AN EXAMINATION OF THE DEVONIAN FISHES OF THE MICHIGAN BASIN

The Devonian, popularly known as the “Age of Fishes,” is marked by multiple major transitions in vertebrate biodiversity, including the rise of jawed fishes, the origin of tetrapods and severe mass extinction. Despite the evolutionary importance of this interval, Devonian vertebrates from the United States are poorly known relative to specimens of the same age from the United Kingdom, China and Australia and even Antarctica, with the possible exceptions of select, very early or late faunas such as the MacKenzie formation, Cleveland Shale and Red Hill. Fish fossils are actually common in fossil-bearing Paleozoic strata throughout the Midwest with many outcrops discovered over a century ago. We surveyed the Devonian fishes of the Michigan Basin for the first time in decades in order to provide a framework for renewed study of these taxa, ecosystems and localities and information for use in global occurrence databases. This has revealed several notable biogeographic patterns. Surprisingly, there is a total lack of antiarch placoderms in Michigan’s Late Devonian sediments, and they are rare in Michigan's Middle Devonian sediments. This is despite the extreme abundance of the group globally and in other parts of Devonian North America, as well as high preservation potential. The probability is low that this lack of fossil material is due to collection bias; it is likely indicative of another factor which limited antiarch dispersal. There was also a massive drop in fish diversity and abundance observed between Michigan's Middle and Late Devonian sediments. This pattern is most likely due to a change in environment potentially caused by a drop in sea-level or sediment dumping from the formation of the Appalachian Mountains. It is unlikely that this pattern was caused by collection bias. In other respects, the Middle Devonian Michigan fauna shows connections with both the Appalachian and Illinois Basins. Yet there is little overlap between these regions in the Late Devonian. Uplift in the Appalachians caused by contemporaneous orogeny may have created a barrier to exchange between the two regions greatly decreasing the abundance and diversity of vertebrate and invertebrate faunas. The Michigan Basin thus stands out as a rare endemic zone among the generally cosmopolitan composition of other marine ecosystems in the Late Devonian.