First Families of Chicago:

Local life about 400 million years ago
and the fossils they left behind

Revised and Copyright © 2006 Ellin Beltz

Presented at "Make Tracks with Paleontology" -- January 15, 2000
sponsored by Project Exploration

A brief overview of local geology

The top of the oldest rock below Chicago lies about 2,000 feet below the city's pavements. It is a Precambrian red granite, an igneous rock that formed billions of years ago. We assume that there must have been some rock above it, because today granites are forming thousands of feet below modern volcanos. But we don't know what was above it because its top surface shows that it was exposed as a landscape. A gap in the rock record is called an "unconformity;" this one represents millions to hundreds of millions of years of erosion.

IL Bedrock
Geologists draw figures called "columns" to show the relationship of various rocks. Geologists also prepare maps of what the rocks would look like at the surface, if all the surface material was removed.

Click on the small map of Illinois (from the Illinois State Geological Survey) to go to a large version.

When you color in your bedrock map of Illinois, use a different color for each age of rock and color in the key and your geological column to match.

IL Column
Click on the picture of the Illinois Stratigraphic Column to go to a larger version which has been annotated to correspond with localities mentioned in this text.

Geological columns are only approximations of reality. Some attempt to show all the possible rocks in a given area; others try to show just the formations which remain. This diagram is of the latter type. No full geologic column is ever exposed to view. Parts may be filled in by studying rocks brought up during excavation or drilling.

The vast majority of geologic time is Precambrian. During the Precambrian, what would become Illinois was part of an island arc. Perhaps that ancient island arc formed the same way they are today - by magma erupting upward from the subduction zone of two colliding oceanic plates. Deep below the volcanic upper surface, the pink granitic bedrock of Illinois and Chicago crystallized. Formally, geologists now assign the Chicago region to the eastern granite/rhyolite province of North America. Granites form deep within the Earth and are slowly uplifted to the surface during collisions. Most granites are jointed by the stresses of being near the surface; perhaps our regional jointing dates to this early time. Finally everything on top of this granite and the surface of the granite itself were eroded.

During this time too, the Baraboo, Wisconsin Syncline formed, providing more evidence of an ancient tectonic collision in the Midwest.

On top of the eroded upper surface of Chicago region Precambrian basement rocks are rocks of Cambrian age. The Mount Simon Sandstone provided well water for the city stockyards and many surrounding communities for over a century before they were depleted. The only outcrops of Cambrian Period rocks near Chicago are in Ogle County, near Dixon, IL. They are on the upthrown side of the Sandwich Fault and can reach 30-40 feet of outcrop.

Cambrian sandstones received their pure quartz sands from the oldest areas of North America, the Canadian Shield and the Laurentian Mountains. During the Cambrian, the Chicago area was alternately a beach and near-shore marine system. Not very many fossils are preserved in these environments because sand is so abrasive.

Over the Cambrian rocks lie the rocks of the Ordovician Period. These are most easily observed in LaSalle County at Starved Rock State Park, Matthiessen State Park, and along the Illinois and Michigan (I and M) Canal at Split Rock. The most famous formation of this Period is the St. Peter Sandstone which forms the bluffs along the Illinois River at Starved Rock. The contact between the topmost Ordovician rock, the Neda oolite, and the bottom-most Silurian rock is exposed near the campground in Kankakee State Park.

Northeastern IL Bedrock

Click on the map to go to a larger image which shows bedrock and county boundaries.

Chicago is built upon a vast Silurian Reef.
Chicago was an ancient reef about 400 million years ago
These pictures are of a diorama at the Milwaukee Public Museum of plants and animals which lived on and formed Silurian-age reefs. Fossil crinoids, brachiopods, trilobites, ammonoids, gastropods and corals are found in Thornton Quarry rock. Modern reef environments are found in warm, shallow oceans. The water is usually clear of silt or even wind-blown sediments. Many of the reef inhabitants are filter feeders and sediment prevents feeding.

