COPROLITES AND ENTEROSPIRAE FROM THE MISSISSIPPIAN MICHIGAN FORMATION, WESTERN MICHIGAN

Fossilized excrement (coprolites) and fossilized contents of the digestive tract (enterospirae) of aquatic vertebrates have been identified from the Mississippian Michigan Formation in the subsurface of western Michigan. The Michigan Formation, exposed in abandoned gypsum mines around Grand Rapids, Kent County, consists of cyclically layered, thick units of gypsum, and thin beds of shale, dolomitic sandstones/siltstones, and detritus-rich dolomite. Based on the presence of mud cracks, ripple marks, angular to subrounded sand grains, and detrital, terrestrial plant debris, the depositional environment is interpreted to have been a shallow, marginal sea with periodic input of floodwaters from streams.

The coprolites and enterospirae are found in lag deposits of medium-sized sand grains that lie at the base of shale units at the contact with underlying gypsum. The lag deposits also include fragments of unidentified fish teeth, fish scales, and shark teeth and spines. The vast majority of the recovered specimens were determined to be coprolites, and, although recovered, enterospirae appear to be quite rare.

The coprolites have a fluorapatite groundmass, range from 2-30 mm in length, and have variable shapes from spherical to cylindrical to irregular. Within some of the lag deposits, elongate coprolites have a preferred orientation. The coprolites were deposited under anoxic or reducing conditions as evidenced by the lack of boring of the specimens, paucity of associated benthic organisms, and the formation of pyrite and glauconite within and around the specimens. The coprolites are interpreted to have been formed by predators, vegetarians, and bottom feeders based on the presence of bone, plant debris, or silt inclusions within the coprolites.

One enterospirae, 5.2 mm in maximum width, shows clear, submucosal folds, visible in hand sample. This enterospirae also exhibits a coiled nature, and includes a well preserved fish(?) scale. Most described enterospirae are known as heteropolar spiral coprolites, and are attributed to the spiral valve present in sharks and rays. This specimen recovered from the Michigan Formation is not a spiral coprolite, and thus is not attributed to the shark or ray families. Instead, it is likely the enterospirae of a fish or other aquatic vertebrate.