Category: Fossils

Ocean habitats, including the shallow seas that covered the Midwest during the Cambrian Period, have given us many fossils.  The following species are all found in Illinois fossil records, and all have forms still alive today.  Bryozoans live in colonies with lacy, branching, or screw-shaped skeletons peppered with many tiny holes in which the animals reside.  Brachiopods attach to any surface by a fleshy appendage extended from their hard shells and were abundant in ages past compared to present day.  Snails are plentiful in the fossil records along the banks of all major rivers.  Cephalopods including squid, octopus, and cuttlefish, were shelled animals in their ancient forms.  They could grow to nineteen feet, but all of the shelled forms are now extinct.  Pelecypods, also known as bivalves, include mussels, clams, oysters, and scallops, and are found in the oldest rocks in the Midwest.

Insect fossils are rare, although more than 100 species have been found from the Pennsylvanian period in coal deposits of Illinois.  Many species lived in swampy areas at the edge of ancient seas.  Most fossils, often found in ironstone, a sedimentary rock, are of now extinct species.  Some of these species include assassin bugs, katydids, millipedes, and centipedes.  One such extinct insect was the giant millipede, Arthropleura, estimated to be eight-and-a-half feet in length.  Dragonflies, damselflies, and cockroaches in both ancient and present-day forms have been identified. 

Vertebrate fossils found in the Illinois region from extinct species are mammoths and mastodons, plus animals still commonly occurring today including horses, deer, and humans.  Smaller species of fish, amphibians, reptiles, birds, and mammals are all present in the fossil record of the Midwest.  As you move further west, larger species from the Mesozoic and Cenozoic ages become common finds.  These include dinosaurs, camels, and saber-toothed tigers.

Plant species found in the fossil record are abundant.  Many fast growing fern and tree species litter the coal deposits brought up from strip mining in Illinois.  Giant scouring rushes and seed ferns, now all extinct, are relatives of today’s horsetail plants.  Many of these were common ferns in ancient times and grew to heights of fifty feet or more.

The Tully Monster, Tullimonstrum gregarium, was named the Illinois state fossil in 1989.  It was a soft-bodied animal that lived during the Pennsylvanian Period, 300 to 315 million years ago.  With a smooth, tapered body and large tail fins, it had an oval, segmented body that grew to about 12″ in length, and was at home in the shallow, tropical seas that covered this area.  The first fossil was found in 1958, and no specimen of this animal has ever been found outside of Illinois.

You can view and study the Tully Monster and other fossil specimens at the Jurica-Suchy Nature Museum, and many other representative specimens can be viewed in most natural history museums.  While fossils have been well-studied over the past several decades, there are many facets of each specimen and the environments and habitats they lived in still to be discovered.

Gallery of fossils from upper left clockwise: Fossil variety at JSNM; Nothosaurus vertebra, Rudersdorf, Germany at JSNM by DonArnold, Oct 2023; Fossil variety at JSNM; Pterosaur phyllurus ramphrhynicus, Jurassic period, at JSNM by DonArnold, Oct 2023; Ptychocarpus unitus, Mazon Creek, IL at JSNM by DonArnold, Oct 2023; Fossil variety at JSNM;

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Fossils, remnants of organisms that lived in the past, can be found worldwide.  Located throughout Illinois in strip mines, rock quarries, and along cliffs, bluffs, and river banks, many fossils are found grouped together from a specific age in geologic time (see the sidebar below).  Knowing the various groups of animals that lived during that age helps us to identify other remains found in the same area.

In order for a fossil to form, an organism that dies must be quickly covered with sediment or amber before decay or erosion sets in, or it is eaten by a scavenger.  Soft tissues dissolve within hours of death, and they are rarely found in fossils.  The harder parts of an animal or plant species such as bones, teeth, shells, and woody tissues may be covered quickly and preserved.  Some fossils are formed by sediment covering the organism, then drying while parts of the organism slowly dissolve.  This results in a mold which can be filled with more sediment that hardens to create a fossil.  Other fossils are formed when insects damage a plant, which releases its sap, called amber, to heal the damage.  Amber may trap and cover the insect, then dry and harden creating an insect fossil inside.

Let’s investigate a few fossils found in the Midwest.  During the Ordovician and Cambrian Periods, Illinois and much of the Midwest was covered with shallow seas.  Many of the fossils found today are from sea creatures that lived on that ocean bottom.  Foraminifera, excellent indicator species for finding oil, are one-celled organisms that live in coastal waters and river estuaries.  They ingest sand and minerals and secrete a jellylike protoplasm that hardens into a shell, forming a chamber.  As they grow, subsequent secretions add more chambers in different shapes and sizes including spherical, tubular, and spiral.  Foraminifera anchor themselves to the bottom of the ocean and feed on food particles of algae, bacteria, and detritus flowing through the chambers in the water current.

Sponges and corals may live singly or in colonies of hundreds of individuals.  They are found anchored to the bottom of both saltwater and freshwater seas.  Corals have a hard skeleton divided into many open chambers.  Sponges have hard, dense skeletons that still allow water to flow freely through.  Both organisms breathe by extracting oxygen and feed by filtering bacteria and detritus from water currents flowing around them.  They also filter carbon, nitrogen, and phosphorous from the current creating a healthier habitat for other aquatic organisms.  In northern Illinois, fossils are commonly found from the Cambrian era.

Echinoderms, including star fish and sea urchins, can be found sparingly in the fossil record in Illinois.  They have an external skeleton of calcium with tube-like feet extending below used to propel themselves over the ocean bottom or pry open shells of prey organisms.  Blastoids, an echinoderm also known as a “sea bud,” with a mouth on top of a small round structure, are commonly found in river banks and bluffs.  Crinoids, often called sea lilies, have several clustered discs with a long stem anchoring them to the ocean floor.  After death, even as the stem dissolves, single discs are often preserved as fossils in limestone beds.

We have numerous fossils on display at the Jurica-Suchy Nature Museum, and many more can be found in other natural history museums.  In my next blog we will continue to explore other types of fossils found in the Midwest.