The area which we call the Chicago Region is itself part of a larger geologic region, regionally known as the Niagaran Escarpment, marked by outcrops of Silurian dolomites and limestones. Niagara Falls cuts through this rock which is exposed in a giant ring around the middle three Great Lakes and centered on the state of Michigan.

Silurian rocks are are exposed
  • along the Kennedy Expressway at Addison (southbound lanes)
  • in the Thornton Quarry at Homewood
  • in the Sag Bridge abandoned quarries (Cook County Forest Preserves)
  • along Rock Creek in the Kankakee River State Park
  • along the Illinois River, and
  • near the towns of Lemont, Romeoville and Joliet.

Blocks of Silurian limestones and dolomites were used in construction of the old Chicago Water Tower, the Chicago Tribune Building, the Michigan Avenue Bridge and as parts of Chicago's lake front revetments and inner city Metra overpasses.

The Ordovician and Silurian rocks record a long period of time during which the Chicago area was covered by a warm shallow ocean. During the Silurian, great reefs grew around the present-day state of Michigan. The restricted sea within the reefs occasionally dried up - resulting in vast salt deposits under Michigan - and some interesting differences in the fossil fauna of Illinois, Wisconsin, Indiana and Michigan.

Under the City of Chicago, the rock record pauses in another great unconformity. Erosion has removed all of the Devonian, Mississippian, Pennsylvanian, Permian -- as well as the entire dinosaur era and mammal era except for Pleistocene glacial till. Whether the Chicago region had or did not have these layers is speculative. However, nearby areas may provide some clues.

  • In the area around Starved Rock, Pennsylvanian rocks are exposed in Buffalo Rock State Park, at the boat launch ramp in Lowell, and along the Vermillion River downstream from Lowell. The former tourist attraction "Bailey Falls," a small waterfall leading into the Vermillion River is no longer a good field trip spot due to quarrying operations. Truly hardy field trippers walk the 1.5 mile Illinois and Michigan Canal Trail from Utica to Split Rock; the less adventurous can see local rocks as building stones in the town of Utica at the Illinois and Michigan (I and M) Canal Museum. Silicified Pennsylvanian fossils can be collected along road cuts in the area to the northwest of Mattheissen State Park.

  • Mazon Creek concretions are Pennsylvanian and may be collected with a permit from paths through strip coal mine debris piles around the Braidwood nuclear power plant cooling ponds.

  • Some Pennsylvanian rocks have also been found in drill cores taken from the "Des Plaines Disturbance," the very bottom of an ancient extraterrestrial object impact crater. Once described as "cryptovolcanic structures," many of these ancient impact craters have been described from around the world. More recent work on the high angle thrust faults and other features of cryptovolcanic structures revealed that they had been damaged from the top down, rather than the bottom up. As the Des Plaines Disturbance is buried under glacial till and was not intersected by excavations for Deep Tunnel, little more is known about this interesting but hidden feature of Chicago region geology.

Fossils below Chicago

Much of what we know about ancient life under the Chicago area comes from the fossil record. There are few to no fossils from the Precambrian Era and Cambrian Period in our area. And even in areas which are fossiliferous, only a few of the animals and plants which lived there in the past have been preserved and observed in the present. Some ecosystems are better represented in the fossil record than others and some plants and animals preserve better than others. So our fossil record shows us a fairly good picture of animals with hard parts (shells, spicules and segments) that lived in the warm shallow oceans of the Paleozoic Era.

Throughout geological time, certain animals only lived at particular times and no others. These are called "index fossils." Others occur in many periods in the fossil record. Some may stay the same for a long time ("conservative species") while others change features of their anatomy frequently like the trilobites.

The Illinois State Geological Society published illustrations of

Ordovician Fossils
Typical Ordovician fossils
Fossil animals are classified by the same system as living animals:

Remember the silly phrase "King Philip cries out for great soup!"
Silurian Fossils
Typical Silurian fossils.

