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.