These ferns grew in the lush tropical forests of Illinois, Missouri, and Pennsylvania some 300 million years ago. They were trapped in mud concretions, which fossilized over time. Fossil hunters look for small oval or round rocks, and if they are skilled enough to crack the rocks open correctly, the leaf ends up in one half (called the positive impression), and its imprint (or negative impression) is visible on the other half. The backs are not polished, but they are smooth to the touch. They are not heavy.
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