A siderite concretion that was split open to reveal an impression of a 3D fern. Mazon Creek, Illinois, is a famous fossil locality containing plants and animals from 309 million years ago. These fossils formed from a large flooding event that rapidly buried organisms, allowing for soft parts to be preserved. This fern fossil was collected in the 1920s in Grundy County and it show really high quality. Pecopteris is genus of seed ferns, extinct species of fern that reproduced from seed as opposed to sporophyte reproduction of most modern ferns. Been in storage almost 100 years! From a long ago collection. Concretion size: 5,5 x 4 cm Negative counterpart. Annularia sphenophylloides Moscovian / Desmoinesian Francis Creek Shale Member, United States of America, Illinois, Mazon Creek Region Rare of this quality.