The region around Cincinnati, Ohio, is known throughout the world for the abundant and beautiful fossils found in limestones and shales that were deposited as sediments on the sea floor during the Ordovician Period, about 450 million years agosome 250 million years before the dinosaurs lived. In Ordovician time, the shallow sea that covered much of what is now the North American continent teemed with marine life. The Cincinnati area has yielded some of the world's most abundant and best-preserved fossils of invertebrate animals such as trilobites, bryozoans, brachiopods, molluscs, echinoderms, and graptolites. So famous are the Ordovician fossils and rocks of the Cincinnati region that geologists use the term "Cincinnatian" for strata of the same age all over North America. This book synthesizes more than 150 years of research on this fossil treasure-trove, describing and illustrating the fossils, the life habits of the animals represented, their communities, and living relatives, as well as the nature of the rock strata in which they are found and the environmental conditions of the ancient sea.
About the Author
David L. Meyer is Professor of Geology at the University of Cincinnati. He lives in Cincinnati, Ohio.
Richard Arnold Davis is Professor of Biology and Geology at the College of Mount St. Joseph in Cincinnati. He lives in Cincinnati, Ohio.
Steven M. Holland is Professor of Geology at the University of Georgia, Athens. He lives in Athens, Georgia.
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A Sea Without Fish
Life in the Ordovician Sea of the Cincinnati Region
By David L. Meyer, Richard Arnold Davis
Indiana University PressCopyright © 2009 Richard Arnold Davis and David Lachlan Meyer
All rights reserved.
The vicinity of Cincinnati, in the Ohio River Valley of southwestern Ohio, including adjacent northern Kentucky and southeastern Indiana, is among the most fossil-rich regions in North America, if not the entire world. The profusion of fossils in the local limestone and shale attracted many pioneering geologists and paleontologists of the nineteenth century, and much fundamental work in American paleontology and stratigraphy was accomplished here. Hundreds of fossil species were first discovered and named from these rocks. Early geologists gave the entire series of strata exposed here the name "Cincinnatian," and this name was applied to strata of similar age throughout North America. Cincinnatian fossils are displayed in museums all over the world. Researchers, students, and amateur fossil collectors regularly visit the Cincinnati region to collect fossils. Many of those who have grown up in the region are aware of the abundance of fossils, yet few appreciate the uniqueness of this richness and its broader significance to our understanding of the Earth's past. The purpose of this book is to explore the richness of Cincinnatian fossils and the stories they tell about life over 450 million years ago, when shallow seas inundated North America and the site of Cincinnati was in the Southern Hemisphere.
Why are fossils so abundant in the rocks of Cincinnati's hills? Beyond sheer abundance, what is their significance for our knowledge of the history of life, evolution, and ancient environments? There is no single answer to these questions, but rather several answers can be given which collectively reveal the significance of Cincinnatian fossils. These answers can be found under four categories: organic evolution, environment, preservation, and history.
Fossils found in Cincinnati's limestones and shales are the remains of animals that lived during an interval of Earth history called the Ordovician Period. The Ordovician is the second oldest period of the larger time interval known as the Paleozoic Era (Figure 1.1). The beginning of the Paleozoic Era (meaning "time of ancient animals") is marked by the oldest rocks containing abundant fossils of multi-celled animals (metazoans). Radiometric dating of volcanic ash beds interbedded with these fossiliferous rocks places the beginning of the Paleozoic at about 543 million years ago. Similar methods date the beginning of the Ordovician Period at about 490 million years ago and its end at about 443 million years ago. The span of Ordovician time represented by the Cincinnatian strata amounted to less than 10 million years, and fell approximately during the latter part of the Ordovician, termed the Late Ordovician. In the Cincinnati region, a total thickness of over 250 meters (820 feet) of interbedded limestone and shale was deposited during the Late Ordovician, constituting the Cincinnatian and containing fossils throughout. Further discussion of the nature and subdivisions of Cincinnatian rocks, and estimates of their age, are the subject of chapter 4.
