AN ARTHROPOD-ZOOPHYCOID ASSEMBLAGE IN THE UPPER DEVONIAN (FAMENNIAN) CHAGRIN SHALE MEMBER OF THE OHIO SHALE

A series of siltstone-shale couplets located about six meters from the top of the Chagrin Member (Upper Devonian, Famennian) of the Ohio Shale along Euclid Creek in Cuyahoga County, Ohio, records a repeating pattern of arthropod and zoophycoid activity. Ichnofossils present in these beds include Chagrinichnites brooksi, C. osgoodi, Zoophycos, Cruziana, Palaeophycus, Bifungites, Rhizocorallium, and ?Skolithos. Other ichnofossils found include vertical burrows, oblique burrows, and arthropod traces other than Chagrinichnites and Cruziana. The siltstone layers have a greater abundance and diversity of ichnospecies than do the shale layers. Zoophycos can be found in both, but is much more common in the siltstones. Cruziana, rare in these beds, is nearly always found closely associated with Zoophycos.

The C. brooksi form present is a shallow, elongate, concave V-shaped burrow which may be interpreted as a trace made by the phyllocarid Echinocaris rather than the previously presumed maker of this form, Palaeopalaemon. These shallow C. brooksi burrows contrast with C. osgoodi specimens present in the siltstones in and below this sequence which appear to represent escape structures. Body fossils are very rare within this bed, but brachiopods have been found in the shales directly below.

These siltstone-shale couplets represent a period of relative shallowing near the top of the Chagrin Member. This is in sharp contrast with the presumed deepening represented by the black shales of the Cleveland Member of the Ohio Shale which directly overlie the Chagrin Member.