Bryozoan fossils are the preserved remains of aquatic animals that lived in colonies. Often called “moss animals,” these tiny invertebrates constructed communal skeletons from calcium carbonate, which readily fossilized after the colony’s death. These structures settled on the seafloor and were buried by sediments like mud and sand. Over millions of years, minerals replaced the original skeletal material, preserving the delicate structures we find today. Bryozoans are a common type of fossil, and their remains contribute to many limestone deposits around the world.
Identifying Bryozoan Fossils
The most definitive feature for identifying a bryozoan fossil is the presence of countless tiny pores on its surface. Each of these microscopic openings, called a zooecium, housed an individual animal known as a zooid. These pores are uniform in size and distribution across the colony’s surface and are often best viewed with a hand lens. The individual openings are less than a millimeter in diameter, a detail that helps distinguish them from other colonial fossils.
Bryozoan colonies grew in a variety of distinct shapes.
- Branching, where the fossil resembles small twigs.
- Fenestrate, or lacy, which creates a delicate, net-like sheet. A well-known example is the genus Archimedes, which has a spiral, screw-like central axis.
- Encrusting, appearing as thin sheets that grew over other objects like shells or rocks.
- Mounded, which form irregular, lumpy masses.
Amateur collectors often confuse bryozoan fossils with those of colonial corals. The primary distinguishing feature is the size of the individual openings. Bryozoan zooecia are much smaller, appearing as mere pinpricks, while the cups that held coral polyps are visibly larger, typically more than a millimeter across. While both can form branching or mounded colonies, the scale of the animal housings provides a clear point of differentiation.
Geological Context and Age
Bryozoans have a long history preserved in the fossil record, first appearing during the Ordovician Period, approximately 485 million years ago. This makes them the last major phylum to appear in the fossil record. Their calcite skeletons mineralized readily, making them common in marine sediments from the Ordovician onward. They were particularly abundant and diverse throughout the Paleozoic Era, which ended about 251 million years ago.
The wide distribution and rapid evolution of different bryozoan forms during the Paleozoic make them useful as index fossils. Geologists use the presence of specific bryozoan species to determine the age of sedimentary rock layers. Finding a particular type of bryozoan can help correlate rock strata across vast distances, providing insights into ancient environments and geological timelines.
Finding Bryozoan Fossils
Fossil hunters are most likely to find bryozoans in sedimentary rock that formed in ancient marine environments. Limestones, shales, and mudstones are productive hunting grounds because these rocks formed from the fine-grained sediments that easily buried and preserved the delicate skeletons. The fossils are often found weathered out of these rock layers, appearing as loose fragments on the ground.
Certain geographic regions are famous for their abundance of bryozoan fossils. In the United States, rocks from the Pennsylvanian and Permian periods in eastern Kansas are known for containing well-preserved specimens. Similarly, the area encompassing parts of Ohio, Kentucky, and Indiana is rich in Ordovician rocks that yield a tremendous number and variety of bryozoan fossils.