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.
