WISCONSIN GEOLOGICAL AND NATURAL HISTORY SURVEY James M. Robertson, Director and State Geologist
John W. Attig, geologist Mindy C. James, publications manager William G. Batten, geologist Marcia J. Jesperson, Map Sales associate Kenneth R. Bradbury, hydrogeologist Kathy A. Kane, network administrator Bill C. Bristoll, information manager Irene D. Lippelt, water resources specialist Bruce A. Brown, geologist Frederick W. Madison, soil scientist Lee Clayton, geologist M.G. Mudrey, Jr., geologist Michael L. Czechanski, cartographer Stanley A. Nichols, biologist Donna M. Duffey, Map Sales associate Deborah L. Patterson, cartographer Timothy T. Eaton, hydrogeologist Roger M. Peters, geologist Thomas J. Evans, geologist Kathy Campbell Roushar, cartographer Stephen J. Gaffield, hydrogeologist Apichart Santipiromkul, information processing Madeline B. Gotkowitz, hydrogeologist specialist David J. Hart, hydrogeologist Curtis L. Thomas, geotechnician Ronald G. Hennings, hydrogeologist (emeritus) Virginia L. Trapino, program assistant Rilla M. Hinkes, office manager Alexander Zaporozec, hydrogeologist (emeritus) Thomas S. Hooyer, geologist Kurt K. Zeiler, GIS specialist Susan L. Hunt, graphic artist Kathie M. Zwettler, administrative manager plus approximately 10 graduate and undergraduate student workers.
RESEARCH ASSOCIATES
Gregory J. Allord, USGS George J. Kraft, Central Wis. Groundwater Center Mary P. Anderson, UW–Madison James T. Krohelski, USGS Michael F. Bohn, Wis. Dept. of Nat. Res. Michael D. Lemcke, Wis. Dept. of Nat. Res. Stephen M. Born, UW–Madison J. Brian Mahoney, UW–Eau Claire Philip E. Brown, UW–Madison Kevin McSweeney, UW–Madison Charles W. Byers, UW–Madison Christine Mechenich, Central Wis. Groundwater Center William F. Cannon, USGS David M. Mickelson, UW–Madison Douglas S. Cherkauer, UW–Milwaukee Donald G. Mikulic, Ill. State Geol. Survey Kevin Connors, Dane Co. Land Conserv. Dept. William N. Mode, UW–Oshkosh Robert H. Dott, Jr., UW–Madison (emeritus) Maureen A. Muldoon, UW–Oshkosh C. Patrick Ervin, Northern Ill. Univ. Robert E. Pearson, Wis. Dept. of Transportation Daniel T. Feinstein, USGS James O. Peterson, UW–Madison Robert F. Gurda, Wis. State Cartographer's Office Jeffrey K. Postle, Wis. Dept. Ag., Trade & Consumer Nelson R. Ham, St. Norbert Coll. Protection Mark T. Harris, UW–Milwaukee Kenneth W. Potter, UW–Madison Karen G. Havholm, UW–Eau Claire Todd W. Rayne, Hamilton Coll. Randy J. Hunt, USGS Daniel D. Reid, Wis. Dept. of Transportation Mark D. Johnson, Gustavus Adolphus Coll. Allan F. Schneider, UW–Parkside (emeritus) Joanne Kluessendorf, Weis Earth Science Museum J. Antonio Simo, UW–Madison James C. Knox, UW–Madison Kent M. Syverson, UW–Eau Claire Jeffrey A. Wyman, UW–Madison The Wisconsin Geological and Natural History Survey also maintains collaborative relationships with a number of local, state, regional, and federal agencies and organizations regarding educational outreach and a broad range of natural resource issues. Common Paleozoic Fossils of Wisconsin
Ross H. Nehm Bryan E. Bemis
2002
Wisconsin Geological and Natural History Survey Educational Series 45 Cover art: Ross H. Nehm
Published by and available from Wisconsin Geological and Natural History Survey 3817 Mineral Point Road • Madison, Wisconsin 53705-5100 ☎ 608/263.7389 FAX 608/262.8086 www.uwex.edu/wgnhs/ James M. Robertson, Director and State Geologist
ISSN: 1052-2115 This report is an interpretation of the data available at the time of preparation. Every reasonable effort has been made to ensure that this interpretation conforms to sound scientific principles; however, the report should not be used to guide site-specific decisions without verification. Proper use of the report is the sole responsibility of the user. The use of company names in this document does not imply endorsement by the Wisconsin Geological and Natural History Survey. Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, University of Wisconsin–Extension, Cooperative Extension. University of Wisconsin–Extension provides equal opportunities in employment and programming, including Title IX and ADA requirements. If you need this information in an alternative format, contact the Office of Equal Opportunity and Diversity Programs or the Wisconsin Geological and Natural History Survey (☎ 608/262.1705).
Mission of the Wisconsin Geological and Natural History Survey The Survey conducts earth-science surveys, field studies, and research. We provide objective scientific information about the geology, mineral resources, water resources, soil, and biology of Wisconsin. We collect, interpret, disseminate, and archive natural resource information. We communicate the results of our activities through publications, technical talks, and responses to inquiries from the public. These activities support informed decision-making by government, industry, business, and individual citizens of Wisconsin.
ii OVERVIEW 1 Wisconsin's fossils 1 Geologic time 1 How and where fossils form 2 What fossils tell us 3 About evolution and extinction 3 About ancient environments 3 About the age of rock 4
COLLECTING FOSSILS 4 Finding fossils in Wisconsin 4 Eastern Wisconsin 5 Central and western Wisconsin 5 Tools for collecting fossils 6 Removing fossils from rock 7 Recording information about fossils 7
IDENTIFYING FOSSILS 7 Stromatolites 8 Corals 9 Brachiopods: Lamp shells 11 Bivalve mollusks: Clams 15 Gastropod mollusks: Snails 15 Cephalopod mollusks: Squid and octopus 18 Extinct arthropods: Trilobites 18 Crinoids: Sea lilies 21 Bryozoans: Moss animals 22 Other fossils 22
SOURCES OF INFORMATION 24 Societies 24 Books 24
ACKNOWLEDGMENTS 25
ABOUT THE AUTHORS 25
iii FIGURES
1. How fossils form 2
2. Block diagram of near-shore sedimentary environments 3
3. Where to find Paleozoic invertebrate fossils 4
4. A paleontologist collecting Middle Ordovician fossils from a roadcut in southwestern Wisconsin 5
5. Tools for fossil collecting. 6
6. Removing fossils from rock 7
7. Reconstruction of stromatolite mounds in the Cambrian Period 8
8. A stromatolite, showing the characteristic layered and domed struc- ture 8
9. Coral anatomy 9
10. Brachiopod anatomy 11
11. Anatomy of the five common groups of articulate brachiopods in Wis- consin 14
12. Bivalve mollusk anatomy 15
13. Gastropod mollusk anatomy 15
14. Cephalopod mollusk anatomy 18
15. Morphologies of fossil cephalopods 18
16. Anatomy of a trilobite 19
17. Crinoid anatomy 21
18. Bryozoan anatomy 22
19. Stromatoporoid anatomy 22
20. Hyolithid anatomy 22
Inside back cover: Geologic time scale
TABLE
1. A classification of three specimens 8
iv PLATES
1. Coral fossils 10
2. Brachiopod fossils 12
3. Brachiopod fossils 13
4. Bivalve mollusk fossils 16
5. Gastropod mollusk fossils 17
6. Cephalopod mollusk fossils 19
7. Trilobite fossils 20
8. Trilobite fossils 21
9. Miscellaneous fossils 23
10. Trace fossils 24
v
Common Paleozoic Fossils of Wisconsin
OVERVIEW Wisconsin’s fossils Many people view the Earth and its lifeforms as of time is the era, which is further subdivided what we see today. However, both have into periods. changed dramatically through time; the evi- Geologic time is divided into three eons. dence for this comes from something most of From oldest to most recent, these eons are the us see every day, but think little about: rock. Archean, Proterozoic, and Phanerozoic. The Much of the rock in Wisconsin contains fossils, Archean and Proterozoic (sometimes collec- the remains of ancient organisms. After these tively referred to as the Precambrian) encom- organisms died, parts of their bodies were pre- pass geologic history prior to 570 million years served in rock. Paleontologists—people who ago. Fossils representing these two eons, when study ancient life on the basis of fossilized the earliest forms of primitive life developed, plants and animals—use them as clues about are uncommon. The oldest known single-celled the Earth’s past. organisms—bacteria that were beautifully pre- The fossils found in rock throughout Wis- served in rock that is 3.5 billion years old— consin were formed from creatures that lived in were discovered in Africa. Fossils of soft-bodied the warm, shallow seas that once covered the multicellular organisms about 700 million years state. In this guide we introduce the fascinating old have been found in Australia’s Ediacara marine (sea) creatures that existed in what is Hills. now called Wisconsin, discuss how they were The Phanerozoic has been divided into preserved as fossils and what they tell us about three principal eras; from oldest