GEOFF KIDD / SCIENCE PHOTO LIBRARY GEOFF KIDD / SCIENCE PHOTO LIBRARY
Pecopteris fern fossil. Pecopteris-leaved ferns appeared in the Devonian period, flourished the early Carboniferous period (Pennsylvanian) and on to c. 250 million years ago. They are named because of the regular comb-like arrangement of the leaflets. This example is from the Mazon Creek deposit, Illinois, USA and shows both positive and negative sides of a split nodule. These nodules are concretions of siderite or iron carbonate and naturally split along the middle.
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