University of Michigan Museum of Paleontology
The Silica Formation: Famous Devonian Fossils from Northwestern Ohio
Driving along the straight country roads just west of the quaint town of Sylvania in northwestern Ohio, the scenery is dominated by the flat, bucolic farmland that is so characteristic of this part of the state. The well-manicured fields give no indication that a vast sea teeming with marine life covered the area during the middle of the Devonian Period, some 387 million years ago. Here and there one can spot clues—the large blocks of marine limestone that are occasionally used as ornamental stone outside some homes and apartment complexes. In fact, just a few feet below the surface lie thick deposits of marine sedimentary rocks that are famous the world over for the exquisitely preserved fossils they contain. The best known is the Silica Formation (known informally as the 'Silica shale'), a gray shale and limestone unit about 55 feet in thickness. Regionally, a blanket of glacial deposits derived from the last ice age covers the bedrock, but thanks to extensive quarrying operations that began in the 1920s, these were removed in places and the rocks beneath exposed. Countless tons of limestone and shale have been extracted in the decades since. The vast majority of the fossils from the Silica Formation were collected from just a few quarries near Sylvania, in Lucas County. The formation is also exposed in an active quarry about ninety miles southwest, near Paulding, in Paulding County, but access there is restricted and collecting has been limited. In the late 1960s, a quarry was opened near Milan in Washtenaw County, Michigan, but operations there were soon abandoned and the quarry flooded. Over the decades, many paleontologists and their students, as well as thousands of amateurs, have collected from these quarries, and their efforts have helped to document the geologic setting and the rich fauna of this famous formation.
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Specimen of the crinoid (sea lily) Arthroacantha carpenteri showing beautifully preserved details of delicate structures so typical in the Silica Formation. UMMP 61328. Photo courtesy of the University of Michigan Museum of Paleontology.
Devonian Paleogeography and Regional Setting
In the Middle Devonian (about 384-392 mya), warm shallow seas covered much of what is now North America. Ohio and Michigan were located in the subtropics, south of the equator. Where Michigan now lies, the earth's crust beneath the sea had been subsiding while thick layers of marine sediments were being deposited. The result today is a bowl-shaped accumulation of sedimentary rocks over two miles thick known as the Michigan Basin. The limestones and shales of the Silica Formation were deposited along the basin's southernmost flank. The sea in the Michigan Basin was connected to other basins further west in Illinois and Iowa, as well as to the Appalachian Basin to the east, through a seaway in what is now Ontario (see detail map). Different environments are represented in the Silica Formation, reflecting subtle changes in factors such as water depth, wave energy, and the influx of muds coming from land masses to the east. The diversity and abundance of organisms fluctuated in response to these environmental changes, providing the opportunity to study the local paleoecology of these faunas.
Stunning Preservation of a Diverse Fauna
Fossils from the Silica Formation are often exceptionally well-preserved, and fine details of very fragile species and delicate structures are common. Over 250 species have been described and the fauna is dominated by marine invertebrates, the most common of which are brachiopods, bryozoans, corals, and arthropods such as trilobites and crustaceans, including over 100 species of tiny crustaceans known as ostracods. The large brachiopod Paraspirifer bownockeri and the trilobite Eldredgeops rana (formally known as Phacops rana) are among the more iconic fossils. Other major invertebrate groups represented include mollusks (gastropods, bivalves, and cephalopods), and echinoderms (starfish, brittle stars, crinoids, and blastoids). There is an excellent record of diverse encrusting organisms that colonized the hard shells of other invertebrates, such as the brachiopod below. Vertebrates are represented by several groups of fishes. In parts of the formation, the calcareous shells of many species were partially replaced by iron pyrite (sometimes called "fool's gold") during fossilization.
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(Left) The large brachiopod (lamp shell), Paraspirifer bownockeri, covered by the delicate colonial encruster Hederella sp. UMMP 61129. (Right) A spectacular specimen with several individuals of the trilobite Eldredgeops rana. Gift of the Don White family. Photos courtesy of the University of Michigan Museum of Paleontology.
