Summary and Conclusions
Approximately 120 specimens of flat, carbonaceous body fossils have been collected from the Cleveland and Chagrin shales of northeastern Ohio. The fossils have been found predominantly in the black Cleveland Shale, which probably represents an anoxic basinal environment (Broadhead, et al., 1982). A few have been collected from the underlying Chagrin Shale, a gray-green unit deposited in dysaerobic conditions (Barron and Ettensohn, 1981; Schwimmer, 1988; 1990). In addition, specimens housed at the National Museum of Natural History which were collected by Cooper in 1932 have been reevaluated in association with their time-equivalent relatives from the east. These fossils have been identified as representing seven species of Sidetes Giebel, 1847, and interpreted to be cephalopod jaw elements. This identification is based upon the general outline of the specimens, and particularly the pattern of ornamentation they exhibit. This concentric pattern of fine ridges is unlike that seen in arthropods, gastropods, bivalves, or brachiopods. Some of the specimens are preserved unfolded and flattened, while others are folded in half along the median line. Further evidence for this interpretation is furnished by energy dispersive x-ray spectrometry. These fossils show no trace of having once contained phosphorous within the structures studied. It is unlikely that alteration after burial would have so completely removed the element, as both arthropods and brachiopods from the same unit have remained phosphatic. On this basis, we can conclude that the Sidetes specimens in this study are neither arthropods nor brachiopods. Two specimens showed signs of compaction damage. When restored to their presumed original shape, they resemble reconstructions of Mesozoic ammonite jaws. It appears that these fossils served the same function in some Devonian cephalopods. By contrast, the Silurian Aptychopsis seems to have been a nautiloid operculum rather than a jaw element.