The burrow is 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.
Discovery News reported that Ed Simpson, a palaeontologist at Kutztown University in Pennsylvania, came up with two basic lines of reasoning that the burrow was either formed by water wind or other physical processes, or it was formed by an organism.
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.
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.