Paleontology in Michigan

Paleogeographic reconstruction showing the Appalachian Basin area during the Middle Devonian period.

Michigan's fossil record stretches as far back as the Precambrian.[1] Blue-green algae remains from this age were preserved between Copper Harbor and Eagle Harbor on the shoreline of Lake Superior.[1] By the early part of the Paleozoic, Michigan was located in equatorial latitudes.[2] Michigan was at least partly covered by seawater during the Cambrian.[3] Life in Michigan's Cambrian seas included some brachiopods, cephalopods, gastropods, and trilobites. Trilobites would become more common in Michigan as the Paleozoic progressed.[1]

During the ensuing Ordovician, Michigan remained inundated by seawater.[2] Brachiopods flourished and are among the most common fossils of the period in Michigan.[1] Cephalopods were also common in the Ordovician.[1] One straight shelled species was more than fifteen feet long.[1] Crinoids were also present. So were blastoids, but they were not as common as the others.[1]

Michigan's Middle Ordovician fossil record does not preserve any fish, however some strata of that age found in the Upper Peninsula correspond to deposits of that age on Ontario's St. Joseph Island where such finds have been made. Since pieces of the bony armor that once embedded in the skin of the fish Astraspis were preserved at St. Joseph's, the similarly aged nearby Middle Ordovician rocks of the Upper Peninsula are also likely to preserve similar fish fossils. If this supposition is correct, then Michigan's fish fossil record may go back as far 460 million years ago.[4]

In the next period, the Silurian, Michigan continued to be a marine environment.[2] Tabulate and tetra- corals appear.[5] Accumulations of these corals up to seventy feet thick are known from places like Engadine, Gould City, and Trout Lake.[5]

During the following period, the sea still covered Michigan.[2] Crinoids were very abundant in Michigan during the Devonian.[5] Brachiopods are also found in the Devonian, but are less common at that time than they were during the Ordovician.[1] Bryozoans and corals were also present.[5] Plant fossils of this age have been found but are relatively rare. Among which were fossils likely attributable to the tree Callixylon.[5]

The Middle Devonian is the best documented geologic epoch in the state's Paleozoic fish fossil record. Such discoveries have occurred in both the northern region of the Lower Peninsula and in the southeast. The northern region has been more productive for Middle Devonian fish fossils, with Alpena, Charlevoix, Emmet, and Presque Isle counties all contributing discoveries. The less abundant Middle Devonian fishes of southeast Michigan are known from the rocks of Arenac, Calhoun, Huron, Jackson, and Kent counties.[4]

A significant proportion of Michigan's Devonian fish were placoderms. Usually Michigan strata of this age only preserve their bony armor and gnathal bones. There are three main groups of placoderms that have been found in Michigan, the antiarchs, arthrodires, and ptyctodonts.[4] Arthrodires reported from Michigan rocks include Dunkleosteus, Holonema, Protitanichthys, and Titanichthys.[4] Ptyctodus is a representative example of a Michiganian ptyctodont. Bothriolepis is the only known antiarch from Michigan.[6]

Acanthodian fossils from Michigan are typically isolated specimens of the spines that supported their fins and these are commonly broken. However, the best preserved specimens of Michiganian acanthodians reveal large eyed generalists who ate plankton in the mid-level of the water column using teeth with multiple points.[6]

The Pleistocene American mastodon with a human to scale.

Sharks swam over Michigan during the Devonian. Since shark skeletons were cartilaginous and lacked hard parts conducive to fossilization, typically only their spines and teeth remain. Despite occurring so early in the fossil record, Michigan's cladoselachian sharks closely resembled modern forms. Fossil shark spines found in Michigan are usually the remains of ctenacanths and cladodonts. Bradyodont shark teeth have also been discovered in Michigan, however, it's also possible that these teeth were shed by animals more closely related to holocephalans than true sharks.[6] Tabulate and tetra- corals disappeared from Michigan during the Devonian.[5]

During the Early Carboniferous the sea covering Michigan began a gradual withdrawal.[2] Sharks persisted as members of Michigan's fish communities during the Mississippian.[6] Mississippian fish fossils have been discovered in Arenac, Calhoun, Huron, Jackson, and Kent counties.[4] Gastropod fossils persisted until the end of the Mississippian.[1] Brachiopods further persisted into the Mississippian but did not become as abundant as they were during the Ordovician.[1]

Swamps formed across the state as the Carboniferous period continued and the sea left the state.[2] These swamps were full of ferns and scale trees.[2]Xenacanth fossils are known from such deposits.[6] Plant fossils are common from Michigan rocks of Pennsylvanian age. The flora of Michigan back then included club moss trees, ferns, and horsetails.[5] Contemporary vegetation was preserved in the Midland and Saginaw regions.[7] Fossil lungfish burrows are another interesting find from the Pennsylvanian coal swamp deposits near Grand Ledge in Clinton County but these tend to be poorly preserved. Nevertheless, such fossils are an uncommon find.[6] Other Pennsylvanian fish fossils were preserved in Clinton and Saginaw counties of the central part of the state.[4]

During the Permian, sediments were being eroded away from Michigan rather than deposited.[2] As such there are no local rocks of that age. As such, no Permian fossils are known from Michigan.[1]

The ensuing Triassic period of the Mesozoic era is missing from the state's rock record for the same reason as the Permian. During the Jurassic, local plants left behind spores that would later fossilize. However, these are the only known local fossil from the time period since rocks of this age are buried deep underground and accessible only through core sample drilling. Like the Permian and Triassic, Cretaceous rocks are altogether absent from the state.[2] No dinosaur fossils are known from Michigan as there aren't any surface rocks of the right age to preserve them.[8]

The same erosional forces responsible for the Permian and Mesozoic gaps in Michigan's rock record were active during the ensuing Paleogene and Neogene periods of the Cenozoic era.[2] As such, no Cenozoic fossils older than the Pleistocene are known from Michigan.[1] Nevertheless, Michigan has many deposits made during the Quaternary period. At that time the state's entire landscape was reworked by glacial activity.[2] The area now submerged under the Great Lakes had been a lowland river system. As glaciers advanced and retreated they carved these areas into the Great Lakes and filled them as they melted.[3] The preservation of fossils in Michigan resumed when the last glaciers withdrew from the state. Between 17,000 and 13,000 years ago, much of Michigan's icy covering had disappeared. After the glaciers melted much of the state was covered in large lakes made of glacial meltwater. By 10,000 years ago many of these lakes had dried. Forests of spruce and fir grew on the newly exposed terrain. By the time about 2,000 years had elapsed, pine trees became the dominant members composing Michigan's forest.[9] The most common mammals in Michigan's Pleistocene fossil record were caribou, elk, Jefferson mammoths, American mastodons, and woodland muskoxen. Less common members of Michigan's fossil record included black bears, giant beavers, white-tailed deer, Scott's moose, muskrats, peccaries, and meadow voles.[10]

• Walter Auffenberg was born on February 6, 1928 in Detroit.
• Robert L. Carroll was born on May 5, 1938 in Kalamazoo.
• William R. Hammer was born in Detroit.
• Nicholas Hotton III was born in 1920 or 1921.

• Chester A. Arnold died on November 19, 1977 in Ann Arbor at age 76.
• Claude W. Hibbard died in Ann Arbor on October 9, 1973.