The tiny colonial animals called bryozoans (bry'-oh-zoh'-ahns) generally build stony skeletons of calcium carbonate.
Bryozoans grow in a variety of shapes and patterns, mound-shaped, lacy, tree-shaped, or even screw-shaped. The skeleton has numerous tiny holes, each of which is the home of a minute animal. They spend their lives attached to the sea floor, to stones, or to other animals.
Bryozoans are among the common fossils. The oldest ones come from Cambrian rocks over 500 million years old, and their descendants live today. During the Mississippian Period bryozoa were so common that their broken skeletons formed entire limestone beds.
Fossil bryozoa may be found either in shales or limestones, and they occur throughout Illinois.