Fossil Bryozoans: The Tiny Colonial Animals of National Park Rocks

Bryozoans are not as familiar as many other invertebrates, but they turn up among the fossils of many national parks, and they still live today. They are colonial animals, yet their fossils are usually small: each animal was microscopic, and a whole colony was often just a few centimeters or inches across, with delicate structures that broke easily. Many colonies took the form of fine lace — the fenestrate bryozoans — or of branching twigs.

Fenestrate bryozoan fossils, Grand Canyon-Parashant National Monument, Arizona. Image from the National Park Service's page.

What bryozoans are

The phylum Bryozoa gets its name from ancient Greek roots meaning "moss animals," and bryozoans have also been called sea mats and lace corals. They are filter-feeding colonial animals that live mostly in the sea. They prefer warm tropical water but are found worldwide. They first appear in the fossil record in the Early Ordovician. Both mineralized and non-mineralized bryozoans live today, but the non-mineralized kinds have left very few fossils.

They are easy to mistake for corals, since both build colonies of many small individuals. But each bryozoan animal, called a zooid, is much smaller than a coral animal and more complex: it has a one-way digestive tract, as people do and corals do not, and it filters its food from the water rather than catching small prey and relying on symbiotic algae.

Bryozoan colonies grew as simple mats and crusts, mounds or domes, branching twigs, dense bushy forms, and nets or lace. Identifying a fossil to genus or species is usually hard, because many species built similar and highly variable colonies, and telling them apart often takes a microscope.

Bryozoan fossil from the Mississippian Redwall Limestone, Grand Canyon National Park, Arizona. Image from the National Park Service's page.

Bryozoans in the national parks

Bryozoan fossils are known from at least 64 units of the National Park System, mostly parks with exposed Paleozoic marine rocks. Along with brachiopods and crinoids, they are a significant part of many shallow-sea fossil communities of the Paleozoic. Because they are small and usually broken, they tend to play a supporting role rather than take center stage, but a few parks have especially good records:

• Mississippi National River and Recreation Area, Minnesota, has many kinds of small colonies in its Ordovician rocks.
• Carlsbad Caverns National Park, New Mexico, and Guadalupe Mountains National Park, Texas, preserve bryozoans as an important part of a Permian reef.
• Grand Canyon National Park, Grand Canyon-Parashant National Monument and Lake Mead National Recreation Area, in Arizona and Nevada, have extensive records from the Mississippian, Pennsylvanian and Permian. Bryozoans are among the fossils of the Kaibab Formation on the rim of the Grand Canyon.

Bryozoans of varied shapes from the Ordovician Decorah Shale, Mississippi National River and Recreation Area, Minnesota. Image from the National Park Service's page.

Twig-style branching bryozoans in the Pakoon Limestone, Grand Canyon-Parashant National Monument. NPS 2020 field inventory.

Screws and lyres

One of the best-known bryozoans is Archimedes, from the middle to late Paleozoic. Its fossils look like screws: they are the central axis that once supported a spiraling lacy frond, which is rarely found still attached. Archimedes has been found in several parks, including Mammoth Cave National Park in Kentucky.

The bryozoan Archimedes, Mammoth Cave National Park. National Park Service photos.

Mammoth Cave also has another showy type: lyre-like bryozoans, with a V-shaped base holding up a lacy colony, named for the lyre, a stringed instrument with a fixed crossbar.

Lyre-like bryozoans, Mammoth Cave National Park. National Park Service photo.

Sources

• National Park Service, "Fossil Bryozoans," from the series Invertebrate Fossils in National Parks: https://www.nps.gov/articles/000/fossil-bryozoans.htm
• A photograph credited to a published identification guide is not reproduced. The source spells "twig-style" as "twig-stye."