to youngest, the ancient Earth, illustrate common Wisconsin they are the Paleozoic (upon which we focus in fossils, and suggest how and where to collect this booklet), Mesozoic, and Cenozoic. them. Diversity of lifeforms and the complexity within them developed during the early part of Geologic time the Phanerozoic, in the Paleozoic Era, 570 to When did the creatures that were to become 245 million years ago. This era has been divid- fossils live? To appreciate fully just how long ed into (from oldest to most recent) the Cam- ago these organisms existed, it is helpful to de- brian, Ordovician, Silurian, Devonian, Missis- velop an understanding of the concept of geo- sippian, Pennsylvanian, and Permian Periods. logic time. Many major groups of shell-bearing inverte- Scientific methods for determining ages in- brates (animals with hard shells and no back- dicate that the Earth is about 4.6 billion years bones) appeared throughout the oceans during old. It is difficult for most of us to imagine such an immense proliferation of lifeforms at the be- a vast amount of time, so geologists have devel- ginning of the Paleozoic. This era ended with oped a timetable (see inside back cover) that the largest extinction in the Earth’s history: 80 breaks geologic time into major units. The percent of all types of marine invertebrates be- most expansive units, covering the longest came extinct at the end of the Paleozoic, during amounts of time, are called eons. The next unit the Permian Period. dates (solidifies) the sediment into layered rock, and, if conditions are right, the remains of or- ganisms preserved in this rock become fossils (fig. 2). Fossils can be preserved in such sedimenta- ry rock types as sandstone, shale, limestone, and dolomite. Sandstone, which is composed of sand-sized particles, forms in water in areas such as surf Figure 1. How fossils The Mesozoic Era (245 to 66 million years zones and beaches. Wave action is strong in form. A: An animal dies ago) is also called the Age of Reptiles because these areas, and rock formed there contains few and settles to the sea of the extensive proliferation of land and sea well preserved fossils—skeletal parts are easily floor. B: Animal is buried and soft tissues decay. reptiles. Dinosaurs were one of the dominant shattered in these high-energy environments. C: Hard parts are slowly animal groups during the Mesozoic Era. Al- Fossil-bearing Cambrian sandstone is exposed replaced by minerals. though Wisconsin contains only a sparse geo- at the surface in central and northwestern Wis- D: Sediment becomes rock and erosion ex- logic record of the Mesozoic, we know from consin. Middle Ordovician sandstone is found poses fossil. the fossil record in other areas that many ma- primarily along river valleys in southern Wis- rine and terrestrial animals, including dino- consin, but it is generally devoid of fossils. Wis- saurs, became extinct near the end of this era. consin sandstone can be white, tan, or iron- Mammals and flowering plants became com- stained brown. mon during about the last two-thirds of the Unlike sandstone, shale forms in areas of Mesozoic. slow-moving or still water; as a result, fossils in The Cenozoic Era encompasses the past 66 shale are generally better preserved than those million years of Earth’s history. The most re- in sandstone. Shale is composed of silt and clay cent Ice Age began about 2.5 million years ago, and can be split into thin sheets. It is most during the Cenozoic. common in the upper Ordovician and Devo- nian rock of eastern Wisconsin and is generally How and where fossils form green, blue green, or black. Because fossils form under specific conditions, Limestone forms from the precipitation only a small percentage of organisms is pre- (settling out) of calcium carbonate from ocean served. Hard skeletal parts such as shells, water in warm, shallow environments away bones, and teeth are the most commonly fossil- from the input of sand, silt, and clay. Limestone ized remains because they are more durable can also be composed of skeletal fragments and than the soft tissues, which decay rapidly. frameworks of organisms that built their shells Fossils are preserved in sedimentary rock, with calcium carbonate from the water. which is formed from what is essentially scrap Limestone becomes dolomite when the ele- material: Weathering processes break up and ment magnesium replaces some of the original erode rocks at the Earth’s surface, and wind calcium. Much limestone in Wisconsin has been and water carry away the scraps—pebbles, sand, altered to dolomite. Because of this alteration, silt, and clay. Many of these particles, called sed- fossils in dolomite are not as well preserved as iment, then settle on sea bottoms. As sea crea- those in limestone. Fossiliferous dolomite and tures die and also settle to the seafloor, they limestone are found in southern and eastern eventually are buried by sediment (fig. 1). Wisconsin. Limestone and dolomite range from Eventually, compaction of the sediment consoli- buff or gray to dark gray or blue gray.
2 | Common Paleozoic Fossils of Wisconsin Conditions at a burial site determine how don’t, are the focus of much research. an organism is preserved. Fossils of marine in- Fossils also show us that many groups of vertebrates in Wisconsin are most commonly creatures eventually become extinct. Trilobites, preserved as replacements or molds. Replace- once plentiful in the Cambrian seas of Wiscon- ments are produced when minerals settle out of sin, are extinct today, along with countless water and replace the original skeletal parts. other creatures. Why are trilobites extinct, but Molds are three-dimensional impressions left in direct descendants of other creatures that lived rock after the skeletal parts of an organism dis- during the Paleozoic, such as snails, abundant solve. A mold can be either internal or external. today? Some creatures survive periods during The impression of the outside of a preserved which many other organisms become extinct. shell is called an external mold; if minerals or Does this happen because they are better sediment fill the space between two shells, but adapted to their environment than those that the shells dissolve, an internal mold is formed. became extinct, or are they just lucky? Paleon- Under certain conditions, the remains of tologists are working to answer these questions. some ancient plants and animals become fossil About ancient environments fuels. Coal is formed from the accumulation What was the area we now call Wisconsin like and burial of plant material under conditions 400 million years ago? It is possible to deter- lacking oxygen, such as in swamps. Over mil- mine the environmental conditions of the past lions of years, high pressures and temperatures on the basis of the organisms that lived in an force oxygen and hydrogen out of the remains, area at a certain time. Using information from leaving carbon, which we burn as fuel. Natural living relatives of extinct creatures and assum- gas and petroleum form similarly, but they may ing that similar, related creatures lived in similar include animal remains as well and form under habitats, we can make inferences about the past. higher pressures and temperatures. About 85 For example, one indicator of marine environ- percent of the energy used in the United States ments is coral. Today, corals and coral reefs re- comes from these fossil fuels. quire specific conditions to flourish—warm temperatures (25oC to 29oC [77oF to 84oF]), What fossils tell us shallow depths (less than 56 meters [165 feet]), About evolution and extinction and normal salinity (the total quantity of dis- As you become familiar with Wisconsin’s Pa- solved salts in water; normal salinity is approxi- leozoic fossils, you may notice that fossils of mately 35 parts per thousand). The abundance certain organisms have different morphologies, of coral fossils in Wisconsin’s Silurian-age rock or forms, through time. Some organisms de- suggests that a warm, shallow sea of normal sa- velop new features (for example, shells that are linity covered Wisconsin during that time. thicker or shaped differently). These changes through time are known as evolution. How and why these changes come about, or why they
Figure 2. Block diagram of near-shore sedimentary environments. A: Eroding land is the source of sediment. B: Sand becomes sandstone. C: Silt and clay become shale. D: Farther from sediment sources, skeletal remains and calcium carbonate from ocean water become limestone.