A lot of plants and animals simply do not fossilize at all; some are found only rarely. Many groups of plants and animals that used to exist have gone extinct. Some have evolved and gone extinct and never been found yet in the fossil record. Some do not fit current classification schemes because we cannot figure out to which other group they belong. Much remains to be learned about the past; it is a fascinating study for people of all ages. Fossils found in the Chicago region include members of the:

Click on each group - not here! Protozoa: Algae, bacteria, diatoms, radiolarians
Porifera: Sponges
Cnidaria: Corals, Sea Anemones & Jellyfish
Lophophorates: Brachiopods and Bryozoa
Echinodermata: Starfish, crinoids, sea urchins
Mollusca: Productids, gastropods, ammonoids and nautiloids
Arthropoda: Trilobites, crustaceans, insects and arachnids.
Problematica: Tullimonstrum gregarium

All these fossils lived together in ecosystems and the ecosystems changed over time. Sometimes the Chicago area was exposed to erosion, quickly destroying the latest deposited soft sediments - just the places most likely to preserve fossils! So the fossil record is incomplete. It does not record well any animals which did not live in shallow marine or marginal marine environments and it usually only preserves those with hard shells, spines or body parts.

Fossils are preserved as molds or as casts which may be the whole body as in life, parts of the body, the body deformed, or a cast of a different material lying within a mold. Soft-bodied preservation is globally more rare than hard bodied preservation; Illinois has a widespread layer called the "Francis Creek Shale" which bears concretions which may preserve Coal Age plants and animals.

Illinois famous Mazon Creek Concretions are formed of siderite, a calcium carbonate mineral. Several times during the Coal Age animals and plants were drowned suddenly and instantly covered with mud. The negative charge of the clay minerals bonded with the still positive charge on the organic debris and the process of conversion to siderite and other minerals proceeded over vast amounts of time. Calcite in some concretions fluoresces under ultraviolet light and reveals even more detail. Metatamorphosed concretions come from the banks of the Vermillion River north of Lowell. During deformation at the end of the Paleozoic, the Francis Creek shale (and many other rocks) were bent into the structure known as the La Salle Anticline.

Protozoa -- algae, bacteria, fusulinids, diatoms, radiolarians

Protozoa are usually not found "by eye," instead are found after the rock has been prepared and observed with a microscope.

Precambrian Banded Iron Formations of Upper Michigan and Wisconsin's varved quartzites record an early environment dominated by organisms which produced layered structures called "stromatolites." Early metamorphism converted the layers of sediment (winter runoff) and hematite (organic remains). Some finely varved specimens have been suggested as confirmation for glacial conditions during this time.

Protozoa, of course, remain today. In fact, microscopic creatures far outnumber any of the higher orders everywhere on Earth. They just don't fossilize well. Recent work in high acid mine environments suggests that some "odd" metal deposits may have been concentrated by Protozoa. The Keewenaw, Michigan aberrant silver/copper intercrystals may have formed as the result of bacterial concentration of metals.

Foraminifers and radiolarians are single-celled floating oceanic plankton which occur in today's oceans in great blooms when the proper nutrients are available. Their dead bodies drift to the bottom and are preserved as chalks and cherts, respectively. Other modern plankton, diatoms and coccothithophores, enter the fossil record after Chicago region rocks were laid down. Index fossils called "Fusulinids" are a type of foraminifer.

Receptaculites, a spectacular sun-flower shaped inclusion in Paleozoic rocks was long assigned to sponges, but is now considered a member of the fossil algae. Large, round masses of Receptaculites are found in rocks near Lowell in LaSalle County.

Porifera -- Sponges

Sponges are familiar animals; we use natural, modern sponges in the bath and for arty painting projects. They are an ancient group of filter feeders. They draw water and nutrients in, move the water through their internal canals and excrete waste and waste water. They build Y-shaped "spicules" of calcium carbonate or silica to help stiffen their bodies. These spicules preserve very well and are very diagnostic of particular genera of sponges. Sponge spicules are prominent on the Michigan Avenue Bridge in Chicago. Astraeospongia and Astylospongia are typical Silurian sponges.

The classes of sponges are divided on spicule shape, the symmetry of spicule arrangement and mineralogy of their spicules. Sponges were one of the most common reef-building organisms in the Paleozoic.

