<em>Macroneuropteris</em> hair structure
Detail of the hair structure of a Macroneuropteris leaf fossilized in an ironstone concretion, from the Yale Peabody Museum of Natural History

Plant fossils can be found in Cambrian strata, but in Illinois, most are in ironstone concretions from coal deposits of the Pennsylvanian Period. The most common plants were giant ferns growing as tall as fifty feet. Extinct seed ferns and giant scouring rushes, relatives of modern horsetails, have also been found. Scale trees reaching 100 feet had a trunk covered in leaves and limbs that left scars when they fell off. Macroneuropteris is a genus of seed plants best known for the species Macroneuropteris scheuchzeri, a medium-size tree that was common throughout the Late Carboniferous.