Bryozoans

Bryozoans are some of the most abundant fossils in the world. They are also widespread today, both in marine and freshwater environments, living at all latitudes and at depths ranging downward to at least 27,900 feet (8,500 meters).

Marine bryozoans show up in the fossil record in the early part of the Ordovician Period, about 485 million years ago. In Kansas, fossil bryozoans are common in the Pennsylvanian and Permian rocks of the eastern part of the state.

Freshwater bryozoans are virtually unknown as fossils, presumably because they did not have mineralized skeletons. Throughout their nearly 500-million-year history, marine bryozoans have been abundant and widely distributed geographically. They are the most abundant fossils in many limestones, calcareous shales, and mudstones. At least 3,500 living species and 15,000 fossil species are known.

Bryozoans are small animals (just large enough to be seen with the naked eye) that live exclusively in colonies. In fact, the Phylum Bryozoa is the only animal phylum in which all known species form colonies. The name comes from two Greek words, bryon (moss) and zoon (animal), and bryozoans are sometimes called moss animals because some colonies resemble mosses.

Bryozoans are sometimes confused with corals, another colonial group of animals. Like corals, most bryozoans secrete external skeletons made of calcium carbonate, which form the framework of the colony. Bryozoans, however, are more complex organisms than corals and generally don't build reefs.

Individual members of a bryozoan colony are called zooids. Although all zooids in the colony are physically connected, each lives in its own calcium carbonate compartment. Some or all zooids in a colony are feeding zooids, equipped with tentacle-bearing feeding organs. Bryozoans feed by projecting these tentacles into the water through openings in their external skeletons. The tentacles have tiny moving filaments called cilia, which create currents that draw microscopic organisms and plants into the mouth. Feeding zooids also have an alimentary canal, muscles, a nervous system and a U-shaped digestive tract. Simple egg- and sperm-producing structures are also present in some zooids in every colony.

Bryozoan colonies start out with a single zooid, which may be produced either sexually or asexually (through budding) by the parent colony. As this original zooid begins feeding, it buds to form additional genetically identical zooids. These new zooids also bud, forming the colony. Large colonies may consist of hundreds of thousands or even millions of zooids.

Some bryozoans built colonies that grew from the seafloor in branching structures; these fossils look like something like twigs. Other species erected netlike frameworks, while some spread like a crust on shells, rocks, plants, and even other bryozoan colonies. Almost all the fossils are fragments of colonies; only rarely is an entire colony preserved.

Bryozoan fossils turn up frequently in the Pennsylvanian and Permian rocks of eastern Kansas. The Florena Shale in Riley and Pottawatomie counties is an excellent place to find bryozoans, and they are also common in the Plattsmouth Limestone Member (of the Oread Limestone), the Beil Limestone Member (of the Lecompton Limestone), and the Topeka Limestone. Bryozoans are less common in the Cretaceous rocks to the west.

Stratigraphic Range: Lower Ordovician to Holocene.

Taxonomic Classification: Bryozoans belong to Kingdom Animalia, Phylum Bryozoa. The phylum is divided into three classes, the Phylactolaemata (freshwater bryozoans), the Stenolaemata, and Gymnolaemata.

Text and photos from Windows to the Past: A Guidebook to Common Invertebrate Fossils of Kansas, Kansas Geological Survey Educational Series 16.