Educational Series 45 | 3 tively younger than the rock layers below, if the rock has not been overturned by geologic pro- cesses. One way you may be able to determine the geological period (age range) from which you are collecting is the presence of index fossils in the rock layers. Index fossils are fossil groups that lived for very short, specific periods of time. That makes them more useful for relative dating of rocks than long-lived fossil groups because they allow you to narrow down a rock’s age range. Geologists can use several index fossils together to make determinations about relative rock ages. The discovery of radioactivity provided a Figure 3. Where to find Paleozoic invertebrate fossils. Black means of establishing absolute dates of rock. Ra- areas are promising localities for collecting fossils. dioactive parent (original) elements decay at a constant rate to produce progeny (decayed; also Many other organisms can be used to re- referred to as daughter) products. By knowing construct aspects of the Earth’s ancient envi- the half life (decay rate) and measuring the ronment as well. Knowing how life has re- amount of parent and progeny products in a sponded to environmental changes in the past rock sample, it is possible to determine how may help us understand how future environ- much radioactive decay has occurred and there- mental changes, such as possible global warm- fore the time that has passed, which gives us the ing, will influence life on Earth. absolute age of the rock. Paleontologists can use a combination of relative and absolute methods About the age of rock to determine the age of rocks and fossils. Fossils can generally be considered the same age as the rock in which they are found. The OLLECTING FOSSILS age of rock can be determined relatively or ab- C solutely, just as events can. For example, we Finding fossils in Wisconsin know that Wisconsin became a state relatively Fossils can be found in many places in the state. later than the explorations of Marquette and Fossil-bearing sedimentary rock covers much of Joliet. But when exactly did both events hap- Wisconsin, particularly the far southern and pen? Speaking in absolute terms, Marquette and eastern parts of the state (fig. 3). Joliet’s expedition began in 1673 and Wiscon- But you may not have to travel far to find sin became a state in 1848. fossils—the gravel in your driveway, the rocks in Different rock layers can be dated relatively your garden, and the stone in buildings near you on the basis of their position in a “stack” of lay- may have fossils in them. The most abundant ers. For example, if you were to drop some and most easily collected fossils come from sheets of paper on the ground, one on top of roadcuts, natural bluffs, and quarries. Roadcuts the other, it’s obvious that the ones on the top containing fossils can be found throughout Wis- got there more recently than the ones on the consin, particularly in the southwest (fig. 4). bottom. In the same way, rock layers (and the Natural bluffs are along rivers and streams. fossils they contain) on top of others are rela-
4 | Common Paleozoic Fossils of Wisconsin Quarries are excellent places to look for fossils ing permission is required. Silurian corals can as well, but falling rocks can make them dan- also be found on stone fences and rock piles gerous. Do remember that many good collect- throughout the county. ing localities, such as quarries and roadcuts, are ■ Cedarburg and Grafton, Wisconsin. Out- private property—always secure permission crops and quarries on the banks of the Milwau- with quarry and landowners before entering a kee River between Cedarburg and Grafton con- collecting site. Use care along roadcuts. tain Silurian fossils. Here we list only general localities for fossil collecting: If we provided directions to specific ■ Waukesha County. Many of the best Siluri- sites, they might become overcollected. In an fossils from Wisconsin come from quarries many cases, the best finds are those that you in the metropolitan areas of eastern Wisconsin. discover on your own. This is part of the excite- Fossils in these quarries are rare but well pre- ment of fossil collecting! Refer to later sections served. Many beautiful specimens from these of this guide for descriptions of specific fossil quarries are now in museums. groups mentioned below. ■ Green Bay, Wisconsin. Near Green Bay, Eastern Wisconsin outcrops of upper Ordovician Maquoketa Shale ■ Door County. Many bedrock exposures in contain bryozoans and brachiopods, as does a Door County contain Silurian brachiopods (es- roadcut on Highway 57 north of Green Bay. pecially Pentamerus) and rugose and tabulate Other exposures along corals. Fragments of Silurian fossils are also river banks in the area found along the shore of Lake Michigan. also contain Ordovi- cian fossils. ■ Oakfield, Wisconsin. An abandoned brick- work quarry southwest of Oakfield contains up- Central and western per Ordovician orthid brachiopods. Many of Wisconsin these brachiopods have been eroded out of Middle Ordovician do- their matrix and can be picked from the lomite that is rich in ground. Above the shale in the lower quarry lie virtually all types of in- harder dolomite that contains layers packed vertebrate fossils is with shell fragments of bryozoans and brachio- found in many locali- pods. Fossils have been collected from this site ties in central and for over a decade, but it still provides some western Wisconsin. good specimens. The most well pre- served fossils come ■ Mayville, Wisconsin. The large quarries of from the Platteville the Mayville Lime Company near Mayville con- Formation of the Sin- tain a few corals as well as casts and molds of nipee Group. These abundant specimens of the brachiopods Pen- fossils include brachio- tamerus and Virgiana. Collecting permission is pods, bivalves (clams), required. gastropods (snails), tri- ■ Washington County. In a quarry near lobites, hyolithids, Grafton, large blocks of reef material—includ- cephalopods, ostra- Figure 4. A paleontologist collecting ing Silurian gastropods, brachiopods, and cods, crinoid colum- Middle Ordovician fossils from a road- crinoid stems—are sometimes exposed. Collect- nals, and corals. cut in southwestern Wisconsin.
Educational Series 45 | 5 ■ Mount Horeb, Wisconsin. Roadcuts along Highway 151 south of Mount Horeb occasion- ally contain Receptaculites, marine organisms that may be algae. Quarries and roadcuts just south of Mount Horeb contain Ordovician fos- sils, including strophomenid brachiopods, bry- ozoans, ostracods, and trilobite fragments.
■ Fennimore, Wisconsin. Roadcuts west and south of Fennimore contain well preserved Or- dovician brachiopods, trilobites, ostracods, bry- ozoans, and cephalopods.
■ Roadcuts along Highways 18 and 35 east of Wyalusing State Park contain Ordovician fossils from the Platteville Formation of the Sinnipee Group, such as brachiopods, trilobite frag- ments, bryozoans, cephalopods, and clams.
Figure 5. Tools for fossil collecting. A: Bricklayer’s hammer. B: Chisel. Tools for collecting fossils C: Dental tool for removing delicate fossils. D: Newspapers for wrap- ping specimens. E: Backpack for storing tools and specimens. Fossil collecting is an inexpensive hobby. The F: Specimen bag with label. G: Hand lens. following items are helpful when collecting (fig. 5):
■ Sun Prairie, Wisconsin. Two quarries about ■ A bricklayer’s hammer. The long, flat, ta- 6.4 kilometers (4 miles) northeast of Sun Prai- pered end of a bricklayer’s hammer is ideal for rie off Highway 151 contain abundant Ordovi- splitting sedimentary rock layers. cian fossils from the Platteville Formation. ■ A chisel. Chisels are needed for chipping a These quarries have been active recently, and fossil from a large piece of rock. Safety glasses fresh rock has been exposed. In 1991 most of a should always be worn when removing fossils cephalopod (Endoceras) approximately 4 meters by this method. (13 feet) long was discovered along with other excellent fossils. These are active quarries; se- ■ Dental tools. Dental tools are useful in the cure permission from owners before entering. removal of delicate fossils.
