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Skolithos is a common type of trace fossil that has been found in rocks as old as 541 million years. Trace fossils are not the fossilized remains of organisms but rather the burrows, footprints, and other structures that resulted from the animal’s activities.
In the case of skolithos, it’s widely believed that a vermiform (resembling a worm) animal created the straight, vertical, tube structures. These worm-like critters probably lived by filtering plankton from the turbulent water of a shallow marine environment. The vertical tubes may have been a dwelling place to retreat to, though their specific purpose is not known.
In the above picture, captured in north central Arkansas, a sandstone has weathered to reveal skolithos traces permeating the approximately 460 million year old rock. This example is from an exposure of the St. Peter Formation, Buffalo National River Park, Marion County, Arkansas.
To see more views of skolithos traces from Arkansas click here