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WISCONSIN GEOLOGICAL AND NATURAL HISTORY SURVEY

James M. Robertson, Director and State Geologist

John W. Attig, geologistWilliam G. Batten, geologistKenneth R. Bradbury, hydrogeologistBill C. Bristoll, information managerBruce A. Brown, geologistLee Clayton, geologistMichael L. Czechanski, cartographerDonna M. Duffey, Map Sales associateTimothy T. Eaton, hydrogeologistThomas J. Evans, geologistStephen J. Gaffield, hydrogeologistMadeline B. Gotkowitz, hydrogeologistDavid J. Hart, hydrogeologistRonald G. Hennings, hydrogeologist (emeritus)Rilla M. Hinkes, office managerThomas S. Hooyer, geologistSusan L. Hunt, graphic artist

Mindy C. James, publications managerMarcia J. Jesperson, Map Sales associateKathy A. Kane, network administratorIrene D. Lippelt, water resources specialistFrederick W. Madison, soil scientistM.G. Mudrey, Jr., geologistStanley A. Nichols, biologistDeborah L. Patterson, cartographerRoger M. Peters, geologistKathy Campbell Roushar, cartographerApichart Santipiromkul, information processing

specialistCurtis L. Thomas, geotechnicianVirginia L. Trapino, program assistantAlexander Zaporozec, hydrogeologist (emeritus)Kurt K. Zeiler, GIS specialistKathie M. Zwettler, administrative manager

Gregory J. Allord, USGSMary P. Anderson, UW–MadisonMichael F. Bohn, Wis. Dept. of Nat. Res.Stephen M. Born, UW–MadisonPhilip E. Brown, UW–MadisonCharles W. Byers, UW–MadisonWilliam F. Cannon, USGSDouglas S. Cherkauer, UW–MilwaukeeKevin Connors, Dane Co. Land Conserv. Dept.Robert H. Dott, Jr., UW–Madison (emeritus)C. Patrick Ervin, Northern Ill. Univ.Daniel T. Feinstein, USGSRobert F. Gurda, Wis. State Cartographer's OfficeNelson R. Ham, St. Norbert Coll.Mark T. Harris, UW–MilwaukeeKaren G. Havholm, UW–Eau ClaireRandy J. Hunt, USGSMark D. Johnson, Gustavus Adolphus Coll.Joanne Kluessendorf, Weis Earth Science MuseumJames C. Knox, UW–Madison

The Wisconsin Geological and Natural History Survey also maintains collaborativerelationships with a number of local, state, regional, and federal agencies and organizationsregarding educational outreach and a broad range of natural resource issues.

George J. Kraft, Central Wis. Groundwater CenterJames T. Krohelski, USGSMichael D. Lemcke, Wis. Dept. of Nat. Res.J. Brian Mahoney, UW–Eau ClaireKevin McSweeney, UW–MadisonChristine Mechenich, Central Wis. Groundwater CenterDavid M. Mickelson, UW–MadisonDonald G. Mikulic, Ill. State Geol. SurveyWilliam N. Mode, UW–OshkoshMaureen A. Muldoon, UW–OshkoshRobert E. Pearson, Wis. Dept. of TransportationJames O. Peterson, UW–MadisonJeffrey K. Postle, Wis. Dept. Ag., Trade & Consumer ProtectionKenneth W. Potter, UW–MadisonTodd W. Rayne, Hamilton Coll.Daniel D. Reid, Wis. Dept. of TransportationAllan F. Schneider, UW–Parkside (emeritus)J. Antonio Simo, UW–MadisonKent M. Syverson, UW–Eau Claire

plus approximately 10 graduate and undergraduate student workers.

RESEARCH ASSOCIATES

Jeffrey A. Wyman, UW–Madison

Common Paleozoic Fossilsof Wisconsin

Ross H. NehmBryan E. Bemis

2002

Wisconsin Geological and Natural History SurveyEducational Series 45

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ISSN: 1052-2115

Published by and available fromWisconsin Geological and Natural History Survey3817 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

This report is an interpretation of the data available at the time of preparation. Every reasonableeffort 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. Properuse of the report is the sole responsibility of the user.

The use of company names in this document does not imply endorsement by the WisconsinGeological and Natural History Survey.

Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, incooperation with the U.S. Department of Agriculture, University of Wisconsin–Extension,Cooperative Extension. University of Wisconsin–Extension provides equal opportunities inemployment and programming, including Title IX and ADA requirements. If you need this informationin an alternative format, contact the Office of Equal Opportunity and Diversity Programs or theWisconsin 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 objectivescientific information about the geology, mineral resources, water resources, soil, and biologyof Wisconsin. We collect, interpret, disseminate, and archive natural resource information. Wecommunicate the results of our activities through publications, technical talks, and responses toinquiries from the public. These activities support informed decision-making by government,industry, business, and individual citizens of Wisconsin.

Cover art: Ross H. Nehm

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OVERVIEW 1Wisconsin's fossils 1Geologic time 1How and where fossils form 2What fossils tell us 3

About evolution and extinction 3About ancient environments 3About the age of rock 4

COLLECTING FOSSILS 4Finding fossils in Wisconsin 4

Eastern Wisconsin 5Central and western Wisconsin 5

Tools for collecting fossils 6Removing fossils from rock 7Recording information about fossils 7

IDENTIFYING FOSSILS 7Stromatolites 8Corals 9Brachiopods: Lamp shells 11Bivalve mollusks: Clams 15Gastropod mollusks: Snails 15Cephalopod mollusks: Squid and octopus 18Extinct arthropods: Trilobites 18Crinoids: Sea lilies 21Bryozoans: Moss animals 22Other fossils 22

SOURCES OF INFORMATION 24Societies 24Books 24

ACKNOWLEDGMENTS 25

ABOUT THE AUTHORS 25

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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 roadcutin 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

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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

of time is the era, which is further subdividedinto periods.

Geologic time is divided into three eons.From oldest to most recent, these eons are theArchean, Proterozoic, and Phanerozoic. TheArchean and Proterozoic (sometimes collec-tively referred to as the Precambrian) encom-pass geologic history prior to 570 million yearsago. Fossils representing these two eons, whenthe earliest forms of primitive life developed,are uncommon. The oldest known single-celledorganisms—bacteria that were beautifully pre-served in rock that is 3.5 billion years old—were discovered in Africa. Fossils of soft-bodiedmulticellular organisms about 700 million yearsold have been found in Australia’s EdiacaraHills.

The Phanerozoic has been divided intothree principal eras; from oldest to youngest,they are the Paleozoic (upon which we focus inthis booklet), Mesozoic, and Cenozoic.

Diversity of lifeforms and the complexitywithin them developed during the early part ofthe Phanerozoic, in the Paleozoic Era, 570 to245 million years ago. This era has been divid-ed into (from oldest to most recent) the Cam-brian, Ordovician, Silurian, Devonian, Missis-sippian, Pennsylvanian, and Permian Periods.Many major groups of shell-bearing inverte-brates (animals with hard shells and no back-bones) appeared throughout the oceans duringan immense proliferation of lifeforms at the be-ginning of the Paleozoic. This era ended withthe largest extinction in the Earth’s history: 80percent of all types of marine invertebrates be-came extinct at the end of the Paleozoic, duringthe Permian Period.

OVERVIEW

Wisconsin’s fossils

Many people view the Earth and its lifeforms aswhat we see today. However, both havechanged dramatically through time; the evi-dence for this comes from something most ofus see every day, but think little about: rock.Much of the rock in Wisconsin contains fossils,the remains of ancient organisms. After theseorganisms died, parts of their bodies were pre-served in rock. Paleontologists—people whostudy ancient life on the basis of fossilizedplants and animals—use them as clues aboutthe Earth’s past.

