- Calvin J. Frye and Rodney M. Feldmann
Kirtlandia, No. 46 (August 1991):49-71,
The Cleveland Museum of Natural History.
Enigmatic, flat body fossils have been collected from at least 9 localities in dark shales of Late Devonian age in northeastern Ohio. The fossils have been found mainly in the Cleveland Shale, a black shale interpreted to represent an anoxic environment, and more rarely in the Chagrin Shale, which was deposited in a dysaerobic environment. The benthic fauna of these shales is sparse and restricted. These Ohio fossils are comparable to similar structures found within the Woodford Shale of Oklahoma, a formation of equivalent age and depositional environment as the Chagrin and Cleveland shales.
The enigmatic fossil remains comprise at least seven authentic species referable to the genus Sidetes Giebel, 1849. Five of these species are found in Ohio. Spathiocaris tenuicosta Cooper, 1932 is morphologically indistinguishable from Sidetes chagrinensis (Ruedemann, 1916), and is, therefore, placed in synonymy. Similarly,Spathiocaris striatula Cooper, 1932 is the junior subjective synonym of Sidetes lata (Ruedemann, 1916) and Spathiocaris woodfordi Cooper, 1932 and Spathiocaris plicifera Cooper, 1932 are junior subjective synonyms of Sidetes newberryi (Whitfield, 1882). All of the fossils are extremely thin and, typically, flat structures marked with fine, nearly concentric corrugations or folds and range from 0.7 cm to 8 cm in length. At various times they have been considered to be brachiopods, barnacle plates, cephalopod aptychi, or the phyllopod crustaceans, Spathiocaris or Aptychopsis. Scanning electron microscopy reveals no ultrastructure within the fossils. Electron-dispersive X-ray spectroscopy indicates they contain neither calcium, strontium, nor phosphorus. Brachiopods and arthropods from the same units do contain phosphorus. Their general morphology and ornamentation is also unlike that of brachiopods or arthropods, permitting their assignment to the Cephalopoda. They appear to be the aptychi of ammonites, structures which probably served as the animal's lower jaw. Their probable preservation as carbon films remnant of degraded organic material is consistent with what is known of cephalopod aptychi. The reconstruction of two specimens that had been cracked and flattened during compaction shows the original form of the structures to have been broadly curved and scooplike. This is consistent with reconstructions of Mesozoic ammonite jaws, and strengthens the assignment of these fossils with the cephalopods.