■ Dickeyville, Wisconsin. A roadcut about 6.4 ■ Old newspapers. Ensuring that a specimen kilometers (4 miles) northwest of Dickeyville in is not damaged requires nothing more than a Grant County on Highways 35 and 61 contains careful wrapping with newspapers. weathered strophomenid brachiopods, bryozo- ■ Pencil and paper. These items are essential ans, ostracods, crinoid columnals, and other in- for labeling specimens and precisely recording vertebrate fossils. where your fossil was found. ■ Platteville, Wisconsin. Roadcuts directly ■ A backpack. A backpack provides a place to southwest of Platteville in Grant County are for tools, specimens, and maps. fossiliferous. A quarry off Highway 151 north of Dickeyville contains the brachiopod Pionode- ■ Specimen bags. Use separate specimen ma and other common Ordovician fossils. bags for each locality. 6 | Common Paleozoic Fossils of Wisconsin ■ A magnifying glass or hand lens. The interesting features of the quarry or outcrop, details of many fossils cannot be seen with the such as unique layering patterns. naked eye, and magnification is helpful for identification. IDENTIFYING FOSSILS Throughout time, humans have sought to Removing fossils from rock name things in the world; in biology and pale- Considerable care must be taken when remov- ontology, this naming process is called tax- ing fossils from rock. Many fossils are fragile onomy. Taxonomy involves naming groups of and cannot be removed without damage. It is organisms, and a taxon (plural, taxa) is a unit of generally best to take a large sample home and any rank in this nam- there carefully and patiently work the specimen ing process. Group- out. Many superb specimens have been de- ing organisms by stroyed by impatient collectors attempting to shared characteristics remove them quickly in the field. Chiseling a is known as systemat- shallow trough around the fossil and then pop- ics. Organisms de- ping it out usually works well (fig. 6). Large scribed within large rocks can be trimmed to smaller sizes by hang- groups, such as king- ing the excess area over the edge of another doms, contain organ- rock and striking the large rock firmly. isms with general similarities; those Recording information about fossils within smaller Amateurs have found many important fossil groups, such as fami- specimens. However, without detailed locality lies, have specific information, fossils are virtually useless to pale- similarities. ontologists. When collecting fossils, label speci- The most precise mens and record the following information: way to identify a fos- sil is by its two-part ■ Locality. Be sure to record the county, scientific name. The nearby roads, and distinguishing features of first part of the name your collecting site. is the genus (plural, ■ Rock description. Include a description of genera); the second, the fossil-bearing rock type (that is, sandstone, the species. A genus shale, or limestone/dolomite), including color. includes species that Figure 6. Removing fossils from rock. are related to each A: Trimming a large rock to a smaller size. ■ Layer in rock. It is useful to describe the other; the members B: Close-up of chiseling out a fossil. specific fossil location in relation to layering vis- of a species are usual- ible in the rock. For example, you may notice ly morphologically similar and are able to breed that rock in some outcrops is punctuated by with each other and produce fertile offspring. thin, horizontal layers composed primarily of The scientific name is usually written in italics. fossil fragments. Describing the location of fos- For example, in the case of Calymene celebra, sils relative to such layers allows you (and oth- Wisconsin’s state fossil, Calymene is the genus ers) to find your exact collecting locality again. and celebra is the species. ■ Interesting features. It may be helpful to This complete hierarchy is shown in the ex- include a sketch or description of any other ample of trilobites in column two of table 1, in
Educational Series 45 | 7 find. Two of the most complete sources are the Treatise on Invertebrate Paleontology, a series of more than forty volumes containing descrip- tions of all known invertebrate genera, and Shimer and Schrock’s Index Fossils of North America. These and other references are listed in the Sources of information section at the end of this guide.
Stromatolites Approximately 3 billion years ago, long before multicellular organisms roamed the earth, lush mounds of algae began to thrive along warm, shallow shorelines (fig. 7). These algal mats trapped sediment, which built up layer by layer into dome-shaped mounds (fig. 8). Such mounds are called stromatolites. Although they are not animals, stromatolites may have been a food source for some of Figure 7. Reconstruc- which Calymene celebra is a member of a larger the animal groups we tion of stromatolite group, the trilobites. Trilobites are themselves describe. Stroma- mounds in the Cambrian members of the animal kingdom. Period (500 million years tolites were ago) of Wisconsin. In this guide, we use scientific and com- abundant in mon names (such as lamp shells for brachio- the Precam- pods) to describe the major fossil groups in brian and Wisconsin. We discuss how, where, and when Cambrian, but these intriguing organisms lived and illustrate their numbers Figure 8. A stromatolite, show- the common genera found within these groups. decreased in ing the characteristic layered Many other types of invertebrate fossils later times. A and domed structure. also lived, died, and were preserved in Wiscon- few stromato- sin rock; therefore, further sources of informa- lites are still forming today. Shark Bay, Austra- tion may be necessary to identify the fossils you lia, is well known for its abundant living stro-
Table 1. A classification of three specimens (after Easton, 1960).
TAXON TRILOBITE HUMAN CAR Kingdom Animalia Animalia machine Phylum Arthropoda Chordata transportation Class Trilobita Mammalia automobile Order Phacopida Primatida gas-powered Family Calymenidae Hominidae sedan Genus Calymene Homo four door Species celebra sapiens brand of automobile Individual specimen number social security number registration number
8 | Common Paleozoic Fossils of Wisconsin Figure 9. Coral anatomy. A: Tabulate coral. B: Rugose, or “horn coral.” C: Living coral animal. matolites. In Wisconsin, stromatolites were most common in Cambrian and Ordovician seas, and therefore are most common in rock of those ages.
Corals to honeycombs; the distinctive rugosan corals Corals are marine animals with simple body resemble cow horns. structures (fig. 9; plate 1). The mouth of a Tabulate corals coral’s sac-like body is surrounded by a ring of Tabulate corals are the most abundant coral tentacles. The living coral animal, the polyp, se- fossils in the Silurian rock of Wisconsin and are cretes a cup-like skeleton called the corallite. usually the largest reef corals. They form mas- Many corallites cemented together make up the sive colonies, about 0.3 meter (1 foot) wide or entire skeleton, or corallum. Inside the coral- larger. They can be identified by the presence of lite, a radial divider, called a septum (plural, tabulae. Fieldstones, commonly found in septa) grows vertically from the attachment fencerows along the edges of farm fields in east- base and helps support the soft tissues. Many ern Wisconsin, in many cases contain well pre- coral polyps contain algal cells, which use pho- served tabulate corals. Beach pebbles and gravel tosynthesis to produce food for themselves and along the Lake Michigan shoreline also abound the coral. with coral fragments. Corals can live together in large colonies, or reefs, which can be hundreds of miles across. Favositid tabulates: Honeycomb corals. The fa- Coral reefs are among the most complex eco- vositid corals are quite common. They usually systems on Earth because many thousands of formed large colonies. The corallite is prismatic species other than corals make the reef their in shape, resembling honeycombs. Favositids home. Corals themselves require specific living have mural pores, tiny holes in the wall of the conditions, so fossil coral reefs tell us a great skeleton, which connect different corallites. deal about the environmental conditions at the These pores are distributed in characteristic time of reef formation. Living coral reefs are patterns and numbers, which are useful for dis- confined to subtropical regions in shallow wa- tinguishing the various types of favositids. ters that are warm and clear. Thus, Wisconsin’s Favositids lived from the Ordovician to the Per- Silurian reefs provide evidence that the state mian, at which time they became extinct. They was once covered by a warm, subtropical sea. are most abundant in middle Silurian to lower Although ancient corals formed the reef itself, Devonian rock. Favosites is the most common many other organisms flourished in the small fossil coral in Wisconsin. habitats the reef provided. Brachiopods covered Halysitid tabulates: Chain corals. Halysitids the reef structure, gastropods fed on the abun- resemble interlocking strings of delicate chains. dant algae and detritus, cephalopods hunted for Halysites is a common chain coral in Wisconsin, prey, and crinoids swayed in the agitated wa- and it is used worldwide as an indicator of Sil- ters. urian rock. Halysites is best seen in weathered Two groups of reef-forming fossil corals rock because the rock between the chains dis- are found in Wisconsin: the tabulates and rug- solves, leaving the chains beautifully exposed. osans (fig. 9). Tabulate corals grow upward, depositing horizontal plates known as tabulae. Syringoporid tabulates: Tube corals. Syrin- Tabulates formed mounds that appear similar goporids are easy to identify because of the
Educational Series 45 | 9 Plate 1. Coral fossils. A: Top view of large, prismatic corallites of Strombodes, a common coral in Silurian rock of eastern Wisconsin [8.5 cm]. B: Closeup of the chains of Halysites, also common in Silurian rock [7 cm]. C: Top view of small, prismatic corallites of Favosites, the honeycomb coral, a common fossil in Silurian rock [7 cm]. D: Side view of Syringopora (common in Silurian rock), showing the lateral collecting tubes [6.5 cm]. E: Lambeophyl- lum, a common coral in Ordovician rock. [2 cm]. F: Streptelasma, an Ordovician rugose coral that has a deep ca- lyx and is taller than Lambeophyllum [4 cm].
Please note that in all the plates that the numbers in brackets following each genus refer to the general length of the longest dimension. These measurements are in the metric system because that is the system that paleontologists use. One centimeter equals about 0.39 inch.
10 | Common Paleozoic Fossils of Wisconsin presence of distinct tubes connecting the corallites. They are quite delicate, but in many places they have been preserved in astonishing Figure 10. Brachiopod anatomy. A: External detail. Syringopora is a com- anatomy. B: Internal anatomy. C. Living brachio- mon syringoporid in Wisconsin; it pods attached to rocks by their pedicles and lived from the Silurian to the Per- two animals living on soft sediments. mian.
