Earth Science Club of Northern Illinois (ESCONI)

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Fossil Friday #300: Orthotarbus robustus from the River

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ESCONI

This is the “Fossil Friday” post #300.  Expect this to be a regular feature of the website.  We will post fossil pictures you send in to esconi.info@gmail.com.  Please include a short description or story.  Check the #FossilFriday Bluesky/Twitter hash tag for contributions from around the world!

Illustration of two spider-like creatures, featuring distinct shapes and colors, one in green and the other in brown.

Is there a better way to post our 300th Fossil Friday than with a Mazon Creek spider? This beautiful spider is an Orthotarbus robustus. It was described by Alexander Petrunkevitch in 1945. Technically, O. robustus isn’t a spider. It belonged to a an extinct group of spider-like animals called Architarbids.  Architarbids lived during the Mississippian and Pennsylvanian Periods. 

Today’s fossil was sent in by Andy Ohst. He collected this rare and beautiful specimen from the Mazon River in the Fall of 2025. Thanks for sharing, Andy!

Two round, dark stones placed above a small U.S. nickel coin, all on a textured fabric surface.
Close-up of a dark, round fossilized rock with faint impressions visible on its surface, placed on a textured beige background.
A close-up image of a flat, irregularly shaped black stone with faint markings on its surface, lying on a textured fabric background.
Close-up of a round, dark stone with a rough texture, resting on a light-colored fabric surface.

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