Sponges (Phylum Porifera) are simple, filter-feeding animals that range from Precambrian to Recent. They are mostly marine but some forms live in freshwater. Sponges are best known as fossils from microscopic needle-like to geometric skeletal structures called spicules. These structures are found in marine rocks in Ohio ranging from Ordovician to Recent. Flint or chert deposits, particularly in Devonian and Pennsylvanian rocks in Ohio are thought to have derived their silica from siliceous sponge spicules. The extensive bed of flint in the Pennsylvanian-age Vanport limestone at Flint Ridge in Licking and Muskingum Counties is thought to be silica derived from a massive, long-existing bank of sponges.
Complete sponges are relatively rare in Ohio rocks, although this may be in part because they are difficult to recognize. However, a group known as stromatoporoids is well represented and easily recognizable in Ohio Ordovician, Silurian, and Devonian rocks. These organisms were originally thought to be related to corals but are now recognized as an informal group of calcareous sponges. They form hemispherical shapes that may be as large as a soccer ball.