Blastoid (Pentremites) Fossil - Illinois

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 crinoid relatives, they were equally as common in rocks around the world. Blastoids were protected by a set of interlocking plates that formed the main body, or theca. In life, a typical blastoid's theca was attached to a stalk or column made up of stacked disc-shaped plates.