Earliest Burrow of Four-Legged Animal Found

The fossilized remains of a groundhog's kind of quarters has now been tracked back to an amphibian that lived a whopping 350 million years ago.

Researchers have found the earliest burrow of what had to be a four-legged critter at a time before many reptiles and long, long before mammals were around. The burrow was likely home to a hefty creature that also left its footprints in related rocks, but no bones.

The discovery of the burrow came when a professor and his students were out looking for signs of ancient animals in the rocks exposed by road cuts in Pennsylvania.

"We were all prospecting for tracks," said Ed Simpson, a paleontologist at Kutztown University in Pennsylvania. "We came upon this structure and said, 'This is weird.'"

The borrow is, of course, now filled with sediments which have turned to stone. But it retains the gently funnel-shaped opening which leads to a downward dropping tunnel, which rises again to the main borrow. There are also features on the outside of the burrow that make a strong case for it having once been open to the surface, near a river.

Simpson asked his students to come up with ideas of what the structure was, and then justify their ideas. The two basic lines of reasoning were that it was either formed by water wind or other physical processes, or it was formed by an organism. Students Lauren Storm and Mattathias D. Needle studied the possibilities, Simpson said, and their names are now the first two on the paper reporting the discovery in the journal Palaeo.

"It's very difficult to get an erosive feature like this," Simpson explained. The only thing that comes close is a pothole, which is formed in a river by rocks, but the hole was too deep and the upward arc of the tunnel can't be easily explained as a pothole.

Another possibility is that a buried log could have rotted away, said geologist David Loope of the University of Nebraska in Lincoln. But doesn't seem likely to have produced this structure either, he said.

A primitive burrow seems far more feasible, Loope agreed. What's more, there are probably lots more like it in sedimentary rocks all over the world, if only people were looking for them.

"There's so much out there to see that so many of us walk past," said Loope. So it really helps to identify and describe structures like this so that people know what to search for, he said.

As for the structure of the burrow itself, it represents a very early stage of development of large burrows that have an evolutionary history of their own, said Simpson.

"These might be the first step for a more complicated burrow system," said Simpson. "If you look at the (later) Permian fossils they actually make corkscrew burrows." These perhaps allowed their inhabitants to fight off intruders by turning corners all the way down -- as in a castle turret.

Regarding the animal that dug the burrow, "it was a tetrapod definitely," said Simpson, referring to early reptile or amphibian with four legs, a spine and a tail -- as opposed to some sort of giant worm or other invertebrate. The best bet was that it was an amphibian, because of the age, he said.