Blastoid (Pentremites) Fossils - Illinois

This is a plate of Pentremites blastoid fossils collected from St. Clair County, Illinois. These fossils are Upper Mississipian in age or approximately 325 million years old. The blastoid fossils have been cleaned using air abrasives and remounted on a slab of matrix from the locality to create a beautiful display.

Blastoids are an extinct type of stemmed echinoderm related to crinoids. They would have attached themselves to the bottom of the ocean with a stalk and were filter feeders. The body of theca of blastoids resembles and acorn and is composed of interlocking armored plates. They first appeared in the Ordovician period, and reached peak diversity during the Mississipian before going extinct in the Permian extinction.