An ancient member of the reef-builders, the Archaeocyathans, may be related to the sponges. They are usually associated with algal fossils and seem to have been more resistent to silty water than other sponges. They disappeared entirely during the Cambrian, although their remains might be found along the Sandwich Fault in northwestern Illinois.

Cnidaria -- Corals, Sea Anemones & Jellyfish

The two major groups of early Paleozoic corals, tabulate corals and rugose corals, appear in the fossil record in the Ordovician. Streptelasma is a typical Ordovician coral. Tabulate corals tend to grow in colonies and include the well-known Silurian chain corals (Halysites) and petrified honey-comb corals (Favosites). Rugose corals lived in colonies which shared a single outer covering, they are sometimes called "horn corals" because they are shaped like the horn on a trumpet or french horn. All corals and anemones belong to the Class Anthozoa, the "flower animals" because while they have hard outer coverings, inside each chamber is a fragile filter feeding animal, known as a polyp. These polyps can retract fully within the outer chamber as a defense against grazing.

The closely related sea fans, sea whips, sea pens, and soft corals are rarely found as fossils although as ever closer attention is paid to microfossils, their parts and pieces will probably be found.

Jellyfish are known from Mazon Creek concretions (often named "blobs" by collectors) and from a Mississippian outcrop in Brown County. They are not common elsewhere in North America.

Lophophorates -- Brachiopods and Bryozoa

Ancient and modern bryozoa build colonial shells of calcium carbonate and other materials. The individual animals are rarely preserved, but their feathery skeletons are commonly found in Paleozoic rocks in Illinois. Bryozoa can be seen in the Chicago Tribune building and on the Michigan Avenue Bridge. They changed their body styles fairly regularly; Ordovician bryozoa are branching, Silurian bryozoa are lacy and later produced an unusual spiral form known as "Archimedes corkscrew" after the ancient Greek philosopher's water lifting invention.

Bryozoa are common in all modern oceans, from tropics to the arctic regions. They eat plankton, mostly diatoms and radiolarians. They prefer clear water as their tiny feeding filters are easily clogged by silt.

Brachiopods are an ancient and common group of filter-feeding, shelled animals has members are alive today that we can study to better understand the ecology and behavior of ancient brachiopods. While only the hard shells are usually preserved, some brachiopod fossils reveal internal anatomy and show us that most had long, fleshy anchors which they used to fasten themselves to rocks or other fixed objects.

There are two classes of brachiopods: the articulate brachiopods and the inarticulate brachiopods. Articulate brachiopods have two shells with a well-developed hinge and muscle system as well as a fleshy anchor or "pedicle." The top and bottom shells may be quite different. Each half of the top shell is identical to the other half of the top shell; while each half of the bottom shell mirrors its other half. The other class, the inarticulate brachiopods, have only one shell and pedicle. This class is represented now by Lingula, a mud-dwelling genus which is relatively unchanged from the Ordovician Period to the present day. All of the bivalve fauna on the typical fossil illustrations are brachiopods; the more familiar mollusks developed later and are not represented in Paleozoic Illinois fossil fauna.

Echinodermata -- Starfish, crinoids, blastoids

Modern echinoderms include the mobile predators known as starfish, sea urchins and sand dollars as well as sea cucumbers and crinoids which are filter feeders. In the Paleozoic, crinoids and three extinct classes (Cystoids, Edrioasteroids and Blastoids) were common members of the near shore and reef habitats. The hard skins of Echinoderms and their body segments fossilize particularly well and they are divided into about 20 classes.

Modern and ancient Echinoderms show radial symmetry, a hydraulic vasular system and tube feet. They have no eyes or respiratory, circulatory or excretory systems. They are a sister group to chordates (including vertebrates) because their developing cells are not locked into a predetermined pathway, but can change into other cells and other organs. Some Echinoderms can regrow lost arms; an ability which is being studied for human limb regeneration. Most Echinoderms display a pentameric (five pointed) symmetry.

Paleozoic Echinoderms found in midcontinent rocks include crinoids, blastoids, cystoids, carpoids, brittle stars, sea stars ("starfish"), and sea cucumbers.