The fossils found in rock throughout Wis-consin were formed from creatures that lived inthe warm, shallow seas that once covered thestate. In this guide we introduce the fascinatingmarine (sea) creatures that existed in what isnow called Wisconsin, discuss how they werepreserved as fossils and what they tell us aboutthe ancient Earth, illustrate common Wisconsinfossils, and suggest how and where to collectthem.

Geologic time

When did the creatures that were to becomefossils live? To appreciate fully just how longago these organisms existed, it is helpful to de-velop an understanding of the concept of geo-logic time.

Scientific methods for determining ages in-dicate that the Earth is about 4.6 billion yearsold. It is difficult for most of us to imagine sucha vast amount of time, so geologists have devel-oped a timetable (see inside back cover) thatbreaks geologic time into major units. Themost expansive units, covering the longestamounts of time, are called eons. The next unit

Common Paleozoic Fossilsof Wisconsin

2 | Common Paleozoic Fossils of Wisconsin

The Mesozoic Era (245 to 66 million yearsago) is also called the Age of Reptiles becauseof the extensive proliferation of land and seareptiles. Dinosaurs were one of the dominantanimal groups during the Mesozoic Era. Al-though Wisconsin contains only a sparse geo-logic record of the Mesozoic, we know fromthe fossil record in other areas that many ma-rine and terrestrial animals, including dino-saurs, became extinct near the end of this era.Mammals and flowering plants became com-mon during about the last two-thirds of theMesozoic.

The Cenozoic Era encompasses the past 66million years of Earth’s history. The most re-cent Ice Age began about 2.5 million years ago,during the Cenozoic.

How and where fossils form

Because fossils form under specific conditions,only a small percentage of organisms is pre-served. Hard skeletal parts such as shells,bones, and teeth are the most commonly fossil-ized remains because they are more durablethan the soft tissues, which decay rapidly.

Fossils are preserved in sedimentary rock,which is formed from what is essentially scrapmaterial: Weathering processes break up anderode rocks at the Earth’s surface, and windand water carry away the scraps—pebbles, sand,silt, and clay. Many of these particles, called sed-iment, then settle on sea bottoms. As sea crea-tures die and also settle to the seafloor, theyeventually are buried by sediment (fig. 1).Eventually, compaction of the sediment consoli-

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, anddolomite.

Sandstone, which is composed of sand-sizedparticles, forms in water in areas such as surfzones and beaches. Wave action is strong inthese areas, and rock formed there contains fewwell preserved fossils—skeletal parts are easilyshattered in these high-energy environments.Fossil-bearing Cambrian sandstone is exposedat the surface in central and northwestern Wis-consin. Middle Ordovician sandstone is foundprimarily along river valleys in southern Wis-consin, but it is generally devoid of fossils. Wis-consin sandstone can be white, tan, or iron-stained brown.

Unlike sandstone, shale forms in areas ofslow-moving or still water; as a result, fossils inshale are generally better preserved than thosein sandstone. Shale is composed of silt and clayand can be split into thin sheets. It is mostcommon in the upper Ordovician and Devo-nian rock of eastern Wisconsin and is generallygreen, blue green, or black.

Limestone forms from the precipitation(settling out) of calcium carbonate from oceanwater in warm, shallow environments awayfrom the input of sand, silt, and clay. Limestonecan also be composed of skeletal fragments andframeworks of organisms that built their shellswith calcium carbonate from the water.

Limestone becomes dolomite when the ele-ment magnesium replaces some of the originalcalcium. Much limestone in Wisconsin has beenaltered to dolomite. Because of this alteration,fossils in dolomite are not as well preserved asthose in limestone. Fossiliferous dolomite andlimestone are found in southern and easternWisconsin. Limestone and dolomite range frombuff or gray to dark gray or blue gray.

Figure 1. How fossilsform. A: An animal diesand settles to the seafloor. B: Animal is buriedand soft tissues decay.C: Hard parts are slowlyreplaced by minerals.D: Sediment becomesrock and erosion ex-poses fossil.

Educational Series 45 | 3

Conditions at a burial site determine howan organism is preserved. Fossils of marine in-vertebrates in Wisconsin are most commonlypreserved as replacements or molds. Replace-ments are produced when minerals settle out ofwater and replace the original skeletal parts.Molds are three-dimensional impressions left inrock after the skeletal parts of an organism dis-solve. A mold can be either internal or external.The impression of the outside of a preservedshell is called an external mold; if minerals orsediment fill the space between two shells, butthe shells dissolve, an internal mold is formed.

Under certain conditions, the remains ofsome ancient plants and animals become fossilfuels. Coal is formed from the accumulationand burial of plant material under conditionslacking oxygen, such as in swamps. Over mil-lions of years, high pressures and temperaturesforce oxygen and hydrogen out of the remains,leaving carbon, which we burn as fuel. Naturalgas and petroleum form similarly, but they mayinclude animal remains as well and form underhigher pressures and temperatures. About 85percent of the energy used in the United Statescomes from these fossil fuels.

What fossils tell us

About evolution and extinctionAs you become familiar with Wisconsin’s Pa-leozoic fossils, you may notice that fossils ofcertain organisms have different morphologies,or forms, through time. Some organisms de-velop new features (for example, shells that arethicker or shaped differently). These changesthrough time are known as evolution. How andwhy these changes come about, or why they

don’t, are the focus of much research.Fossils also show us that many groups of

creatures eventually become extinct. Trilobites,once plentiful in the Cambrian seas of Wiscon-sin, are extinct today, along with countlessother creatures. Why are trilobites extinct, butdirect descendants of other creatures that livedduring the Paleozoic, such as snails, abundanttoday? Some creatures survive periods duringwhich many other organisms become extinct.Does this happen because they are betteradapted to their environment than those thatbecame extinct, or are they just lucky? Paleon-tologists are working to answer these questions.

About ancient environmentsWhat was the area we now call Wisconsin like400 million years ago? It is possible to deter-mine the environmental conditions of the paston the basis of the organisms that lived in anarea at a certain time. Using information fromliving relatives of extinct creatures and assum-ing that similar, related creatures lived in similarhabitats, we can make inferences about the past.For example, one indicator of marine environ-ments is coral. Today, corals and coral reefs re-quire specific conditions to flourish—warmtemperatures (25oC to 29oC [77oF to 84oF]),shallow depths (less than 56 meters [165 feet]),and normal salinity (the total quantity of dis-solved salts in water; normal salinity is approxi-mately 35 parts per thousand). The abundanceof coral fossils in Wisconsin’s Silurian-age rocksuggests that a warm, shallow sea of normal sa-linity covered Wisconsin during that time.

Figure 2. Block diagram of near-shore sedimentaryenvironments. A: Eroding land is the source ofsediment. B: Sand becomes sandstone. C: Silt andclay become shale. D: Farther from sedimentsources, skeletal remains and calcium carbonatefrom ocean water become limestone.


Many other organisms can be used to re-construct aspects of the Earth’s ancient envi-ronment as well. Knowing how life has re-sponded to environmental changes in the pastmay help us understand how future environ-mental changes, such as possible global warm-ing, will influence life on Earth.

About the age of rockFossils can generally be considered the sameage as the rock in which they are found. Theage of rock can be determined relatively or ab-solutely, just as events can. For example, weknow that Wisconsin became a state relativelylater than the explorations of Marquette andJoliet. But when exactly did both events hap-pen? Speaking in absolute terms, Marquette andJoliet’s expedition began in 1673 and Wiscon-sin became a state in 1848.

