Bryozoans (sometimes referred to as Entoprocta and Ectoprocta) are microscopic sea animals that live in colonial structures that are much larger than the individual animal. Because these structures are usually composed of secreted calcite, they commonly form fossils. Bryozoans were so common in Kentucky's ancient past that they may be the most common form of fossil found in the State.

Bryozoan fossils occur in many forms, including finger-shaped, fan-shaped, mats, spiralling fans, and massive irregular mounds. Many of the fossils, if examined closely with a magnifying glass, will show the individual pits where the individual bryozoans lived. Each animal was attached to the inside of its pit and could not leave the pit. Bryozoa feed on microscopic organisms floating in the water, which they grab with tiny tentacles. Bryozoan fossils can be found in Kentucky's Ordovician, Silurian, Devonian, Mississippian, and Pennsylvanian rocks.

Bryozoa fossil

Fenestrate bryozoa colonies, like the three diffenent types shown above, are lace-like in construction. The individual bryozoan animals lived in microscopic tubes or pores on the lace branches. These are from Mississippian limestones in Kentucky. A Mississippian fenestrate bryozoan colony, called Archimedes (above), lived attached to a screw-shaped support structure. In this figure, only the structure is readily seen; the edge view of several lace-work sheets of the colony can barely be discerned extending laterally from the supports. In life, this support would have been upright and attached to the sea bottom or a shell of some sort.

Massive bryozoa colonies

Massive bryozoa colonies like this one are very commonly found in Ordovician limestones in the Blue Grass Region. This one is ornamented with little bumps (monticules).

Other types of bryozoans may have branching, tear, or chocolate-drop shapes.

Bryozoans are sometimes misidentified as bones by amateur collectors because they may have a bone-like appearance. Constellaria bryozoans have small circular, “star-shaped,” or “asterisk-like” bumps (monticules) that bone would not have. In section (look at the broken end of a fossil or edge of the rock containing the fossil), bryozoans will tend to be filled with calcite or concentric rings rather than exhibiting the spongy texture of bone.

fragments of marien bryozoans
cracks in branching or stick-like bryozoans and corals may mimic the appearance of articulated small bones


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Last Modified on 2023-01-05
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