Gymnolaemate bryozoans are animals that live on ocean bottoms, growing on rocks, seaweeds, or even other animals. They are tiny, but a microscope would reveal their cylindrical external skeleton enclosing soft body parts within. The oldest gymnolaemate fossils are from the late Ordovician (about 450 million years ago). Early gymnolaemates did not have exoskeletons. What they left in the fossil record was instead fossilized evidence of their habits of drilling into shells. While there may have been Paleozoic gymnolaemates that didn’t bore into shells, they did not leave a trace. By the early Cretaceous (about 145 million years ago), gymnolaemates with calcified skeletons began to diversify and leave a substantial fossil record behind. They continued to diversify to the present, into more than 1000 genera. Most bryozoans living today are gymnolaemates.

Bryozoans have an ample fossil record. Thousands of fossils have been found that date to the early Ordovician (about 480 million years ago) and later. However, bryozoans don’t appear in older rocks that contain fossil evidence of most other animals without backbones (invertebrates). The earliest bryozoans may have lacked the hard body parts that fossilize. Also, because they are microscopic in size, bryozoans could be easy to miss if it weren’t for their colonial behavior. Bryozoans live in huge colonies of thousands or millions of identical individuals that form moss-like coverings on the sea bottom. Beginning in the Ordovician, fossil bryozoans are found as limestone made of their broken up skeletons. Many buildings include marble (derived from limestone) made from bryozoan remains. As Bryozoans got really abundant during the Mississippian (about 350 million years ago), they became an important food source for bottom-feeding organisms, and they continue to play that role today.