Different rock layers can be dated relativelyon the basis of their position in a “stack” of lay-ers. For example, if you were to drop somesheets of paper on the ground, one on top ofthe other, it’s obvious that the ones on the topgot there more recently than the ones on thebottom. In the same way, rock layers (and thefossils they contain) on top of others are rela-

tively younger than the rock layers below, if therock has not been overturned by geologic pro-cesses.

One way you may be able to determine thegeological period (age range) from which youare collecting is the presence of index fossils inthe rock layers. Index fossils are fossil groupsthat lived for very short, specific periods of time.That makes them more useful for relative datingof rocks than long-lived fossil groups becausethey allow you to narrow down a rock’s agerange. Geologists can use several index fossilstogether to make determinations about relativerock ages.

The discovery of radioactivity provided ameans of establishing absolute dates of rock. Ra-dioactive parent (original) elements decay at aconstant rate to produce progeny (decayed; alsoreferred to as daughter) products. By knowingthe half life (decay rate) and measuring theamount of parent and progeny products in arock sample, it is possible to determine howmuch radioactive decay has occurred and there-fore the time that has passed, which gives us theabsolute age of the rock. Paleontologists can usea combination of relative and absolute methodsto determine the age of rocks and fossils.

COLLECTING FOSSILS

Finding fossils in Wisconsin

Fossils can be found in many places in the state.Fossil-bearing sedimentary rock covers much ofWisconsin, particularly the far southern andeastern parts of the state (fig. 3).

But you may not have to travel far to findfossils—the gravel in your driveway, the rocks inyour garden, and the stone in buildings near youmay have fossils in them. The most abundantand most easily collected fossils come fromroadcuts, natural bluffs, and quarries. Roadcutscontaining fossils can be found throughout Wis-consin, particularly in the southwest (fig. 4).Natural bluffs are along rivers and streams.

Figure 3. Where to find Paleozoic invertebrate fossils. Blackareas are promising localities for collecting fossils.


Quarries are excellent places to look for fossilsas well, but falling rocks can make them dan-gerous. Do remember that many good collect-ing localities, such as quarries and roadcuts, areprivate property—always secure permissionwith quarry and landowners before entering acollecting site. Use care along roadcuts.

Here we list only general localities for fossilcollecting: If we provided directions to specificsites, they might become overcollected. Inmany cases, the best finds are those that youdiscover on your own. This is part of the excite-ment of fossil collecting! Refer to later sectionsof this guide for descriptions of specific fossilgroups mentioned below.

Eastern Wisconsin■ Door County. Many bedrock exposures inDoor County contain Silurian brachiopods (es-pecially Pentamerus) and rugose and tabulatecorals. Fragments of Silurian fossils are alsofound along the shore of Lake Michigan.

■ Oakfield, Wisconsin. An abandoned brick-work quarry southwest of Oakfield contains up-per Ordovician orthid brachiopods. Many ofthese brachiopods have been eroded out oftheir matrix and can be picked from theground. Above the shale in the lower quarry lieharder dolomite that contains layers packedwith shell fragments of bryozoans and brachio-pods. Fossils have been collected from this sitefor over a decade, but it still provides somegood specimens.

■ Mayville, Wisconsin. The large quarries ofthe Mayville Lime Company near Mayville con-tain a few corals as well as casts and molds ofabundant specimens of the brachiopods Pen-tamerus and Virgiana. Collecting permission isrequired.

■ Washington County. In a quarry nearGrafton, large blocks of reef material—includ-ing Silurian gastropods, brachiopods, andcrinoid stems—are sometimes exposed. Collect-

ing permission is required. Silurian corals canalso be found on stone fences and rock pilesthroughout the county.

■ Cedarburg and Grafton, Wisconsin. Out-crops and quarries on the banks of the Milwau-kee River between Cedarburg and Grafton con-tain Silurian fossils.

■ Waukesha County. Many of the best Siluri-an fossils from Wisconsin come from quarriesin the metropolitan areas of eastern Wisconsin.Fossils in these quarries are rare but well pre-served. Many beautiful specimens from thesequarries are now in museums.

■ Green Bay, Wisconsin. Near Green Bay,outcrops of upper Ordovician Maquoketa Shalecontain bryozoans and brachiopods, as does aroadcut on Highway 57 north of Green Bay.Other exposures alongriver banks in the areaalso contain Ordovi-cian fossils.

Central and westernWisconsinMiddle Ordovician do-lomite that is rich invirtually all types of in-vertebrate fossils isfound in many locali-ties in central andwestern Wisconsin.The most well pre-served fossils comefrom the PlattevilleFormation of the Sin-nipee Group. Thesefossils include brachio-pods, bivalves (clams),gastropods (snails), tri-lobites, hyolithids,cephalopods, ostra-cods, crinoid colum-nals, and corals.

Figure 4. A paleontologist collectingMiddle Ordovician fossils from a road-cut in southwestern Wisconsin.


■ Sun Prairie, Wisconsin. Two quarries about6.4 kilometers (4 miles) northeast of Sun Prai-rie off Highway 151 contain abundant Ordovi-cian fossils from the Platteville Formation.These quarries have been active recently, andfresh rock has been exposed. In 1991 most of acephalopod (Endoceras) approximately 4 meters(13 feet) long was discovered along with otherexcellent fossils. These are active quarries; se-cure permission from owners before entering.

■ Dickeyville, Wisconsin. A roadcut about 6.4kilometers (4 miles) northwest of Dickeyville inGrant County on Highways 35 and 61 containsweathered strophomenid brachiopods, bryozo-ans, ostracods, crinoid columnals, and other in-vertebrate fossils.

■ Platteville, Wisconsin. Roadcuts directlysouthwest of Platteville in Grant County arefossiliferous. A quarry off Highway 151 northof Dickeyville contains the brachiopod Pionode-ma and other common Ordovician fossils.

■ Mount Horeb, Wisconsin. Roadcuts alongHighway 151 south of Mount Horeb occasion-ally contain Receptaculites, marine organismsthat may be algae. Quarries and roadcuts justsouth of Mount Horeb contain Ordovician fos-sils, including strophomenid brachiopods, bry-ozoans, ostracods, and trilobite fragments.

■ Fennimore, Wisconsin. Roadcuts west andsouth of Fennimore contain well preserved Or-dovician brachiopods, trilobites, ostracods, bry-ozoans, and cephalopods.

■ Roadcuts along Highways 18 and 35 east ofWyalusing State Park contain Ordovician fossilsfrom the Platteville Formation of the SinnipeeGroup, such as brachiopods, trilobite frag-ments, bryozoans, cephalopods, and clams.

Tools for collecting fossils

Fossil collecting is an inexpensive hobby. Thefollowing items are helpful when collecting(fig. 5):

■ A bricklayer’s hammer. The long, flat, ta-pered end of a bricklayer’s hammer is ideal forsplitting sedimentary rock layers.

■ A chisel. Chisels are needed for chipping afossil from a large piece of rock. Safety glassesshould always be worn when removing fossilsby this method.

■ Dental tools. Dental tools are useful in theremoval of delicate fossils.

■ Old newspapers. Ensuring that a specimenis not damaged requires nothing more than acareful wrapping with newspapers.

■ Pencil and paper. These items are essentialfor labeling specimens and precisely recordingwhere your fossil was found.

■ A backpack. A backpack provides a place tofor tools, specimens, and maps.

■ Specimen bags. Use separate specimenbags for each locality.

Figure 5. Tools for fossil collecting. A: Bricklayer’s hammer. B: Chisel.C: Dental tool for removing delicate fossils. D: Newspapers for wrap-ping specimens. E: Backpack for storing tools and specimens.F: Specimen bag with label. G: Hand lens.


