Pentremites
(Blastoid) with Bryozoans
Echinodermata,
Blastoidea, Spiraculata, Pentremitidae
Paleozoic,
Mississippian, Lower Carboniferous (320 to 360 million years ago)
Paint Creek
Formation, Ridenhower Member - Millstadt, Illinois
Blastoids are an extinct marine invertabrate Class belonging to the Phylum Echinodermata. Blastoids are restricted to paleozoic sediments and range from the Ordovician the Permian with their peak development and diversity in the Mississippian Period. Fossil Blastoids are easy to recognize by the symmetrically arranged calcareous plates, they look like a roundish bulb with five (5 Ambulacra) areas that meet at the top, and are commonly called fossil Rose Buds or Hickory Nuts in the South. The Calyx is composed normally of 13plates that are firmly united. Pinnules are rarely fossilized and are attached to the outer edge of the ambulacra. Pinnules caught the food for this invertabrate and transported its catch toward the mouth, top and center. The Stem elevated the Calyx above the sea floor, rising it higher in the water column than most other invertabrates on the crowded sea floor, and keeping its delicate pinnules from harm. Pentremites is the most common Blastoid form, it flurished in the upper Mississippian (Chesterian) age of the eastern USA. 1Bryozoans: What makes this specimen interesting is not the 5 pentremites (blastoids), but the presence of bryozoans (the "seafan" looking formations on left of specimen).
These "moss" animals, unlike corals, have a true mouth and anus, and a ciliated filter-feeding mechanism called a lophophore. Yet they are tiny animals, only about 1/24 inch long. They are very common in carbonates and shales and tend to be well preserved in Paleozoic limestones and shales. On each bryozoan, look for tiny pin-pricks, which are the openings in which the bryozoan animals lived. 2
References:
1. http://www.fossilcrinoid.com/blastoidd.html - Fossil Chrinoids
2. Rocks and Fossils, The Nature Company Guides (1997)