Rosselia – or Rosselia socialis – is a trace fossil that’s common to rocks deposited in a variety of shallow marine environments such as estuaries, tidal flats, lagoons, etc.. This picture was taken in a quarry in the Pennsylvanian Bloyd Formation, near Greers Ferry, Arkansas. Rosselia is a funnel-shaped burrow with concentric cone-like layers, and a sandy plug near the center. The picture shows a side view, or cross-section, of several burrows.
Like many trace fossils, Rosselia was made by a soft-bodied animal that was rarely if ever fossilized. We only know it existed because of the burrows it left behind. They may represent places an animal lived, fed, or perhaps both.
One theory suggests the burrows were occupied by worm-like animals that fed by filtering nutrients from sediment, then excreting the sediment outward around their bodies in concentric muddy layers. When new beds of sand were deposited, the animal would crawl to the top of the sand bed and make a new burrow; this behavior is clearly evident in burrows at the center of the photo.
Be sure to check out more pictures of Rosselia here!
Above is a commonly misunderstood geologic feature called an Asterosoma. Almost everyone, upon first seeing an Asterosoma, thinks it’s either a fossilized plant, flower, or some kind of fossilized animal – usually an octopus.
Asterosomas are actually trace fossils left behind by ancient marine animals (most likely worms or shrimp) that burrowed through mud in a delta or tidal-flat. This one was found in the Carboniferous section of north Arkansas and is roughly 300 million years old. These trace fossils are called Asterosoma because of their star-like shape.
In cross-sectional view, multiple Asterosomas sometimes overlie one another connected by a central vertical tube – like a garland of Asterosomas. This suggests that, as new sediment was periodically washed into the environment, the animal may have burrowed its way back to the top of the mud and wallowed out another home for itself. The animal itself was too soft-bodied to be preserved in the rock record.