Professor Stig M. Bergström of the Ohio State University is among the world's most knowledgeable and widely-traveled specialists on Ordovician fossils and stratigraphy. He indicated to us that "there is nothing that can be compared elsewhere in the world" to the diversity of shelly fossils in the Cincinnatian (Bergström, pers. comm.). Metazoan marine life first began to diversify during the so-called Cambrian explosion that marked the onset of the Paleozoic, but accelerated during the Cambrian and Ordovician Periods to reach a peak late in the Ordovician when the Cincinnatian strata were deposited. In fact the Ordovician Period is recognized as a unique time of evolutionary diversification, termed the Ordovician Radiation (Droser et al. 1996) or the Ordovician Biodiversification Event (Webby, Paris, Droser, and Percival 2004). The Ordovician marked a convergence of what Sepkoski (1981) called three "evolutionary faunas": metazoan groups that first appeared during the Cambrian but persisted into the Ordovician ("Cambrian Fauna"), groups that began to diversify during the Ordovician ("Paleozoic Fauna"), and groups that first appeared in the Ordovician that diversified after the end of the Paleozoic ("Modern Fauna") (Figure 1.2). At the end of the Ordovician there occurred a global mass extinction that eliminated species on a large scale. Thus the Cincinnatian time was significant in the history of life as a Golden Age of evolutionary diversification just before a major crisis of mass extinction. In many ways the Late Ordovician is comparable to the Late Cretaceous Period, another Golden Age preceding a crisis (Figure 1.2; Seilacher 1998). Few if any fossil species found in the Cincinnatian strata survived into the succeeding Silurian Period. Chapters 5–14 introduce each of the major groups of organisms found as fossils in the Cincinnatian.
The environment of Late Ordovician time in the Cincinnati region contributed to the abundance and richness of fossils in several fundamental ways. Cincinnatian fossils and rocks bear profound testimony to the existence of widespread shallow seas (called epicontinental or epeiric seas) over most of the North American continent at this time (Plate 1). Using many sources of evidence, geologists have compiled a record of the rise and fall of sea level during the past half billion years of Earth history (Figure 1.3). The Late Ordovician was one of the times of maximum rise of sea level over the entire globe, rivaled only by the Late Cretaceous (according to the reconstruction by Hallam ). The cause of this flooding has been attributed to high rates of sea floor spreading which swelled the mid-ocean ridges, displacing immense volumes of seawater from the deep ocean basins onto the continental plates. The Atlantic Ocean as we know it did not exist, but instead, a narrower ocean called the Iapetus Ocean separated North America from continental plates later to constitute Europe and Africa (Plate 1). The nearest landmasses to the Cincinnati region were the rising Appalachian mountain chain, about 300 miles to the east, and the low-lying Canadian Shield to the north. Just before and during the Late Ordovician, a phase of major tectonic (mountain-building) activity, the Taconic Orogeny, resulted in severe crustal deformation and uplift along the region bordering New York and New England. Islands were raised high above sea level as lofty and jagged mountain chains resembling the modern Alps or Himalayas. Weathering and erosion attacked these ranges, and rivers carried huge loads of fresh water, sediments, and nutrients into the shallow sea.
