Sponges are the most simple form of animal found as fossils in Kentucky. They all live in water and most live in seawater. They have a wide variety of shapes, but all have pores through which they pump large quantities of seawater. Most modern sponges do not secrete a hard skeleton that would be preserved after death. Some forms secrete calcium carbonate or silica, and these are the types that have been preserved as fossils in Kentucky.
Brachiospongia digitata, from Nettleroth (1889)
Lithographic Plates from Kentucky Fossil Shells--A Monograph of the Fossil Shells of the Silurian and Devonian [and Ordovician] Rocks of Kentucky
One of the most unusual sponge fossils in Kentucky is Brachiospongia digitata, a silica sponge. Its diameter ranges from 6 to 12 inches. This form is found in Ordovician limestones and siltstones in the central part of the Blue Grass Region.
Perhaps the most common sponge fossils found in Kentucky are the stromatoporoids, or stroms for short. Stroms are calcareous sponges that form mounds 2 to 3 feet across on the seafloor. Stroms still exist today in moderately deep water. Ordovician fossil stromatoporoids can be seen along roadcuts in the Blue Grass Region, especially near Lexington, where the white mounds contrast with the gray limestones. Devonian stroms can be seen at the Falls of the Ohio near Louisville.