Fossil imprints have been found that closely resemble the Sensitive Fern; these imprints date back to the time of the dinosaurs. Thus, the Sensitive Fern appears to have changed very little during millions of years. The Sensitive Fern is the only member of its genus; its nearest living relative (in Illinois, anyway) is Matteuccia struthiopteris (Ostrich Fern), which looks very different. The Sensitive Fern can be readily distinguished from other ferns by its peculiar fertile leaves – each fertile leaf resembles an elongated cluster of dark brown beads on a stalk. These fertile leaves are very persistent and can be observed near the infertile leaves throughout most of the year. The common name of this species refers to the deciduous nature of the infertile leaves and their sensitivity to frost; they do not coil back in response to being touched.