Great volumes of sediment, consisting of coarse gravels, sands, silts, and muds (termed siliciclastics) were deposited as river deltas and redistributed by oceanic currents near the coastline in the Appalachian Basin. The total thickness of the Late Ordovician strata in the Appalachian Basin in Virginia reaches about 1000 meters (over 3000 feet) whereas the same time interval is represented in the Cincinnati region by strata less than 300 meters (less than 1000 feet) thick (Figure 1.4; Kay 1951). Offshore, only the muddy components of this heavy sediment input remained suspended as clay particles, and were carried by currents to reach the Cincinnati area. These muds were thus imports to the region that eventually lithified (turned to stone) to form shales. In the Cincinnati area, shales are interbedded with limestones, which are composed of calcareous shells and skeletons of "native" marine invertebrates. In the western United States and Canada, the Late Ordovician contains mostly limestones secondarily converted to dolomites. Thus, the Cincinnati region represents an intermediate zone of mixed shales and limestones between the great thickness of siliciclastics to the east and pure limestones farther west. Both sediments intermingled in the Cincinnati region, producing a varied and patchy sea floor that was muddy in places and shelly in others. Such a variegated bottom environment offered more potential types of living spaces for bottom-dwelling organisms (the benthos), and provides a further reason why high diversity developed in the region. Because there was very little vegetation on land during the Late Ordovician, erosion may have carried a heavier load of dissolved inorganic nutrients into the sea. These nutrients may have acted as a fertilizer to stimulate the production of benthic biomass. In addition, climate, oceanographic conditions, and available food supply must have been crucial to support prolific marine life in the Cincinnatian sea; these factors are explored in detail in chapter 15.
When we look at rock layers as crowded with well-preserved fossils as those of the Cincinnatian, we tend to think we are looking at a complete picture of life on the Ordovician sea floor—a snapshot—in terms of both the diversity of species present and their abundance. Unfortunately, the correspondence between this fossil assemblage and the original living community from which it was derived is rarely that simple and direct. The fossil record provides a mere glimpse of ancient life, one that is heavily biased by many factors. In order to assess the impact of these factors on the quality of the fossil sample, paleontologists have devoted an entire subdiscipline, called taphonomy, to the investigation of processes affecting organic remains from death to ultimate fossilization. Taphonomy emphasizes the wide variation in the preservation potential of organisms. An appreciation of the significance of variable preservation can be gained by considering aspects of life, death, and post-mortem history that entered into the complex equation that determined the ultimate fossil record of the Ordovician sea.
Nature of the Living Organism
Biological factors affecting preservation potential include presence of "hard parts," their chemistry, mineralogy, and construction, and the mode of life of the organism. By far the most important requirement for fossilization is possession of mineralized hard parts such as shells or skeletons. Soft body parts including skin, muscle, hair, and internal organs almost always decay rapidly following death. Many common marine invertebrates like worms lack hard parts altogether or have only hardened jaw structures. In some marine environments, animal communities are dominated in numbers of species or individuals by such soft-bodied species with little or no fossilization potential. One of the best-known exceptions to the dominant preservation of hard parts is the Cambrian Burgess Shale of British Columbia, with its amazing wealth of soft-bodied worms, arthropods, and other invertebrates, along with shell-bearing forms (Gould 1989). In the Cincinnatian, there is virtually no preservation of soft-bodied species or soft parts of shell- or skeleton-bearing species. The only records known to us of soft-body preservation in the Cincinnatian are a worm described by Ulrich (1878) and the recent discovery of fossilized "tube feet" in a brittle star (Glass 2006). Our knowledge of the Cincinnatian biota is thus heavily biased in favor of species with hard parts, the shells and skeletons, complete or partial, known as body fossils. Fortunately, this is offset to some degree by evidence of the activity of soft-bodied species from trace fossils (burrows, tracks, and trails—the subject of chapter 14). However, it must be kept in mind that potentially great numbers of species in the biota will never be known because they left no fossil record whatsoever.
