Bryozoans (Phylum Bryozoa) are colonial, filter-feeding animals that are mostly marine but a few live in freshwater. They range from Ordovician to Recent and are common in marine limestones and shales in several geologic systems present in Ohio. They are particularly abundant in Ordovician rocks in southwestern Ohio.
Bryozoans consist of a skeletal structure of calcium carbonate that has numerous tiny holes or openings dotting the surface. These holes once housed individual bryozoan animals that derived their nutrients from the seawater. Bryozoan colonies range from small encrusting forms on shells of other invertebrates to branching, twig-like forms, to fan-shaped forms. Most colonies were only a few inches in diameter but a colony of an Ordovician form found in the Cincinnati region ( Florence, Kentucky) is more than 26 inches in diameter and is one of the largest known bryozoan colonies.
Although many bryozoan species are common fossils in some rocks, and some can easily identified, many species are only identifiable using microscopic techniques. Commonly, branching forms are broken into many pieces.