, said he's "comfortable with the hypothesis that this was a natural accumulation of multiple individuals — probably in a burrow" that was presented in the paper. 

"This fossil shows us that by at least 38 million years ago, snakes were using burrows — either for avoiding heat or cold [or] as places of sanctuary, which is a behaviour that we see in modern snakes in similar environments," Gardner said. 

Gardner said the fact that the fossil was found almost 50 years ago puts a certain limit on the data that can be derived from it. It's likely that little information was recorded about it, he added, including the local geology of the site. 

Retracing the fossil back to the location it was found could yield yet more finds, Gardner said. 

"There could be more fossil snake skeletons curled up there, in a burrow."