Shells and skeletons preserved in Cincinnatian strata are predominantly composed of calcium carbonate (CaCO3) in the mineral form calcite. Some shells of brachiopods (see chapter 8) and the microfossils known as conodonts (see chapter 13) are preserved as calcium phosphate. Despite the abundance of calcium carbonate in Cincinnatian fossils, not all shells having this chemical composition are equally well preserved. The reason for this is that some organisms form calcium carbonate shells or skeletons not as calcite but as a different mineral called aragonite. Aragonite, with a different crystallographic structure than calcite, becomes unstable in seawater after death of the organism and recrystallizes as calcite. In some cases this transformation occurs as a solid-state replacement of aragonite by calcite, altering the microstructure but retaining the macroscopic structure of a shell. Aragonitic shells can also be lost entirely by dissolution even before burial in sediment. In other cases, a shell may become buried, and as the internal soft parts decay, sediment seeps into the shells, replacing the soft parts and forming a perfect mold of the interior. After the aragonitic shell dissolves, the sediment infilling remains and can be lithified by calcitic cement. In this manner an internal mold or steinkern is formed which perfectly preserves the internal spaces of a shell, often molding features of the inner shell surface like muscle scars, even though the actual original aragonitic shell disappears. In other cases the shell may not be infilled, and once the shell dissolves, a void remains as an external mold of the outer surface of the shell, or the external mold can be infilled with sediment to form a cast. These are often the only ways a record of an aragonitic shell is preserved, and we have no way of gauging how many aragonitic shells dissolved leaving no trace whatsoever. Thus it is very difficult to estimate the original abundance of species forming aragonitic shells.
Even among species forming calcitic shells, preservation can be highly selective. Thinner, more delicate shells are more likely to be destroyed before they can be buried. In groups like trilobites (see chapter 11), the exoskeleton is composed of the protein chitin, with varying amounts of calcium carbonate. Juvenile, or newly molted, trilobites had weakly calcified exoskeletons, and were thus less preservable than more heavily calcified individuals. Thus, within a single species, preservational potential is unequal. Species having shells formed of one or two valves (snails, clams, or brachiopods) have a higher preservation potential than species with multi-parted skeletons such as crinoids or trilobites. Multi-parted skeletons are held together with connective tissue, which is susceptible to scavenging and decay, causing the skeleton to become disarticulated and scattered by currents. The consequence of all these variable factors of shell composition and structure is that all organisms producing a calcitic shell capable of preservation do not have an equal potential for actual preservation. Preservation is highly selective even among shells chemically and mineralogically stable enough to survive post mortem.
The mode of life of organisms determines preservation potential even before animals die. For aquatic species, bottom-dwellers (benthos) have a higher likelihood of preservation than swimming (nektonic) or floating (planktonic) species. Among the benthos, species that burrow into the sediment for a living (infauna) obviously have a much higher potential for preservation than do surface dwellers (epifauna). Among the epifauna, species living permanently attached to the bottom often have a higher potential for preservation than free-living, mobile species, simply because they are unable to escape sudden burial by sediment.
Excerpted from A Sea Without Fish by David L. Meyer, Richard Arnold Davis. Copyright © 2009 Richard Arnold Davis and David Lachlan Meyer. Excerpted by permission of Indiana University Press.
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Table of Contents
PrefaceAcknowledgmentsList of Repositories of Fossils Illustrated in This Book1. Introduction2. Science in the Hinterland: The Cincinnati School of Paleontology3. Naming and Classifying Organisms4. Rocks, Fossils, and Time 5. Algae: The Base of the Food Chain 6. Poriferans and Cnidarians: Sponges, Corals, and Jellyfish 7. Bryozoans: "Twigs and Bones"8. Brachiopods: The Other Bivalves 9. Molluscs: Hard but with a Soft Center 10. Annelids and Wormlike Fossils 11. Arthropods: Trilobites and Other legged Creatures 12. Echinoderms: A World Unto Themselves 13. Graptolites and Conodonts: Our Closest Relatives? 14. Trace Fossils: Tracks, Trails, and Burrows 15. Paleoeography and Paleoenvironment, by Steven M. Holland 16. Life in The Cincinnatian Sea Appendix 1: ResourcesAppendix 2: Individuals and Institutions Associated with the Cincinnati RegionGlossaryReferences CitedIndex
What People are Saying About This
"The authors provide a comprehensive view of the grand panorama of Ordovician paleontology in the Cincinnati region. This volume belongs in the libraries of those interested in the Ordovician Period, the geology and paleontology of the Cincinnati area, and the history of science."