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Article Excerpt
ABSTRACT
Fossilized vertebrate excrement (coprolites) and digestive-tract contents (cololites) are identified in the Mississippian Michigan Formation, exposed in old gypsum mines in the Grand Rapids area (Kent County). In western Michigan the formation consists of cyclical gypsum beds, thin shales, dolomitic sandstones/siltstones, and detritus-rich dolomites. These rocks, which contain mud cracks, ripple marks, and terrestrial plant debris, suggest deposition in a shallow, marginal sea with alternate desiccation and stream floodwater input. The coprolites and cololites are in lenses of sandstone at the shale contact with underlying gypsum. Coprolites are common, whereas cololites are rare. The sandstone lenses also contain shark teeth and spines and other fish teeth and scales.
The medium to dark gray or light-gray to brown coprolites range from 2 to 30 mm in length and vary in shape (spherical to elongate to irregular). They were deposited under anoxic conditions as evidenced by a lack of borers, a paucity of other benthic organisms, and the presence of pyrite and glauconite within the specimens. The coprolites were formed by predators, herbivores, and bottom feeders, based on inclusions of bone, plant debris, or silt. The vertebrates that produced the coprolites cannot be identified, but the coprolites are found in association with a variety of fish and shark remains. The rare cololites are recognized on the basis of their coiling. One cololite, 5.2 mm in width, is coiled, has submucosal folds within the coils, and includes a well-preserved fish scale. Another specimen may be spiraled, and thus possibly attributed to the shark or ray families.
INTRODUCTION
The Michigan Natural Storage Company (MNSC) underground facility, a former underground gypsum mine in Kent County, Michigan (Figure 1), provides exposures of the Mississippian-age Michigan Formation. In western Michigan this formation consists of six, cyclically bedded, thick layers of gypsum and thin beds of shale, dolomitic siltstones and sandstones, and detritus-rich dolomite. The strata have been informally divided into six units numbered 1 (youngest) through 6 (South et al. 1995). Generally, each unit consists of a layer of gypsum overlain by shale and then dolomite. The MNSC mine exposes units 1 and 2 in approximately 10 km of tunnels over a 300,000 square meter area at a depth of more than 24 m. The tunnels are generally about 3 m in height. Figure 2 shows the entire unit 2, a portion of the overlying unit 1 strata, and the dolomite and shale of the underlying unit 3. The contact between the top of the gypsum in unit 2 and the overlying shale is a diastem. Lenses of sandstone, which contain the coprolites and cololites of this study, occur along this contact (Figure 2). Using the terminology of McAllister (1985), in this paper coprolites are defined to be preserved excreted fecal material, whereas cololites are defined to be fossilized intestinal contents that were not excreted before the death of the animal.
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Although the strata are not particularly fossiliferous, an interesting diversity of fossils is found throughout the section. In addition to the coprolites and cololites, the sand lenses and associated shale contain shark teeth and spines, fish scales, fish teeth, and occasional trace fossils (burrows). Plant fragments occur at the shale-dolomite contact in unit 2. Although Dorr and Eschman (1970) reported the gypsum as being unfossiliferous, within the gypsum in unit 2 we have found small, nodular masses with laminae that could be of algal origin.
This study examines the abundant, small (up to 30 mm), medium to dark gray or light gray-brown, spherical to irregularly shaped structures found within the thin sand lenses at the base of the shale in unit 2 (Figures 2 and 3). Dorr and Moser (1964) first reported these structures to be coprolites, but no published study verifies that they were the fecal matter of vertebrates....
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