Crinoids are especially common in Illinois rocks. They are usually preserved as pieces of the stem and the feeding arms; rarely is a whole animal found intact. The stem pieces look like shirt buttons, but they only have one hole -- in the center. Why we find mostly disarticulated stem pieces may be explained by the behavior of a modern crinoid; when threatened, it frees the topmost, flower-like part of its body, leaving the stem and holdfast to die. The top part of the crinoid can float freely and reproduce. It is unknown if Paleozoic crinoids could separate, but we do know one way that they moved around was to attach themselves upside down to floating debris. Typical Silurian echinoderms include Caryocrinites, Eucalyptocrinites and Pisocrinus.

Paleozoic Echinoids are rare. Echinoids, including the sand dollar, sea biscuits, sea urchins, heart urchins and others with spines are best preserved in Mesozoic and Cenozoic environments. Echinoid body plates are held together by loose sutures and tend to separate immediately postmortem.


Four classes of mollusks were relatively common members of Paleozoic ecosystems: Productids, Gastropods, Cephalopods and Nautiloids.
  • Productid molluscs (also called "Bivalvia" or "Pelecypoda") are represented today by clams, oysters and scallops. They all have two shells, each a mirror image of the other, joined by a strong muscle along a hinge. Productids are filter-feeders and often live in communal beds in warm, shallow marine environments. There are also fresh water forms alive now.

  • Gastropods are slugs, land and marine snails, sea hares and nudibranchs. The most familiar probably are the cowry, oyster, the conches and all the showy curled shells so prized by collectors. In reality, the shells provide a safe haven for a soft, slug-like animal which can retreat and slap shut a door called the "operculum" over its body cavity. Gastropods are often found in Paleozoic rocks in Illinois; their protective opercula rarely fossilize. Hormotoma and other Ordovician gastropods are often preserved only as interior casts. They can be tiny, grazing amidst crinoid debris when buried and petrified.

  • Cephalopods include extinct ammonoids, and their relatives, the living squids, octopi and nautiloids. Modern cephalopods are intelligent, fast-swimming or jet-propelled multi-tentacled ocean predators. Their ancestors probably dominated the marine environments of their times.

    Modern cephalopods have lost or modified the ancestral exterior shell. Commonly seen in shops, the "cuttlebones" produced for captive birds are the inner shell of the cuttlefish, a modern cephalopod.

    Cephalopods with exterior shells form complicated but diagnostic sutures between their growth chambers and grow either
    1. straight-shelled -- orthoconic
    2. slightly curved -- crytoconic; or
    3. involute -- where each coil covers the previous and makes a pattern which progresses according to a Fibonacci spiral. Unlike gastropods, ammonoid shells do not coil up and out from a central point; rather they are symmetric around a midline.

    Ammonoids were free swimming carnivores of the ancient seas. The animal within the shell was similar to modern squid or octopus. They had tentacles and a well-developed eye and may have traveled by jet propulsion.

    As a group, these highly successful, diverse and apparently intelligent animals seem to have gone extinct about 65 million years ago.

    Straight ammonoid molds up to a foot long are found around the Kankakee River State Park. The casts are usually shorter sections and they are less commonly found than the molds. Occasionally the silicified casts can be found along streams that feed the Kankakee.

  • Nautiloids are limited to one genus today, the chambered nautilus which has a squid-like animal within a tightly coiled shell. Their ancestors were members of the Paleozoic fauna from the Cambrian onward, although they are not common in the Illinois fossil record. Nautiloids are involute and planisymmetric (the same around a midline).