■ A magnifying glass or hand lens. Thedetails of many fossils cannot be seen with thenaked eye, and magnification is helpful foridentification.

Removing fossils from rock

Considerable care must be taken when remov-ing fossils from rock. Many fossils are fragileand cannot be removed without damage. It isgenerally best to take a large sample home andthere carefully and patiently work the specimenout. Many superb specimens have been de-stroyed by impatient collectors attempting toremove them quickly in the field. Chiseling ashallow trough around the fossil and then pop-ping it out usually works well (fig. 6). Largerocks can be trimmed to smaller sizes by hang-ing the excess area over the edge of anotherrock and striking the large rock firmly.

Recording information about fossils

Amateurs have found many important fossilspecimens. However, without detailed localityinformation, fossils are virtually useless to pale-ontologists. When collecting fossils, label speci-mens and record the following information:

■ Locality. Be sure to record the county,nearby roads, and distinguishing features ofyour collecting site.

■ Rock description. Include a description ofthe fossil-bearing rock type (that is, sandstone,shale, or limestone/dolomite), including color.

■ Layer in rock. It is useful to describe thespecific fossil location in relation to layering vis-ible in the rock. For example, you may noticethat rock in some outcrops is punctuated bythin, horizontal layers composed primarily offossil fragments. Describing the location of fos-sils relative to such layers allows you (and oth-ers) to find your exact collecting locality again.

■ Interesting features. It may be helpful toinclude a sketch or description of any other

Figure 6. Removing fossils from rock.A: Trimming a large rock to a smaller size.B: Close-up of chiseling out a fossil.

interesting features of the quarry or outcrop,such as unique layering patterns.

IDENTIFYING FOSSILS

Throughout time, humans have sought toname things in the world; in biology and pale-ontology, this naming process is called tax-onomy. Taxonomy involves naming groups oforganisms, and a taxon (plural, taxa) is a unit ofany rank in this nam-ing process. Group-ing organisms byshared characteristicsis known as systemat-ics. Organisms de-scribed within largegroups, such as king-doms, contain organ-isms with generalsimilarities; thosewithin smallergroups, such as fami-lies, have specificsimilarities.

The most preciseway to identify a fos-sil is by its two-partscientific name. Thefirst part of the nameis the genus (plural,genera); the second,the species. A genusincludes species thatare related to eachother; the membersof a species are usual-ly morphologically similar and are able to breedwith each other and produce fertile offspring.The scientific name is usually written in italics.For example, in the case of Calymene celebra,Wisconsin’s state fossil, Calymene is the genusand celebra is the species.

This complete hierarchy is shown in the ex-ample of trilobites in column two of table 1, in


which Calymene celebra is a member of a largergroup, the trilobites. Trilobites are themselvesmembers of the animal kingdom.

In this guide, we use scientific and com-mon names (such as lamp shells for brachio-pods) to describe the major fossil groups inWisconsin. We discuss how, where, and whenthese intriguing organisms lived and illustratethe common genera found within these groups.

Many other types of invertebrate fossilsalso lived, died, and were preserved in Wiscon-sin rock; therefore, further sources of informa-tion may be necessary to identify the fossils you

find. Two of the most complete sources are theTreatise on Invertebrate Paleontology, a series ofmore than forty volumes containing descrip-tions of all known invertebrate genera, andShimer and Schrock’s Index Fossils of NorthAmerica. These and other references are listedin the Sources of information section at the endof this guide.

Stromatolites

Approximately 3 billion years ago, long beforemulticellular organisms roamed the earth, lushmounds of algae began to thrive along warm,shallow shorelines (fig. 7). These algal matstrapped sediment, which built up layer by layerinto dome-shaped mounds (fig. 8). Suchmounds are called stromatolites. Although theyare not animals, stromatolites may have been afood source for some ofthe animal groups wedescribe. Stroma-tolites wereabundant inthe Precam-brian andCambrian, buttheir numbersdecreased inlater times. Afew stromato-lites are still forming today. Shark Bay, Austra-lia, is well known for its abundant living stro-

TAXON TRILOBITE HUMAN CAR

Kingdom Animalia Animalia machinePhylum Arthropoda Chordata transportationClass Trilobita Mammalia automobileOrder Phacopida Primatida gas-poweredFamily Calymenidae Hominidae sedanGenus Calymene Homo four doorSpecies celebra sapiens brand of automobileIndividual specimen number social security number registration number

Figure 7. Reconstruc-tion of stromatolitemounds in the CambrianPeriod (500 million yearsago) of Wisconsin.

Figure 8. A stromatolite, show-ing the characteristic layeredand domed structure.

Table 1. A classification of three specimens (after Easton, 1960).


matolites. In Wisconsin, stromatolites weremost common in Cambrian and Ordovicianseas, and therefore are most common inrock of those ages.

Corals

Corals are marine animals with simple bodystructures (fig. 9; plate 1). The mouth of acoral’s sac-like body is surrounded by a ring oftentacles. The living coral animal, the polyp, se-cretes a cup-like skeleton called the corallite.Many corallites cemented together make up theentire skeleton, or corallum. Inside the coral-lite, a radial divider, called a septum (plural,septa) grows vertically from the attachmentbase and helps support the soft tissues. Manycoral polyps contain algal cells, which use pho-tosynthesis to produce food for themselves andthe coral.

Corals can live together in large colonies,or reefs, which can be hundreds of miles across.Coral reefs are among the most complex eco-systems on Earth because many thousands ofspecies other than corals make the reef theirhome. Corals themselves require specific livingconditions, so fossil coral reefs tell us a greatdeal about the environmental conditions at thetime of reef formation. Living coral reefs areconfined to subtropical regions in shallow wa-ters that are warm and clear. Thus, Wisconsin’sSilurian reefs provide evidence that the statewas once covered by a warm, subtropical sea.Although ancient corals formed the reef itself,many other organisms flourished in the smallhabitats the reef provided. Brachiopods coveredthe reef structure, gastropods fed on the abun-dant algae and detritus, cephalopods hunted forprey, and crinoids swayed in the agitated wa-ters.

Two groups of reef-forming fossil coralsare found in Wisconsin: the tabulates and rug-osans (fig. 9). Tabulate corals grow upward,depositing horizontal plates known as tabulae.Tabulates formed mounds that appear similar

to honeycombs; the distinctive rugosan coralsresemble cow horns.

Tabulate coralsTabulate corals are the most abundant coralfossils in the Silurian rock of Wisconsin and areusually the largest reef corals. They form mas-sive colonies, about 0.3 meter (1 foot) wide orlarger. They can be identified by the presence oftabulae. Fieldstones, commonly found infencerows along the edges of farm fields in east-ern Wisconsin, in many cases contain well pre-served tabulate corals. Beach pebbles and gravelalong the Lake Michigan shoreline also aboundwith coral fragments.

Favositid tabulates: Honeycomb corals. The fa-vositid corals are quite common. They usuallyformed large colonies. The corallite is prismaticin shape, resembling honeycombs. Favositidshave mural pores, tiny holes in the wall of theskeleton, which connect different corallites.These pores are distributed in characteristicpatterns and numbers, which are useful for dis-tinguishing the various types of favositids.Favositids lived from the Ordovician to the Per-mian, at which time they became extinct. Theyare most abundant in middle Silurian to lowerDevonian rock. Favosites is the most commonfossil coral in Wisconsin.

