New Perspectives on Faunal Analyses of the Mississippian Michigan Formation, Western Michigan

Exposures of the Mississippian Michigan Formation occur in a former gypsum mine in Wyoming, Michigan. The formation consists of six cyclic sets of alternating units of shale, dolomite, and gypsum, believed to have been deposited in marine to marginal-marine environments (e.g., restricted embayments, salinas, lagoons). Prior to the current study, a limited number of fossils and trace fossils of bottom-dwelling organisms had been found at the site. Despite the paucity of evidence for bottom-dwelling organisms, evidence of a diverse nektonic vertebrate fauna includes fish teeth, fish scales, dorsal-fin spines, abundant phosphatic nodules (commonly reported as coprolites), and a few cololites. This study evaluates alternative hypotheses for the environment of preservation: (1) water stratification with a surface oxygenated layer but anoxic bottom conditions; or (2) alternating oxygen-poor and oxygen-rich conditions, the later perhaps a result of periodic storms and flooding resulting in lag deposits (thanatocoenosis). The presence of an unabraded shark tooth found in its natural eruption state, a partial jaw retaining several teeth, unabraded dorsal spines, and a few delicate cololites supports the conclusion that at least some of the vertebrate remains underwent little transport. On the other hand, skeletal-rich layers may be lag deposits and suggest mixing and oxygenation of the water column. New trace fossils identified as borings within some phosphatic nodules also suggest moderately oxygenated water. Micro-fragments of gastropods, bryzoa, and rugosan coral occur within a few nodules, suggesting the presence of some benthic, marine invertebrates, whereas the occurrence of Lingula sp. in the upper part of the sequence suggests deposition in an embayment or estuary. The potential importance of water-column stratification with hypoxic bottom conditions is still being investigated, but the presence of benthic organisms, borers, and burrowers suggests that a stratified water column may not be sufficient to explain all aspects of the faunal assemblage.