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Geopic of the week: Rosselia trace fossils

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.

 

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