Rugosans: Horn corals Rugosans (plate 1) are cone-shaped corals that the extinction at the end of resemble cow horns. The polyp lived in a space the Permian (about 245 in the center of the cone, known as the calice. million years ago). Now, Rugosan corals first appeared in the Ordovician only about 250 living spe- and are the second most common type of coral cies of brachiopods exist; in Wisconsin. Because most horn corals appear more than 30,000 fossil species to be similar, they can be difficult to identify. have been identified in the fossil Rugose corals can be colonial or solitary. record. Colonial rugosans formed bunches attached to Brachiopods have two valves (shells) that one another. Solitary rugose corals commonly are generally of unequal size and shape (fig. were dislodged and then tipped over. If they 10A), but the right and left halves of each valve survived, in many cases they grew upward mirror each other. Near the tip of the bottom again. Such rugosans have geniculations (con- shell (the pedicle valve), a fleshy stalk (the pedi- tortions) caused by the change in growth direc- cle) emerges through a hole (the pedicle open- tion. Solitary and colonial rugosans are charac- ing) and attaches the animal to the sea bottom terized by external growth bands, which (fig. 10A, C). The other shell is known as the formed much like tree rings. brachial valve, which contains the brachidium. Rugose corals declined after the Silurian The brachidium supports the gill-like lopho- and eventually died out at the end of the Paleo- phore, which has many filaments and cilia zoic Era. (hairs) that create currents to bring microscop- ic food particles to the mouth (fig. 10B). The Brachiopods: Lamp shells large surface area of the filaments is used to ob- Brachiopods (plates 2 and 3) are the most tain oxygen from water and eliminate carbon abundant fossils in Wisconsin. Most people are dioxide. The two valves join at the hinge line, not familiar with living brachiopods because which is on the shell near the pedicle (fig. modern species inhabit extremely deep regions 10A). Muscles hold the two valves together so of the world’s oceans, and their shells are rarely that the soft parts are protected. The two valves found on modern seashores. But during the meet at the commissure. Paleozoic, thousands of different species of Other shell features are useful for identify- brachiopods teemed in the near-shore and ing brachiopods. A sulcus (a groove-like depres- deep-sea environments of Wisconsin. The num- sion) is present on many brachiopod shells, and ber of brachiopod species has decreased since a fold (a raised ridge) can be found on the op-
Educational Series 45 | 11 Plate 2. Brachiopod fossils. A, B, and C: Top, side, and back views of Pentamerus, an exceptionally common and distinctive pentamerid brachiopod in Silurian rock of Wisconsin [4.5 cm]. D: Valcourea, a flat Ordovician orthid brachiopod [2 cm]. E and F: Front and back views of Pionodema, an orthid brachiopod with a strong sulcus. It is found in large concentrations within specific layers in Ordovician rock [2 cm]. G: Interior view of Strophomena, an abundant Ordovician strophomenid brachiopod that is flat and broad [4.5 cm]. H and I: Two examples of Doleroides, a Middle Ordovician orthid brachiopod with a shallow sulcus [2 cm and 3 cm, respectively]. J and K: Side and pedicle valve views of the common Ordovician rhynchonellid brachiopod Rhynchotrema, showing the zig-zag commissure and sulcus [1 cm]. L: Rafinesquina, a Middle Ordovician strophomenid brachiopod. It is simi- lar to Strophomena, but with pointed, lateral projections of the shell near the hingeline [3 cm].
12 | Common Paleozoic Fossils of Wisconsin Plate 3. Brachiopod fossils. A: Westonia, a raindrop-shaped inarticulate brachiopod found in Cambrian rock [1 cm]. B: Atrypa, a common spiriferid brachiopod found in Devonian rock along the margin of Lake Michigan [2.5 cm]. C: Spiri- fer, a Devonian spiriferid brachiopod with wing-like extensions of the shell [6 cm]. D: Platystrophia, an Ordovician orthid brachiopod that looks like a spiriferid with a squared front end [3 cm]. E: Leptaena, a square-shaped Devonian stro- phomenid brachiopod that has concentric ridges [2.5 cm]. F: Bottom view of the flat Silurian pentamerid brachiopod Stricklandinia [3 cm]. G: A cast of Orthis, a rounded orthid brachiopod found in Ordovician rock [2 cm]. H and I: Top and side views of Cyrtina, a small spiriferid brachiopod found in Silurian rock [1 cm]. J and K: Pedicle valve and brachial valve views of Hesperorthis, an abundant orthid brachiopod in Middle Ordovician rock. It is more egg-shaped than Eoor- this (shown on this plate as L) and has a larger interarea [2 cm]. L: Pedicle valve view of Eoorthis, an upper Cambrian or- thid. It is more square-shaped than Hesperorthis and has more pronounced costae than Billingsella (shown on this plate as P) [1.5 cm]. M: Protomegastrophia, a spade-shaped Devonian strophomenid brachiopod that resembles the Ordovi- cian strophomenid brachiopod Strophomena (plate 2G), but is more rounded [3.5 cm]. N and O: Lingulepis, an inarticu- late brachiopod that can be found in Cambrian rock [1.5 cm]. P: Billingsella, a square-shaped orthid brachiopod with fine costae that is found in upper Cambrian rock [1 cm]. Q: Interior view of Sowerbyella, a common strophomenid bra- chiopod in Middle Ordovician rock, showing the raised, π-shaped structure near the hinge [1.5 cm]. R: Interior view of Oepikina, a Middle Ordovician strophomenid brachiopod, showing strong ridges that diverge from the hinge [2 cm].
Educational Series 45 | 13 Figure 11. Anatomy of repre- sentatives of the five common groups of articulate brachio- pods in Wisconsin. A: Pio- nodema, an orthid (external shell). B: Strophomena, a stro- phomenid (internal shell). C: Pentamerus, a pentamerid. D: Rhynchotrema, a rhyn- chonellid. E: Mucrospirifer, a spiriferid.