Halysitid tabulates: Chain corals. Halysitidsresemble interlocking strings of delicate chains.Halysites is a common chain coral in Wisconsin,and it is used worldwide as an indicator of Sil-urian rock. Halysites is best seen in weatheredrock because the rock between the chains dis-solves, leaving the chains beautifully exposed.

Syringoporid tabulates: Tube corals. Syrin-goporids are easy to identify because of the

Figure 9. Coral anatomy.A: Tabulate coral. B: Rugose, or“horn coral.” C: Living coral animal.


Plate 1. Coral fossils. A: Top view of large, prismatic corallites of Strombodes, a common coral in Silurian rock ofeastern Wisconsin [8.5 cm]. B: Closeup of the chains of Halysites, also common in Silurian rock [7 cm]. C: Topview 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 generallength of the longest dimension. These measurements are in the metric system because that is the systemthat paleontologists use. One centimeter equals about 0.39 inch.


presence of distincttubes connecting thecorallites. They are quitedelicate, but in manyplaces they have beenpreserved in astonishingdetail. Syringopora is a com-mon syringoporid in Wisconsin; itlived from the Silurian to the Per-mian.

Rugosans: Horn coralsRugosans (plate 1) are cone-shaped corals thatresemble cow horns. The polyp lived in a spacein the center of the cone, known as the calice.Rugosan corals first appeared in the Ordovicianand are the second most common type of coralin Wisconsin. Because most horn corals appearto be similar, they can be difficult to identify.

Rugose corals can be colonial or solitary.Colonial rugosans formed bunches attached toone another. Solitary rugose corals commonlywere dislodged and then tipped over. If theysurvived, in many cases they grew upwardagain. Such rugosans have geniculations (con-tortions) caused by the change in growth direc-tion. Solitary and colonial rugosans are charac-terized by external growth bands, whichformed much like tree rings.

Rugose corals declined after the Silurianand eventually died out at the end of the Paleo-zoic Era.

Brachiopods: Lamp shells

Brachiopods (plates 2 and 3) are the mostabundant fossils in Wisconsin. Most people arenot familiar with living brachiopods becausemodern species inhabit extremely deep regionsof the world’s oceans, and their shells are rarelyfound on modern seashores. But during thePaleozoic, thousands of different species ofbrachiopods teemed in the near-shore anddeep-sea environments of Wisconsin. The num-ber of brachiopod species has decreased since

the extinction at the end ofthe Permian (about 245million years ago). Now,only about 250 living spe-cies of brachiopods exist;more than 30,000 fossil specieshave been identified in the fossilrecord.

Brachiopods have two valves (shells) thatare generally of unequal size and shape (fig.10A), but the right and left halves of each valvemirror each other. Near the tip of the bottomshell (the pedicle valve), a fleshy stalk (the pedi-cle) emerges through a hole (the pedicle open-ing) and attaches the animal to the sea bottom(fig. 10A, C). The other shell is known as thebrachial valve, which contains the brachidium.The brachidium supports the gill-like lopho-phore, which has many filaments and cilia(hairs) that create currents to bring microscop-ic food particles to the mouth (fig. 10B). Thelarge surface area of the filaments is used to ob-tain oxygen from water and eliminate carbondioxide. The two valves join at the hinge line,which is on the shell near the pedicle (fig.10A). Muscles hold the two valves together sothat the soft parts are protected. The two valvesmeet at the commissure.

Other shell features are useful for identify-ing brachiopods. A sulcus (a groove-like depres-sion) is present on many brachiopod shells, anda fold (a raised ridge) can be found on the op-

Figure 10. Brachiopod anatomy. A: Externalanatomy. B: Internal anatomy. C. Living brachio-pods attached to rocks by their pedicles andtwo animals living on soft sediments.


Plate 2. Brachiopod fossils. A, B, and C: Top, side, and back views of Pentamerus, an exceptionally common anddistinctive pentamerid brachiopod in Silurian rock of Wisconsin [4.5 cm]. D: Valcourea, a flat Ordovician orthidbrachiopod [2 cm]. E and F: Front and back views of Pionodema, an orthid brachiopod with a strong sulcus. It isfound 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 ofDoleroides, 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 thezig-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].


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 orthidbrachiopod 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 brachiopodStricklandinia [3 cm]. G: A cast of Orthis, a rounded orthid brachiopod found in Ordovician rock [2 cm]. H and I: Topand side views of Cyrtina, a small spiriferid brachiopod found in Silurian rock [1 cm]. J and K: Pedicle valve and brachialvalve 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 plateas 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 withfine 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 ofOepikina, a Middle Ordovician strophomenid brachiopod, showing strong ridges that diverge from the hinge [2 cm].


posite valve. Costae are elevated ribs on theshell. Growth lines are concentric rings repre-senting successive periods of growth.

Brachiopods are divided into two maingroups: the articulates and the inarticulates.Articulates have hinge structures on their shells;inarticulates do not. Because articulates greatlyoutnumber inarticulates in Wisconsin, we focuson the five orders of articulate brachiopodsfound in Wisconsin: Orthida (orthids), Stro-phomenida (strophomenids), Pentamerida(pentamerids), Rhynchonellida (rhynchonel-lids), and Spiriferida (spiriferids).

OrthidsWisconsin’s Ordovician rock holds manyorthids (fig. 11A). Several characteristics oforthids distinguish them from other brachio-pods. Both valves of orthid shells are convex,and one valve is larger and deeper than theother. Most orthids have costae. The hingelineof the orthid shells is normally straight andsomewhat long and is bordered by a large inter-area, a flat, shelf-like region. Orthids lived fromthe Cambrian to late Permian, and most Or-dovician rock contains diverse assemblages ofthese brachiopods.

StrophomenidsStrophomenids (fig. 11B) were especially abun-dant during the Ordovician. Strophomenidsusually have large, semicircular shells, with oneconcave and one convex valve. A short, flat in-

terarea borders the hinge region, and the hin-geline is straight, like that of the orthids, butmay contain a row of small teeth. Muscle scarsare conspicuous features of the internal shell.The strophomenids in Wisconsin, particularlythe genus Strophomena, commonly cover entirehorizons (layers) of rock. Strophomenids weredevastated along with other groups during thePermian extinction, but managed to survive alittle longer, into the Triassic, before becomingextinct.

PentameridsPentamerid shells (fig. 11C) are extremelyabundant in the Silurian rock of eastern Wis-consin. These brachiopods are large and egg-shaped, with curved hingelines and pronouncedshell beaks. They possess a unique internalstructure found near the hinge; it is called thespondylium, a raised, spoon-shaped platformused for muscle attachment. Pentamerids arosein the Cambrian, were most common in the Sil-urian, and became extinct in the Devonian.Some quarries in Dodge County contain Sil-urian rock with millions of large pentameridmolds and casts.

RhynchonellidsMost rhynchonellids (fig. 11D) in Wisconsinare small and marble-like in shape. The com-missure is zig-zag in outline. These small shellshave prominent ribs, and a fold and sulcus areusually present. Rhynchonellids arose in the Or-

Figure 11. Anatomy of repre-sentatives of the five commongroups of articulate brachio-pods in Wisconsin. A: Pio-nodema, an orthid (externalshell). B: Strophomena, a stro-phomenid (internal shell).C: Pentamerus, a pentamerid.D: Rhynchotrema, a rhyn-chonellid. E: Mucrospirifer, aspiriferid.


dovician. This group is still present in some ofthe Earth’s oceans.

SpiriferidsSpiriferids (fig. 11E) are among the most beau-tiful brachiopods. They are somewhat triangu-lar; many possess shell extensions that givethem the appearance of having wings. Thespiriferids have a unique spiraling support forthe lophophore as well as a fold and sulcus.This group arose in the late Ordovician, butcan be found mostly in Devonian rock in theMilwaukee metropolitan area. Two maingroups of spiriferids are found in Wisconsin:spiriferidinids and atrypids.