Arthropods are divided into four classes: Crustaceans, Insects, Chelicerates and Trilobites.
  • Crustaceans include crabs, crayfish, lobsters, shrimp, barnacles, ostracodes and other groups. Their fossil relatives go back as far as the Ordovician, and perhaps further.
  • Insects include flies, beetles, butterflies, mosquitoes, centipedes, millipedes and many others.
  • Chelicerates include spiders, ticks, mites, scorpions and horseshoe crabs. One type of extinct chelicerate found in the Midwestern U.S. are called Eurypterids. They were most common in waters with high salinity. They were apparently ferocious predators, perhaps feeding on newly molted members of their own Phylum.
  • Trilobites are extinct; their nearest living relative is the horseshoe crab. Some have suggested that trilobites lived buried in sediment, while others propose that they were free-swimmers. Like many modern groups, Trilobites probably fulfilled a lot of roles in various niches. They were very variable and because they were continually changing their body styles, they are very helpful in dating Paleozoic rocks.
By the Ordovician, trilobites were flexible enough to roll up tightly for defense. The antennae would be inside and only the eyes and outer shell were exposed. Trilobites shed their outer shells when they became too tight. They were probably vulnerable during and immediately after the moulting. Fossilized sheds have been tentatively identified.

Finding your first trilobite is usually a very exciting experience. Then you wonder how many "crinoids" and "horn corals" you've identified that were actually one or two lobes of this three-lobed animal. There is a trilobite mold on the front of the Tribune Tower building about 3-4 feet above the sidewalk and other bits and pieces of trilobites in the Michigan Avenue Bridge, in Joliet and Lemont building stones such as those used on the old Water Tower and Pumping Station and many other places.

Problematica: Tullimonstrum gregarium

Some early Paleozoic fossils, including some members of the Canadian "Burgess Shale Fauna" and the Illinois State Fossil Tullimonstrum gregarium (Pennsylvanian, Mazon Creek) do not fit in any current classification system.

Where to see fossils around Chicago

  • Earth and Life Through Time Exhibit, Field Museum of Natural History, Roosevelt Road and Lake Shore Drive, Chicago
  • Illinois and Michigan Canal Museum, Utica, Illinois
  • Illinois River Lock and Dam Visitor Center, east of Utica, Illinois
  • Lizzadro Museum of Lapidary Art, 220 Cottage Hill, Wilder Park, Elmhurst, IL 630-833-1616. Call for schedule of fossil programs suitable for each age group.
Quarried stone
  • Take my Field Trip -- Geology along Michigan Avenue for a building by building review.
  • The Chicago Tribune Building and the Michigan Avenue Bridge just south of it in downtown
  • On the Intercontinental Hotel, just north of the Tribune Building
  • On various buildings in Joliet and Lemont
  • In many building stones all over the Chicago area, along some Chicago lake front revetments, and many inner city Metra overpass walls.
In the wild
No collecting is permitted on public lands in Illinois. Photographic collecting is always permissible and is highly encouraged to permit other fossil hunters to share your joy of discovery. Entering private lands always requires the permission of the owner. Some of my favorite fossils have come from gravel, including gravel along the edges of parking lots. Keep your eyes open!

You can look for fossils on public lands:
  • Along Rock Creek especially under the road bridge is a massive bryozoan limestone. Trilobites and ammonoids can be found in the river cobbles; also the Ordovician/Silurian contact ("Neda Oolite") is exposed along river near campground. Click here to see my field trips around Kankakee.

  • In some layers in Starved Rock State Park, south of Utica, Illinois and Matthiessen State Park, south of Starved Rock and in road cuts and quarries surrounding both parks. Paleozoic infill in a joint at the Lowell boat launch and fossils in the river. Click here to see my field trips around LaSalle.

  • See coal and an unconformity between the Ordovician and the Pennsylvanian at Buffalo Rock State Park, east of Utica, Illinois along with framboidal Paleozoic iron deposits and concretions associated with the coal. Click here to see my field trips around LaSalle.

  • Many quarries are producing aggregate from fossiliferous rock. Unfortunately few quarries will permit you to pick through their crusher pile or waste rock associated with operations. Always visit the office and ask permission before entering any quarry. Check along nearby railroad tracks or where rocks fall off their trucks to see what's coming out of the quarry. Click here to see my field trips to Thornton Quarry.

  • In gravels and retaining walls along railway embankments. Usually rock is shipped the shortest possible distance because it is dense and expensive to ship. Watch for trains!

References and Literature Cited

Illinois Geology - Useful Publications
Figure sources

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Ellin Beltz /
March 14, 2006
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