posite valve. Costae are elevated ribs on the terarea borders the hinge region, and the hin- shell. Growth lines are concentric rings repre- geline is straight, like that of the orthids, but senting successive periods of growth. may contain a row of small teeth. Muscle scars Brachiopods are divided into two main are conspicuous features of the internal shell. groups: the articulates and the inarticulates. The strophomenids in Wisconsin, particularly Articulates have hinge structures on their shells; the genus Strophomena, commonly cover entire inarticulates do not. Because articulates greatly horizons (layers) of rock. Strophomenids were outnumber inarticulates in Wisconsin, we focus devastated along with other groups during the on the five orders of articulate brachiopods Permian extinction, but managed to survive a found in Wisconsin: Orthida (orthids), Stro- little longer, into the Triassic, before becoming phomenida (strophomenids), Pentamerida extinct. (pentamerids), Rhynchonellida (rhynchonel- Pentamerids lids), and Spiriferida (spiriferids). Pentamerid shells (fig. 11C) are extremely Orthids abundant in the Silurian rock of eastern Wis- Wisconsin’s Ordovician rock holds many consin. These brachiopods are large and egg- orthids (fig. 11A). Several characteristics of shaped, with curved hingelines and pronounced orthids distinguish them from other brachio- shell beaks. They possess a unique internal pods. Both valves of orthid shells are convex, structure found near the hinge; it is called the and one valve is larger and deeper than the spondylium, a raised, spoon-shaped platform other. Most orthids have costae. The hingeline used for muscle attachment. Pentamerids arose of the orthid shells is normally straight and in the Cambrian, were most common in the Sil- somewhat long and is bordered by a large inter- urian, and became extinct in the Devonian. area, a flat, shelf-like region. Orthids lived from Some quarries in Dodge County contain Sil- the Cambrian to late Permian, and most Or- urian rock with millions of large pentamerid dovician rock contains diverse assemblages of molds and casts. these brachiopods. Rhynchonellids Strophomenids Most rhynchonellids (fig. 11D) in Wisconsin Strophomenids (fig. 11B) were especially abun- are small and marble-like in shape. The com- dant during the Ordovician. Strophomenids missure is zig-zag in outline. These small shells usually have large, semicircular shells, with one have prominent ribs, and a fold and sulcus are concave and one convex valve. A short, flat in- usually present. Rhynchonellids arose in the Or-
14 | Common Paleozoic Fossils of Wisconsin Figure 12. Bivalve mollusk anatomy. Internal view of the right valve of a clam. dovician. This group is still present in some of valves are typically of similar the Earth’s oceans. size and shape. The two shells are hinged together by a Spiriferids strong ligament. Small teeth Spiriferids (fig. 11E) are among the most beau- and sockets may also be tiful brachiopods. They are somewhat triangu- present in the hinge area, which add strength to lar; many possess shell extensions that give the hinge and prevent the two valves from slip- them the appearance of having wings. The ping apart. spiriferids have a unique spiraling support for Bivalves are filter feeders: They strain small the lophophore as well as a fold and sulcus. food particles from water. Water enters through This group arose in the late Ordovician, but the inhalent siphon, passes through the gill, can be found mostly in Devonian rock in the which takes in oxygen from the water, and exits Milwaukee metropolitan area. Two main through the exhalent, the outcurrent siphon. groups of spiriferids are found in Wisconsin: The gill traps food particles and transports spiriferidinids and atrypids. them to the mouth. Bivalves live in a variety of Spiriferidinids have straight hingelines and habitats: Some live epifaunally (on top of sea- lateral wing-like shell extensions. Some mem- floor sediment); others live infaunally (in sea- bers of this group, such as Eospirifer, are super- floor sediment), using their muscular foot to ficially similar to pentamerids. Platyrachella, be- burrow within the bottom sediments. The cause of its interarea, looks suspiciously like an shape of the shell can be related to the environ- orthid. For the most part, however, triangular- ment in which the animal lives. shaped spiriferids are found in Wisconsin. Bivalves originated in the Cam- Atrypids are triangular, and they do not brian, but did not become abundant have the wing-like shape of some other spirifer- until the Ordovician. They are not ids. These spiriferids have curved hingelines and particularly common in Paleozoic two convex valves. Most atrypids have ribs and rock of Wisconsin, but they some- a small beak. Atrypa is a common Devonian times can be found within large brachiopod. groupings of other Ordovician fossils. Bivalve mollusks: Clams Gastropod mollusks: Snails Bivalves (plate 4) are aquatic mollusks that pos- Once confined to the oceans, gastro- sess two valves (fig. 12) that protect the soft pods (snails; plate 5) now live in body parts. The valves are secreted by the streams, lakes, and even on land. mantle, a soft tissue that leaves a scar (the pal- Gastropods are the most abundant lial line) where it connected to the inside of the mollusks: There are approximately shell. The muscles that hold the two valves to- 40,000 living species and 50,000 fos- gether also leave scars on the inside of the shell, sil species. Some modern species are as do the siphons, through which food passes in voracious predators equipped with and out of the body. The length and shape of Figure 13. Gastropod mollusk poison. However, the snails that lived anatomy. A: Apertural view of gastro- the siphon scar, or pallial sinus, is helpful for in Wisconsin’s Paleozoic seas were pod shell. B: Anatomy of a living gas- interpreting the burrowing habits of a bivalve. tropod. probably peaceful grazers. Different scars are also characteristic of differ- The shell of a gastropod is usually coiled ent bivalve species. and is used for protection, as it is for most mol- Unlike the valves of brachiopods, clam lusks (fig. 13A). Gastropods can retreat into
Educational Series 45 | 15 Plate 4. Bivalve mollusk fossils. A: Side view of Saffordia, an Ordovician clam [3.5 cm]. B: Ambonychia, a raindrop- shaped Ordovician clam [3.5 cm]. C. The elongate Ordovician clam Ctenodonta [3.5 cm]. D, E, and F: Two top views and one back view of casts of the egg-shaped Ordovician clam Cypricardites [3.5 cm]. G: Tellinomya, an Or- dovician wedge-shaped clam [3.5 cm]. H and I: Top view of a cast and internal view of the hinge structure of the square-shaped Cyrtodonta, a clam found in Ordovician and Silurian rock [4 cm]. J: Amphicoelia, a Devonian clam [3.5 cm].
16 | Common Paleozoic Fossils of Wisconsin Plate 5. Gastropod mollusk fossils. A: Helicotoma, an Ordovician gastropod with raised, angular sutures [2 cm]. B: Liospira, an Ordovician gastropod with a flattened shell [1.5 cm]; C and D: Sinuites, a common Middle Ordovician gastropod with a large, smooth shell [2.5 cm]. E: Clathrospira, a large and common Ordovician gas- tropod with a cone-shaped spire [4.5 cm]. F: Lophospira, a gastropod with angular ridges. It is similar to Trocho- nema (shown on this plate as H), but taller and with an unwound body whorl [4.5 cm]. G: Bottom view of Ma- clurites, an Ordovician gastropod with one side flattened where it rested on the sea floor [4.5 cm]. H: Trochone- ma, an Ordovician gastropod with angular ridges [3.5 cm]. Several different types can be found in Wisconsin. I: Hormotoma, a high-spired gastropod found in Ordovician rock [2 cm]. J: Phragmolites, an Ordovician gastropod possessing suture-like ornamentation [1.5 cm]. K: Subulites, a tall, high-spired Ordovician and Silurian gastro- pod [5 cm].
Educational Series 45 | 17 their shells and close the opening duced shells, fossil shells were well developed. with a cover called the operculum. Cephalopod shells have evolved into many as- The shape of the shell can be tonishing and beautiful forms (fig. 14), exhibit- highly varied because of the way it ing a variety of shapes, such as straight, slightly coils. Each 360o revolution of the curved, crescent, and coiled (fig. 15). shell is called a whorl. The body The shell of a cephalopod is normally tube- whorl is the bottom and largest or cone-shaped with many dividers. These di- whorl; it contains the aperture, viders are called septa, and they partition the in- the opening of the shell. The shell side of the shell into chambers. Each septum spire includes the whorls above the intersects the shell wall at a suture, which can body whorl. Whorls come in con- be seen as a pattern on the outside of the shell. tact with one another at the shell Sutures are characteristic of cephalopod groups sutures. and are therefore helpful for identification. The Figure 14. Cephalopod mollusk anatomy: coiled and straight cephalopod Gastropods have highly devel- body chamber, which lacks sutures, is the area shells (after Boardman and others, 1987). oped sensory organs, including where the animal lived. A small tube called the tentacles and eyes (fig. 13B). The siphuncle runs the length of the shell, and pass- head is attached to the foot, a muscular organ es through the septa. The siphuncle contains used for creeping and feeding. Many gastro- liquid that helps maintain the buoyancy of the pods have a radula, a rasp-like structure used animal in the water. for scraping algae and other food off the seaf- Like their clam and snail relatives, cephalo- loor. Some carnivorous gastropods use the rad- pods possess typical molluscan features: a shell, ula for boring through the shells of other ani- a muscular foot, and a mantle. They also have mals. highly developed sensory organs. The eye of a Gastropods originated in the Cambrian, cephalopod, for example, is similar to that of a but few are found in rock of that age in Wis- human. Unlike the brachiopods and clams, consin; more are found in Silurian and Devo- cephalopods are mobile predators, and some nian rock. Our Ordovician rock contains abun- can swim at speeds of approximately 64 kilo- dant and beautiful gastropod fossils. meters (40 miles) per hour by jetting water from the mantle cavity through a fleshy funnel. Cephalopod mollusks: Cephalopods originated during the Cam- Squid and octopus brian and are common as fossils in Ordovician Cephalopods (plate 6) are a group of swim- and Silurian rock in Wisconsin. Fossil cephalo- ming mollusks, including the living squid, oc- pods from Wisconsin can exceed 4 meters (13 topus, and the chambered Nautilus. Although feet) in length. They are exciting to find be- most living cephalopods have somewhat re- cause of their large size, but they can be pre- served as molds and casts and can be difficult Figure 15. Mor- to identify. phologies of fossil cephalopods. Extinct arthropods: Trilobites A: Straight. B: Crescent-shaped. Trilobites (plates 7 and 8) were a group of C: Coiled. crab-like animals with hard exoskeletons (outer skeletons) similar to those of modern insects. Trilobites have a three-lobed body (fig. 16B): Two grooves divide the body lengthwise into
18 | Common Paleozoic Fossils of Wisconsin Plate 6. Cephalopod mollusk fossils. A: Spyroceras, a common Ordovician and Silurian cephalopod with a straight and ridged shell [10 cm]. B: Richardsonoceras, common in Ordovician rock [11 cm]. C: Crescent-shaped Richard- sonoceras, similar to Gyroceras, but lacking a ribbed shell [5.5 cm]. D and E: Two examples of Whitfieldoceras, an Or- dovician cephalopod with a constricted body chamber [2.5 cm and 5 cm, respectively]. F: Gyroceras, a Silurian cres- cent-shaped cephalopod [6 cm]. G: Actinoceras, a large, straight cephalopod common in Ordovician rock [35 cm or more]. H: Enlarged siphuncle of Actinoceras, a cephalopod common in Ordovician rock and sometimes found in lower Silurian rock [4.5 cm]. I: Beloitoceras, a short and stout Ordovician cephalopod [5 cm]. It is similar to Rich- ardsonoceras, but smaller. J: Trocholites, a coiled cephalopod occasionally found in Ordovician rock [8.5 cm]. K: Kiono- ceras, a Ordovician cephalopod possessing longitudinal ribs [8.5 cm]. L: Cyrtorizoceras, an Ordovician cephalopod possessing a large body chamber [4 cm].
