Ceratopea (Ordovician) Phylum Mollusca, Class Gastropoda
These fossils are the operculum or lid of gastropods. This lid acted as a door to close the opening of the shell when the snail withdrew inside. They are found in dolostones in the Ozark Plateaus Region. During life, the operculum is a massive horn-like structure of calcium carbonate and because of its mineral composition, or the mode in which the mineral matter was deposited, is especially susceptible to silicification (replacement by chert) (Yochelson and Wise, 1972). Only one specimen with operculum and shell preserved together has been found in Arkansas by O.A. Wise (Arkansas Geological Commission).
Operculum and shell preserved together. Scale in centimeters.
*top drawings from Fossils of Arkansas by Tom Freeman, 1965.
Freeman, Tom, 1966, Fossils of Arkansas: Arkansas Geological Commission
Bulletin 22, 53 p., 12 pls., 15 figs., 1 map.
Yochelson, E.L. and Wise, O.A., 1972, A life association of shell and operculum in the Early Ordovician gastropod Ceratopea Ungus: Journal of Paleontology, v. 46, no. 5, p. 681-684.