Spiriferidinids have straight hingelines andlateral wing-like shell extensions. Some mem-bers of this group, such as Eospirifer, are super-ficially similar to pentamerids. Platyrachella, be-cause of its interarea, looks suspiciously like anorthid. For the most part, however, triangular-shaped spiriferids are found in Wisconsin.

Atrypids are triangular, and they do nothave the wing-like shape of some other spirifer-ids. These spiriferids have curved hingelines andtwo convex valves. Most atrypids have ribs anda small beak. Atrypa is a common Devonianbrachiopod.

Bivalve mollusks: Clams

Bivalves (plate 4) are aquatic mollusks that pos-sess two valves (fig. 12) that protect the softbody parts. The valves are secreted by themantle, a soft tissue that leaves a scar (the pal-lial line) where it connected to the inside of theshell. The muscles that hold the two valves to-gether also leave scars on the inside of the shell,as do the siphons, through which food passes inand out of the body. The length and shape ofthe siphon scar, or pallial sinus, is helpful forinterpreting the burrowing habits of a bivalve.Different scars are also characteristic of differ-ent bivalve species.

Unlike the valves of brachiopods, clam

Figure 12. Bivalve molluskanatomy. Internal view ofthe right valve of a clam.

valves are typically of similarsize and shape. The two shellsare hinged together by astrong ligament. Small teethand sockets may also bepresent in the hinge area, which add strength tothe hinge and prevent the two valves from slip-ping apart.

Bivalves are filter feeders: They strain smallfood particles from water. Water enters throughthe inhalent siphon, passes through the gill,which takes in oxygen from the water, and exitsthrough the exhalent, the outcurrent siphon.The gill traps food particles and transportsthem to the mouth. Bivalves live in a variety ofhabitats: Some live epifaunally (on top of sea-floor sediment); others live infaunally (in sea-floor sediment), using their muscular foot toburrow within the bottom sediments. Theshape of the shell can be related to the environ-ment in which the animal lives.

Bivalves originated in the Cam-brian, but did not become abundantuntil the Ordovician. They are notparticularly common in Paleozoicrock of Wisconsin, but they some-times can be found within largegroupings of other Ordovician fossils.

Gastropod mollusks: Snails

Once confined to the oceans, gastro-pods (snails; plate 5) now live instreams, lakes, and even on land.Gastropods are the most abundantmollusks: There are approximately40,000 living species and 50,000 fos-sil species. Some modern species arevoracious predators equipped withpoison. However, the snails that livedin Wisconsin’s Paleozoic seas wereprobably peaceful grazers.

The shell of a gastropod is usually coiledand is used for protection, as it is for most mol-lusks (fig. 13A). Gastropods can retreat into

Figure 13. Gastropod molluskanatomy. A: Apertural view of gastro-pod shell. B: Anatomy of a living gas-tropod.


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 topviews 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 thesquare-shaped Cyrtodonta, a clam found in Ordovician and Silurian rock [4 cm]. J: Amphicoelia, a Devonian clam[3.5 cm].


Plate 5. Gastropod mollusk fossils. A: Helicotoma, an Ordovician gastropod with raised, angular sutures [2cm]. B: Liospira, an Ordovician gastropod with a flattened shell [1.5 cm]; C and D: Sinuites, a common MiddleOrdovician 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 gastropodpossessing suture-like ornamentation [1.5 cm]. K: Subulites, a tall, high-spired Ordovician and Silurian gastro-pod [5 cm].


their shells and close the openingwith a cover called the operculum.The shape of the shell can behighly varied because of the way itcoils. Each 360o revolution of theshell is called a whorl. The bodywhorl is the bottom and largestwhorl; it contains the aperture,the opening of the shell. The shellspire includes the whorls above thebody whorl. Whorls come in con-tact with one another at the shellsutures.

Gastropods have highly devel-oped sensory organs, includingtentacles and eyes (fig. 13B). The

head is attached to the foot, a muscular organused for creeping and feeding. Many gastro-pods have a radula, a rasp-like structure usedfor scraping algae and other food off the seaf-loor. Some carnivorous gastropods use the rad-ula for boring through the shells of other ani-mals.

Gastropods originated in the Cambrian,but few are found in rock of that age in Wis-consin; more are found in Silurian and Devo-nian rock. Our Ordovician rock contains abun-dant and beautiful gastropod fossils.

Cephalopod mollusks:Squid and octopus

Cephalopods (plate 6) are a group of swim-ming mollusks, including the living squid, oc-topus, and the chambered Nautilus. Althoughmost living cephalopods have somewhat re-

duced shells, fossil shells were well developed.Cephalopod shells have evolved into many as-tonishing and beautiful forms (fig. 14), exhibit-ing a variety of shapes, such as straight, slightlycurved, crescent, and coiled (fig. 15).

The shell of a cephalopod is normally tube-or cone-shaped with many dividers. These di-viders are called septa, and they partition the in-side of the shell into chambers. Each septumintersects the shell wall at a suture, which canbe seen as a pattern on the outside of the shell.Sutures are characteristic of cephalopod groupsand are therefore helpful for identification. Thebody chamber, which lacks sutures, is the areawhere the animal lived. A small tube called thesiphuncle runs the length of the shell, and pass-es through the septa. The siphuncle containsliquid that helps maintain the buoyancy of theanimal in the water.

Like their clam and snail relatives, cephalo-pods possess typical molluscan features: a shell,a muscular foot, and a mantle. They also havehighly developed sensory organs. The eye of acephalopod, for example, is similar to that of ahuman. Unlike the brachiopods and clams,cephalopods are mobile predators, and somecan swim at speeds of approximately 64 kilo-meters (40 miles) per hour by jetting waterfrom the mantle cavity through a fleshy funnel.

Cephalopods originated during the Cam-brian and are common as fossils in Ordovicianand Silurian rock in Wisconsin. Fossil cephalo-pods from Wisconsin can exceed 4 meters (13feet) in length. They are exciting to find be-cause of their large size, but they can be pre-served as molds and casts and can be difficultto identify.

Extinct arthropods: Trilobites

Trilobites (plates 7 and 8) were a group ofcrab-like animals with hard exoskeletons (outerskeletons) similar to those of modern insects.Trilobites have a three-lobed body (fig. 16B):Two grooves divide the body lengthwise into

Figure 15. Mor-phologies of fossilcephalopods.A: Straight.B: Crescent-shaped.C: Coiled.

Figure 14. Cephalopod molluskanatomy: coiled and straight cephalopodshells (after Boardman and others, 1987).


three sections. The middle section isthe axial lobe; the other two sectionsare the pleural lobes. The body can alsobe divided into three sections fromhead to tail: the front section is thehead, or cephalon, the middle section isthe thorax, and the tail section isknown as the pygidium. These marineanimals had a series of small, bilobate

Plate 6. Cephalopod mollusk fossils. A: Spyroceras, a common Ordovician and Silurian cephalopod with a straightand 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 ormore]. H: Enlarged siphuncle of Actinoceras, a cephalopod common in Ordovician rock and sometimes found inlower 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 cephalopodpossessing a large body chamber [4 cm].

Figure 16. Trilobiteanatomy. A: Bilobate ap-pendages of a living ar-thropod, similar to that oftrilobites. B: Exoskeletonof trilobite.


