.6" Blastoid (Pentremites) Fossil - Illinois
This is a Pentremites blastoid fossil collected from St. Clair County, Illinois. These fossils are Upper Mississipian in age or approximately 325 million years old. They have been cleaned using air abrasives and remounted on the shale.
Blastoids are an extinct type of filter feeding, stemmed echinoderm that resembles a small hickory nut. They thrived during the Mississippian period before going extinct during the great Permian extinction. While not as diverse as their relative the crinoid, they were equally as common in rocks around the world. Blastoids were protected by a set of interlocking plates, which formed the main body, or theca. In life, the theca of a typical blastoid was attached to a stalk or column made up of stacked disc-shaped plates.