Bryozoan fossils are common throughout Illinois in shale and limestone. The oldest Bryozoan fossils are more than 500 million years old, dating to the Cambrian Period of the Paleozoic Era. Bryozoans live in colonies attached to the floor of the ocean or to objects there. The colony generates a skeleton of calcium carbonate, with small holes serving as living spaces. Some colonies generate mound-shaped structures, while others create tree-shaped, screw-shaped, or lacey forms. In some areas, limestone is primarily composed of bryozoan skeletons.