Plate 7. Trilobite fossils. A: Cephalon of Illaenus, a common trilobite in Ordovician rock. The cephalon is similarin shape to a strophomenid brachiopod [2.5 cm]. B: Complete specimen of the Ordovician trilobite Isotelus. Ithas 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 trilobiteConaspis. 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 trilobiteCeraurus, showing the characteristic tail spines and pronounced “ribs” of the pleural lobes [4 cm].

(two-pronged) legs beneath their exoskeleton(fig. 16A). One part of the leg, the exopodite,was used for walking; the other, the endopodite,was used for gas exchange or “breathing.”

On the cephalon, a series of lines, or facialsutures, are present. These sutures opened when

trilobites molted their skeletons. The trilobitepushed itself out of its old skeleton and grew anew one; most trilobite fossils are instars, ormolted skeletons. The region of the cephalonbetween the eyes and bounded by the sutures iscalled the glabella.


Plate 8. Trilobite fossils. A: Cephalon and pygidium of the Cambrian trilobite Wilburnia. The glabella is similarto 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 anarrower pygidium and lip [3 cm]. C: Cephalon of Ptychaspis, a Cambrian trilobite. It is similar to Cedaria, butmuch larger and with more widely spaced eyes [3 cm]. D: Dalmanites, a Silurian trilobite possessing bumps onthe cephalon [4.5 cm].

Crinoids: Sea lilies

Crinoids (plate 9A, B) are echinoderms, agroup that includes the starfish, sea ur-chins, and sand dollars. Sometimes calledsea lilies, crinoids resemble long-stemmedflowers, but they are marine animals. Aholdfast at the base of the animal’s stemfunctions like a root that holds the animalin place (fig. 17). The animal’s cuplikebody, or calyx, is composed of a mosaicof geometric plates. The calyx hasmany arms that open into a fan-like netso the crinoid can feed on microscopicfood particles. The flexible stem is

Trilobites had compound eyes, much likethose of insects. Some trilobites were adaptedto mud-burrowing, and possessed vestigial, ornonfunctional, eyes. Mud-burrowing trilobiteshad smooth, streamlined bodies.

Trilobites first appeared in the Cambrian,but after a diversification they dwindled andeventually became extinct during the Permian.Trilobite fossils are not common in Wisconsin.Generally, they form a large component ofCambrian fossil assemblages, and Ordovicianrock commonly contains trilobite fragments. InSilurian rock, trilobites may be associated withreef-like habitats. Devonian trilobites are notparticularly abundant. Fossil tracks of trilobitescan be found on rock that was once the Paleo-zoic seafloor. The trilobite Calymene celebra isWisconsin’s state fossil.

Figure 17. Crinoidanatomy.


composed of a series of button-like discs calledcolumnals. When crinoids die, their stems fallapart, or disarticulate, into columnals. The lu-men, a hole in the middle of the stem, con-tained a tube for carrying nutrients to the stemand holdfast. The mouth of the crinoid is onthe top of the calyx.

Crinoids first appeared in the Cambrianand diversified until the Permian extinction,when their numbers were greatly reduced.Complete crinoid fossils may occasionally befound in Wisconsin’s Silurian and Devonianrock, but most crinoid fossils consist of scat-tered columnals. In the Paleozoic, crinoidslived in colonies in shallow waters, buttoday they live in deeper regions of the world’soceans.

Bryozoans: Moss animals

Bryozoans (plate 9C, D) are small, mostly ma-rine animals that form skeletons of a variety ofshapes: tiny twigs, nets, domes, and mossycrusts (fig. 18). The zooecium (skeleton) houseshighly structured colonies of zooids, or cuplikeanimals. Bryozoans, like brachiopods, contain atentacle-bearing lophophore used in feeding

and gas exchange. They also contain a u-shapedgut with a distinct mouth and anus.

Bryozoans are inconspicuous fossils, butcan be seen most easily on slabs of Ordovicianlimestone or dolomite with the aid of a magni-fying glass or hand lens. Bryozoans are difficultto distinguish from one another. In most casesyou may not be able to identify your fossilmore specifically than as a bryozoan.

Bryozoans were able to live almost any-where: on a brachiopod shell, the side of acephalopod, coating sea plants, or in an indi-vidual colony on the sea floor. Many fossilshave patches of bryozoans on them. Bryozoansfirst evolved in the Ordovician and have persist-ed to the present day; they live in ocean habi-tats that are similar to those of Paleozoic seas.

Other fossils

Stromatoporoids: Laminated reef buildersStromatoporoids were colonial marine organ-isms related to sponges. They formed skeletonscomposed of thin layers called laminae. Smallbumps called mamelons were present on theirsurface (fig. 19). Stromatoporoids formed reefsin the shallow Silurian seas of Wisconsin. Theybecame extinct 65 million years ago.

Ostracods: Bivalved arthropodsOstracods (plate 9E) are small, two-valvedarthropods that resemble small, dark, polished

Figure 19. Stromatoporoid anatomy.

Figure 20. Hyolithid anatomy.

Figure 18. Bryozoananatomy. A: Skeleton of abryozoan colony. B: Azooid living in the skel-eton.


Plate 9. Miscellaneous fossils. A: Crinoid columnals, or plates, which make up the stems [0.5 cm]. B: Crinoidcolumn section, or stem [3.5 cm]. C and D: Fragments of bryozoan colonies found in Ordovician, Silurian, andDevonian rock [3 cm]. E: Eoleperditia, an Ordovician ostracod. [1 cm]. F and G: Two views of the Ordovicianhyolithid Hyolithes [3 cm]. H: An Ordovician colony of Receptaculites, believed to be a skeleton-secreting algae[10 cm].

beans. Ostracods first appeared in the Cambrianand still exist today in freshwater, marine, andterrestrial environments. Like other arthropods,ostracods molt their skeletons as they grow. Al-though they are not very common, ostracodsare occasionally found in Wisconsin’s Ordovi-cian and Silurian rock.

Hyolithids: Animals of unknown affinitiesVery little is known about these mysterious ani-mals. Hyolithids (plate 9F, G) are an unusual,extinct group of marine animals that had aconical shell with an operculum (fig. 20). Theyare sometimes found in the digestive tracts offossil marine worms. Middle Ordovician rock inWisconsin commonly contains hyolithids.

Receptaculitids: Skeleton-secreting algaeReceptaculitids (plate 9H) are another extinctgroup of marine organisms that scientists have

had difficulty classifying. Receptaculitids havebeen classified as corals and sponges, but theyare now thought to be colonial algae. They su-perficially resemble favositids, the honeycombcorals. Receptaculitid fossils are most commonin Ordovician rock.

Trace fossils: Trails, borings, and burrowsTrace fossils (plate 10) are the preserved pathsof animals that crawled on and bored or bur-rowed into the seafloor. A variety of paths rep-resenting behaviors—such as feeding, moving,and resting—can be found in sedimentary rock.Certain burrowing behaviors are specific to cer-tain environments, so paleontologists can re-construct ancient environments from the shapeof trace fossils. Trace fossils are common in thePaleozoic rock of Wisconsin and are still beingformed in ocean sediments today.


SOURCES OF INFORMATION

Societies

Wisconsin abounds in groups that are devotedto all aspects of the earth sciences, includingpaleontology. One way to find out if there is aclub near you is to check with the MidwestFederation of Mineralogical and Geological So-cieties, a regional member of the AmericanFederation of Mineralogical Societies. TheMidwest Federation maintains a directory of itsmember societies. (Note that not all of thesesocieties focus on paleontology.) The Internetaddress for the Midwest Federation at the timeof the printing of this publication is<www.amfed.org/mwf/>.

