REINTERPRETATION OF PUTATIVE “ARTHRODIRE EGG CASES” FROM THE FAMENNIAN CLEVELAND MEMBER AS EXTREMELY LARGE THYLACOCEPHALAN ARTHROPODS

Specimens from the Cleveland Member (Famennian) of the Ohio Shale that have been interpreted as “arthrodire egg cases” are here reinterpreted as extremely large thylacocephalan arthropods. These then are the largest arthropods by far from the Ohio Shale. The specimens, preserved within large dolomitic concretions like those famously containing fossil sharks, have a maximum length exceeding 23 cm, about four times the length of smaller, concavicarid thylacocephalans from the Cleveland Member. The large specimens are mostly flattened to somewhat inflated and are seen in lateral to dorsal views. Preservation of the large forms is imperfect, but parts of both the exterior and interior are preserved. Carapaces are only partially preserved, mostly as thin films, but one specimen has a large portion of a carapace rim preserved. Some carapace material is iridescent. Preserved internal structures include a well-developed series of more than 40 transverse, lathlike elements (?branchial structures) extending ventrally from near the dorsum to beyond mid-carapace. Internal structures also include forms that can be classified as waves, reticular ribbons, fibers, rods, and knobs. Preservation of these large thylacocephalans contrasts with that of the smaller Cleveland Member thylacocephalans which are preserved on bedding planes in black shale and within the interiors of sharks preserved in Cleveland Member concretions.