Fruitful Collaborations Among Professional and Amateur/Avocational Paleontologists
The documentation of the fossils from the Silica Formation provides one of the best examples of professional paleontologists and amateur/avocational paleontologists working together towards a common objective. The many dedicated amateurs who visited the quarries at every available opportunity had eventually accumulated collections as well as knowledge and experience about the quarries and fauna that often surpassed that of professionals. Many were interested in learning more about the fossils they were finding and eagerly donated rare or unique specimens to museums for formal study. Dr. Robert Kesling, a curator at the University of Michigan Museum of Paleontology (UMMP), recognized the mutual benefit of working together and, in 1971, he and several amateur collectors established the Friends of the University of Michigan Museum of Paleontology (FUMMP) to permanently foster cooperation between collectors and professionals. The FUMMP, working with Kesling, produced a profusely illustrated book documenting the larger fossils of the Silica Formation (Kesling and Chilman 1975, reformatted and reprinted 2012). Although geared towards the serious amateur, the volume soon became the standard illustrated reference for amateurs and professionals alike. Today, the FUMMP is still going strong and members continue to donate fossils, time, and expertise to professionals and students at the UMMP and throughout North America. Fossils from the Silica Formation and other Michigan Basin localities still figure prominently in their collections and they are collaborating with professionals to make images of some of their finds available online.
A Park Devoted to Fossils
The glory days of collecting in northwestern Ohio and southeastern Michigan have long passed and are now just fond memories for the lucky "old timers". Collecting in active quarries is restricted and the old abandoned quarries are no longer accessible. However, the casual collector and "kids of all ages" can still experience the thrill of finding Silica Formation fossils thanks to the opening of Fossil Park in 2001, on a five acre site just outside of Sylvania. Periodically, shale material from local quarrying operations is dumped at this site where it is freely available to the fossil-hunting public. Although the abundance and diversity of the fossil material is perhaps limited compared to what was once available, it is still possible to find many excellent examples of typical fossils in a safe and easily accessible location.
The UMMP and the Importance of Museum Collections
Paleontologists who wish to study the Silica Formation today rely primarily on the vast collections of fossils found in museums to conduct their research. The UMMP houses one of the largest collections of Silica Formation fossils and is a frequent stopping place for researchers. Curators and students at the University of Michigan regularly use the collections and many specimens are on display in the University's public exhibits. Museum collections such as these are invaluable for answering a myriad of questions about the history of life. Recently, there has been increased interest in Middle Devonian deposits and how to correlate more precisely the rock units from the various basins in North America. Some of the species (or closely related forms) found in the Silica Formation, are also found in rocks deposited in adjacent areas, and paleontologists are working diligently to understand further the evolutionary and biogeographic relationships among these faunas. Much remains to be learned about the faunal history in North America during the Middle Devonian and how it relates to geographically extensive fluctuations in sea-level and other environmental changes. The Devonian Period was also an interval during which a number of major marine predators evolved and diversified and there is interest in how increased predation impacted the marine invertebrate communities in the geologic past. Long after the close of the classic quarries, the fossils of the Silica Formation housed in the UMMP and other museums, as well as those in private collections, continue to play critical roles in helping paleontologists unravel the history of life during this important time.
Article and photographs provided by Dr. Daniel J. Miller (University of Michigan Museum of Paleontology Invertebrate Collections Manager)
Kesling, R.V., and Chilman, R.B. 1975 (reformatted and reprinted 2012). Strata and Megafossils of the Middle Devonian Silica Formation. University of Michigan Museum of Paleontology, Papers on Paleontology, Vol. 8.
2013 Paleozoic Partner feature articles: | January: Fossils of the 2013 National Fossil Day Artwork | February: Paleontological Research Institution, Museum of the Earth | March: Falls of the Ohio State Park | April: Field Museum of Natural History, Mazon Creek Collection | May: Prehistoric Trackways National Monument | June: Cincinnati Museum Center | July: Glacier Bay National Park & Preserve | August: University of Michican Museum of Paleontology, Silica Formation Fossils | September: Yale Peabody Museum of Natural History, Beecher's Trilobite Bed | October: Guadalupe Mountains National Park | November: Utah Geological Survey, Millard County Cambrian Fossils | December: Denver Museum of Nature and Science, High-Altitude Mass Extinction |