One of the largest groups in Wisconsin isthe Wisconsin Geological Society. It is a non-profit organization, located in Milwaukee, thatconducts study groups, meetings, and field tripson geology and paleontology. Its monthly pub-lication, The Trilobite, contains articles of localinterest on rocks, minerals, and fossils. Thisgroup is an excellent place to learn more aboutfossils and geology and meet others interestedin Earth history.

Books

You may be interested in learning about fossilsin more detail. Many books have been writtenfor the interested layperson; check your local

public library for the various titles available.The following books provide detailed technicalinformation about fossils and the principles ofpaleontology. They are usually available at uni-versity libraries.

Boardman, R.S., Cheetham, A.H., and Rowell,A.J., 1987, Fossil Invertebrates: BlackwellScientific Publications, Palo Alto, 713 p.

A detailed, college-level textbook intendedfor paleontology students.

Chamberlin, T.C., 1882, Geology of Wisconsin:Survey of 1873–1879, Volume IV, 779 p.

A classic work on the geology andpaleontology of Wisconsin.

Easton, W.H., 1960, Invertebrate Paleontology:Harper and Row, New York, 701 p.

A detailed and extensively illustratedcollege-level textbook.

Moore, R.C. (director and editor), 1953—,Treatise on Invertebrate Paleontology: TheUniversity of Kansas Press/The GeologicalSociety of America.

A multi-volume, professional-level treatisecontaining every described fossil genus.

Moore, R.C., Lalicker, C.G., and Fischer, A.G.,1952, Invertebrate Fossils: McGraw–HillBook Company, Inc., New York, 766 p.

A general text on invertebrate fossils.

Plate 10. Trace fossils. A: Large, worm-like feeding traces parallel to rock layers [diameter can be as much as 1cm]. B: Large, variably shaped borings perpendicular to rock layers [diameter up to 1 cm or more; length up to 4cm or more]. C: Long, thin burrows perpendicular to rock layers [diameter less than 0.5 cm; length up to 5 cmor more].


Rhodes, F.H.T., Zim, H.S., and Shaffer, P.R.,1962, Fossils: Western Publishing Compa-ny, Inc., Racine, 160 p.

An inexpensive, superbly written, andextensively illustrated book.

Schrock, R.R., and Twenhofel, W.H., 1923,Principles of Invertebrate Paleontology:McGraw-Hill Book Company, Inc., 816 p.

A classic text written by Wisconsinpaleontologists.

Shimer, H.H., and Schrock, R.R., 1945, IndexFossils of North America: John Wiley &Sons, New York, 837 p.

A book that will help you identify fossils notillustrated in this guide.

ACKNOWLEDGMENTS

We thank those who reviewed earlier drafts ofthis guide, especially Maureen Muldoon, M.G.Mudrey, Jr., and the late Juergen Reinhardt, ofthe Wisconsin Geological and Natural HistorySurvey, who provided many useful comments.We also thank Susan Lawton Hunt and RonaldHennings of the Wisconsin Geological and

Natural History Survey, for assistance and en-couragement throughout this project. PaulaSumpter of the Milwaukee Public Museum andKlaus Westphal of the University of Wisconsin–Madison assisted with museum collections. Wethank David Clark and Dana Geary of the Uni-versity of Wisconsin–Madison for sharing theirknowledge of fossils and paleontology with us.RHN thanks Wayne Schaefer, of the Universityof Wisconsin Center–West Bend, Lloyd andEllen Brown, Dan and Ellen Dettwiler, and hisparents for support during the early periods ofthis project.

ABOUT THE AUTHORS

Ross H. Nehm and Bryan E. Bemis grew up inWisconsin and collected fossils before pursuingundergraduate degrees at the University of Wis-consin–Madison. Both received their doctoratesfrom the University of California—Nehm atBerkeley, and Bemis at Davis. Nehm is now anassistant professor at The City College, CityUniversity of New York, in New York City. Be-mis is a research associate at the U.S. Geologi-cal Survey, Menlo Park, California.

Geologic time scale. Shaded area indicates time periods for which there is only sparseevidence in Wisconsin.

Reconstruction of Silurian reef. (Photographof diorama © Milwaukee Public Museum.)

WISCONSIN GEOLOGICAL AND NATURAL HISTORY SURVEY

James M. Robertson, Director and State Geologist

John W. Attig, geologistWilliam G. Batten, geologistKenneth R. Bradbury, hydrogeologistBill C. Bristoll, information managerBruce A. Brown, geologistLee Clayton, geologistMichael L. Czechanski, cartographerDonna M. Duffey, Map Sales associateTimothy T. Eaton, hydrogeologistThomas J. Evans, geologistStephen J. Gaffield, hydrogeologistMadeline B. Gotkowitz, hydrogeologistDavid J. Hart, hydrogeologistRonald G. Hennings, hydrogeologist (emeritus)Rilla M. Hinkes, office managerThomas S. Hooyer, geologistSusan L. Hunt, graphic artist

Mindy C. James, publications managerMarcia J. Jesperson, Map Sales associateKathy A. Kane, network administratorIrene D. Lippelt, water resources specialistFrederick W. Madison, soil scientistM.G. Mudrey, Jr., geologistStanley A. Nichols, biologistDeborah L. Patterson, cartographerRoger M. Peters, geologistKathy Campbell Roushar, cartographerApichart Santipiromkul, information processing

specialistCurtis L. Thomas, geotechnicianVirginia L. Trapino, program assistantAlexander Zaporozec, hydrogeologist (emeritus)Kurt K. Zeiler, GIS specialistKathie M. Zwettler, administrative manager

Gregory J. Allord, USGSMary P. Anderson, UW–MadisonLarry K. Binning, UW–MadisonMichael F. Bohn, Wis. Dept. of Nat. Res.Stephen M. Born, UW–MadisonPhilip E. Brown, UW–MadisonCharles W. Byers, UW–MadisonWilliam F. Cannon, USGSKevin Connors, Dane Co. Land Conserv. Dept.C. Patrick Ervin, Northern Ill. Univ.Daniel T. Feinstein, USGSRobert F. Gurda, Wis. State Cartographer's OfficeNelson R. Ham, St. Norbert Coll.Mark T. Harris, UW–MilwaukeeKaren G. Havholm, UW–Eau ClaireRandy J. Hunt, USGSMark D. Johnson, Gustavus Adolphus Coll.John Klasner, Western Ill. Univ.Joanne Kluessendorf, Weis Earth Science MuseumJames C. Knox, UW–Madison

The Wisconsin Geological and Natural History Survey also maintains collaborativerelationships with a number of local, state, regional, and federal agencies and organizationsregarding educational outreach and a broad range of natural resource issues.

George J. Kraft, Central Wis. Groundwater CenterJames T. Krohelski, USGSGene L. LaBerge, UW–OshkoshEiliv Larsen, Geological Survey of NorwayJ. Brian Mahoney, UW–Eau ClaireKevin McSweeney, UW–MadisonChristine Mechenich, Central Wis. Groundwater CenterDavid M. Mickelson, UW–MadisonDonald G. Mikulic, Ill. State Geol. SurveyWilliam N. Mode, UW–OshkoshMaureen A. Muldoon, UW–OshkoshJames O. Peterson, UW–MadisonJeffrey K. Postle, Wis. Dept. Ag., Trade & Consumer ProtectionKenneth W. Potter, UW–MadisonTodd W. Rayne, Hamilton Coll.Allan F. Schneider, UW–Parkside (emeritus)J. Antonio Simo, UW–MadisonKent M. Syverson, UW–Eau ClaireJeffrey A. Wyman, UW–Madison

plus approximately 10 graduate and undergraduate student workers.

RESEARCH ASSOCIATES

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