Figure 16. Trilobite anatomy. A: Bilobate ap- three sections. The middle section is pendages of a living ar- the axial lobe; the other two sections thropod, similar to that of are the pleural lobes. The body can also trilobites. B: Exoskeleton be divided into three sections from of trilobite. head to tail: the front section is the head, or cephalon, the middle section is the thorax, and the tail section is known as the pygidium. These marine animals had a series of small, bilobate
Educational Series 45 | 19 Plate 7. Trilobite fossils. A: Cephalon of Illaenus, a common trilobite in Ordovician rock. The cephalon is similar in shape to a strophomenid brachiopod [2.5 cm]. B: Complete specimen of the Ordovician trilobite Isotelus. It has a more triangular cephalon and pygidium than Bumastus, and a more pronounced axial lobe [5 cm]. C: Bumas- tus, a Ordovician mud-burrowing trilobite with a broad, rounded cephalon and pygidium [6.5 cm] D: Large cepha- lon of Bumastus [4 cm]. E and F: Stubby, rounded pygidium and disarticulated cephalon of the Cambrian trilobite Conaspis. The glabella is more rounded than that of Crepicephalus (shown on this plate as H) [3 cm]. G: Large, egg- shaped pygidium of the upper Cambrian trilobite Dikelocephalus [6 cm]. The pygidium looks similar to a fish tail. H: Molted parts of the Cambrian trilobite Crepicephalus. The glabella is more squared than that of Conaspis. Ex- tensions of the pygidium make it look similar to a swallowtail butterfly wing [3 cm]. I: The Ordovician trilobite Ceraurus, showing the characteristic tail spines and pronounced “ribs” of the pleural lobes [4 cm].
(two-pronged) legs beneath their exoskeleton trilobites molted their skeletons. The trilobite (fig. 16A). One part of the leg, the exopodite, pushed itself out of its old skeleton and grew a was used for walking; the other, the endopodite, new one; most trilobite fossils are instars, or was used for gas exchange or “breathing.” molted skeletons. The region of the cephalon On the cephalon, a series of lines, or facial between the eyes and bounded by the sutures is sutures, are present. These sutures opened when called the glabella.
20 | Common Paleozoic Fossils of Wisconsin Plate 8. Trilobite fossils. A: Cephalon and pygidium of the Cambrian trilobite Wilburnia. The glabella is similar to that of Conaspis (shown on plate 7 as F), but with a pronounced flared lip along the outer margin [1.5 cm]. B: Articulated specimen of Cedaria, found in Cambrian rock. It is similar to Wilburnia, but smaller and with a narrower pygidium and lip [3 cm]. C: Cephalon of Ptychaspis, a Cambrian trilobite. It is similar to Cedaria, but much larger and with more widely spaced eyes [3 cm]. D: Dalmanites, a Silurian trilobite possessing bumps on the cephalon [4.5 cm].
Trilobites had compound eyes, much like Crinoids: Sea lilies those of insects. Some trilobites were adapted Crinoids (plate 9A, B) are echinoderms, a to mud-burrowing, and possessed vestigial, or group that includes the starfish, sea ur- nonfunctional, eyes. Mud-burrowing trilobites chins, and sand dollars. Sometimes called had smooth, streamlined bodies. sea lilies, crinoids resemble long-stemmed Trilobites first appeared in the Cambrian, flowers, but they are marine animals. A but after a diversification they dwindled and holdfast at the base of the animal’s stem eventually became extinct during the Permian. functions like a root that holds the animal Trilobite fossils are not common in Wisconsin. in place (fig. 17). The animal’s cuplike Generally, they form a large component of body, or calyx, is composed of a mosaic Cambrian fossil assemblages, and Ordovician of geometric plates. The calyx has rock commonly contains trilobite fragments. In many arms that open into a fan-like net Silurian rock, trilobites may be associated with so the crinoid can feed on microscopic reef-like habitats. Devonian trilobites are not food particles. The flexible stem is particularly abundant. Fossil tracks of trilobites can be found on rock that was once the Paleo- Figure 17. Crinoid zoic seafloor. The trilobite Calymene celebra is anatomy. Wisconsin’s state fossil. Educational Series 45 | 21 composed of a series of button-like discs called columnals. When crinoids die, their stems fall
apart, or disarticulate, into columnals. The lu- Figure 19. Stromatoporoid anatomy. men, a hole in the middle of the stem, con- tained a tube for carrying nutrients to the stem and holdfast. The mouth of the crinoid is on the top of the calyx. Crinoids first appeared in the Cambrian and diversified until the Permian extinction, when their numbers were greatly reduced. Complete crinoid fossils may occasionally be found in Wisconsin’s Silurian and Devonian Figure 20. Hyolithid anatomy. rock, but most crinoid fossils consist of scat- tered columnals. In the Paleozoic, crinoids and gas exchange. They also contain a u-shaped lived in colonies in shallow waters, but gut with a distinct mouth and anus. today they live in deeper regions of the world’s Bryozoans are inconspicuous fossils, but oceans. can be seen most easily on slabs of Ordovician limestone or dolomite with the aid of a magni- Bryozoans: Moss animals fying glass or hand lens. Bryozoans are difficult Bryozoans (plate 9C, D) are small, mostly ma- to distinguish from one another. In most cases rine animals that form skeletons of a variety of you may not be able to identify your fossil shapes: tiny twigs, nets, domes, and mossy more specifically than as a bryozoan. crusts (fig. 18). The zooecium (skeleton) houses Bryozoans were able to live almost any- highly structured colonies of zooids, or cuplike where: on a brachiopod shell, the side of a animals. Bryozoans, like brachiopods, contain a cephalopod, coating sea plants, or in an indi- tentacle-bearing lophophore used in feeding vidual colony on the sea floor. Many fossils have patches of bryozoans on them. Bryozoans first evolved in the Ordovician and have persist- ed to the present day; they live in ocean habi- Figure 18. Bryozoan tats that are similar to those of Paleozoic seas. anatomy. A: Skeleton of a bryozoan colony. B: A zooid living in the skel- Other fossils eton. Stromatoporoids: Laminated reef builders Stromatoporoids were colonial marine organ- isms related to sponges. They formed skeletons composed of thin layers called laminae. Small bumps called mamelons were present on their surface (fig. 19). Stromatoporoids formed reefs in the shallow Silurian seas of Wisconsin. They became extinct 65 million years ago.
Ostracods: Bivalved arthropods Ostracods (plate 9E) are small, two-valved arthropods that resemble small, dark, polished
22 | Common Paleozoic Fossils of Wisconsin Plate 9. Miscellaneous fossils. A: Crinoid columnals, or plates, which make up the stems [0.5 cm]. B: Crinoid column section, or stem [3.5 cm]. C and D: Fragments of bryozoan colonies found in Ordovician, Silurian, and Devonian rock [3 cm]. E: Eoleperditia, an Ordovician ostracod. [1 cm]. F and G: Two views of the Ordovician hyolithid Hyolithes [3 cm]. H: An Ordovician colony of Receptaculites, believed to be a skeleton-secreting algae [10 cm].
beans. Ostracods first appeared in the Cambrian had difficulty classifying. Receptaculitids have and still exist today in freshwater, marine, and been classified as corals and sponges, but they terrestrial environments. Like other arthropods, are now thought to be colonial algae. They su- ostracods molt their skeletons as they grow. Al- perficially resemble favositids, the honeycomb though they are not very common, ostracods corals. Receptaculitid fossils are most common are occasionally found in Wisconsin’s Ordovi- in Ordovician rock. cian and Silurian rock. Trace fossils: Trails, borings, and burrows Hyolithids: Animals of unknown affinities Trace fossils (plate 10) are the preserved paths Very little is known about these mysterious ani- of animals that crawled on and bored or bur- mals. Hyolithids (plate 9F, G) are an unusual, rowed into the seafloor. A variety of paths rep- extinct group of marine animals that had a resenting behaviors—such as feeding, moving, conical shell with an operculum (fig. 20). They and resting—can be found in sedimentary rock. are sometimes found in the digestive tracts of Certain burrowing behaviors are specific to cer- fossil marine worms. Middle Ordovician rock in tain environments, so paleontologists can re- Wisconsin commonly contains hyolithids. construct ancient environments from the shape of trace fossils. Trace fossils are common in the Receptaculitids: Skeleton-secreting algae Paleozoic rock of Wisconsin and are still being Receptaculitids (plate 9H) are another extinct formed in ocean sediments today. group of marine organisms that scientists have
Educational Series 45 | 23 Plate 10. Trace fossils. A: Large, worm-like feeding traces parallel to rock layers [diameter can be as much as 1 cm]. B: Large, variably shaped borings perpendicular to rock layers [diameter up to 1 cm or more; length up to 4 cm or more]. C: Long, thin burrows perpendicular to rock layers [diameter less than 0.5 cm; length up to 5 cm or more].
SOURCES OF INFORMATION public library for the various titles available. The following books provide detailed technical Societies information about fossils and the principles of Wisconsin abounds in groups that are devoted paleontology. They are usually available at uni- to all aspects of the earth sciences, including versity libraries. paleontology. One way to find out if there is a club near you is to check with the Midwest Boardman, R.S., Cheetham, A.H., and Rowell, Federation of Mineralogical and Geological So- A.J., 1987, Fossil Invertebrates: Blackwell cieties, a regional member of the American Scientific Publications, Palo Alto, 713 p. Federation of Mineralogical Societies. The A detailed, college-level textbook intended Midwest Federation maintains a directory of its for paleontology students. member societies. (Note that not all of these societies focus on paleontology.) The Internet Chamberlin, T.C., 1882, Geology of Wisconsin: address for the Midwest Federation at the time Survey of 1873–1879, Volume IV, 779 p. of the printing of this publication is A classic work on the geology and
24 | Common Paleozoic Fossils of Wisconsin Rhodes, F.H.T., Zim, H.S., and Shaffer, P.R., Natural History Survey, for assistance and en- 1962, Fossils: Western Publishing Compa- couragement throughout this project. Paula ny, Inc., Racine, 160 p. Sumpter of the Milwaukee Public Museum and An inexpensive, superbly written, and Klaus Westphal of the University of Wisconsin– extensively illustrated book. Madison assisted with museum collections. We thank David Clark and Dana Geary of the Uni- Schrock, R.R., and Twenhofel, W.H., 1923, versity of Wisconsin–Madison for sharing their Principles of Invertebrate Paleontology: knowledge of fossils and paleontology with us. McGraw-Hill Book Company, Inc., 816 p. RHN thanks Wayne Schaefer, of the University A classic text written by Wisconsin of Wisconsin Center–West Bend, Lloyd and paleontologists. Ellen Brown, Dan and Ellen Dettwiler, and his Shimer, H.H., and Schrock, R.R., 1945, Index parents for support during the early periods of Fossils of North America: John Wiley & this project. Sons, New York, 837 p. A book that will help you identify fossils not ABOUT THE AUTHORS illustrated in this guide. Ross H. Nehm and Bryan E. Bemis grew up in Wisconsin and collected fossils before pursuing ACKNOWLEDGMENTS undergraduate degrees at the University of Wis- We thank those who reviewed earlier drafts of consin–Madison. Both received their doctorates this guide, especially Maureen Muldoon, M.G. from the University of California—Nehm at Mudrey, Jr., and the late Juergen Reinhardt, of Berkeley, and Bemis at Davis. Nehm is now an the Wisconsin Geological and Natural History assistant professor at The City College, City Survey, who provided many useful comments. University of New York, in New York City. Be- We also thank Susan Lawton Hunt and Ronald mis is a research associate at the U.S. Geologi- Hennings of the Wisconsin Geological and cal Survey, Menlo Park, California.
Educational Series 45 | 25
WISCONSIN GEOLOGICAL AND NATURAL HISTORY SURVEY James M. Robertson, Director and State Geologist
John W. Attig, geologist Mindy C. James, publications manager William G. Batten, geologist Marcia J. Jesperson, Map Sales associate Kenneth R. Bradbury, hydrogeologist Kathy A. Kane, network administrator Bill C. Bristoll, information manager Irene D. Lippelt, water resources specialist Bruce A. Brown, geologist Frederick W. Madison, soil scientist Lee Clayton, geologist M.G. Mudrey, Jr., geologist Michael L. Czechanski, cartographer Stanley A. Nichols, biologist Donna M. Duffey, Map Sales associate Deborah L. Patterson, cartographer Timothy T. Eaton, hydrogeologist Roger M. Peters, geologist Thomas J. Evans, geologist Kathy Campbell Roushar, cartographer Reconstruction of Silurian reef. (Photograph Stephen J. Gaffield, hydrogeologist Apichart Santipiromkul, information processing of diorama © Milwaukee Public Museum.) Madeline B. Gotkowitz, hydrogeologist specialist David J. Hart, hydrogeologist Curtis L. Thomas, geotechnician Ronald G. Hennings, hydrogeologist (emeritus) Virginia L. Trapino, program assistant Rilla M. Hinkes, office manager Alexander Zaporozec, hydrogeologist (emeritus) Thomas S. Hooyer, geologist Kurt K. Zeiler, GIS specialist Susan L. Hunt, graphic artist Kathie M. Zwettler, administrative manager plus approximately 10 graduate and undergraduate student workers.
RESEARCH ASSOCIATES
Gregory J. Allord, USGS George J. Kraft, Central Wis. Groundwater Center Mary P. Anderson, UW–Madison James T. Krohelski, USGS Larry K. Binning, UW–Madison Gene L. LaBerge, UW–Oshkosh Michael F. Bohn, Wis. Dept. of Nat. Res. Eiliv Larsen, Geological Survey of Norway Stephen M. Born, UW–Madison J. Brian Mahoney, UW–Eau Claire Philip E. Brown, UW–Madison Kevin McSweeney, UW–Madison Charles W. Byers, UW–Madison Christine Mechenich, Central Wis. Groundwater Center William F. Cannon, USGS David M. Mickelson, UW–Madison Kevin Connors, Dane Co. Land Conserv. Dept. Donald G. Mikulic, Ill. State Geol. Survey C. Patrick Ervin, Northern Ill. Univ. William N. Mode, UW–Oshkosh Daniel T. Feinstein, USGS Maureen A. Muldoon, UW–Oshkosh Robert F. Gurda, Wis. State Cartographer's Office James O. Peterson, UW–Madison Nelson R. Ham, St. Norbert Coll. Jeffrey K. Postle, Wis. Dept. Ag., Trade & Consumer Mark T. Harris, UW–Milwaukee Protection Karen G. Havholm, UW–Eau Claire Kenneth W. Potter, UW–Madison Randy J. Hunt, USGS Todd W. Rayne, Hamilton Coll. Mark D. Johnson, Gustavus Adolphus Coll. Allan F. Schneider, UW–Parkside (emeritus) John Klasner, Western Ill. Univ. J. Antonio Simo, UW–Madison Joanne Kluessendorf, Weis Earth Science Museum Kent M. Syverson, UW–Eau Claire James C. Knox, UW–Madison Jeffrey A. Wyman, UW–Madison
The Wisconsin Geological and Natural History Survey also maintains collaborative relationships with a number of local, state, regional, and federal agencies and organizations regarding educational outreach and a broad range of natural resource issues. Geologic time scale. Shaded area indicates time periods for which there is only sparse evidence in Wisconsin.
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