This fossil is part of a Pennsylvanian plant called Sphenopteridium. At 300 million years old, it is actually one of the youngest such specimens ever found. Plants such as Sphenopteridium were common during the Mississipian and Pennsylvanian (355-295 million years ago), and often form a significant portion of many coal deposits. This particular fossil was buried in a quiet pond or lake, and was discovered by scientists splitting the layers of mudstone that resulted. When the rock was split open, two parts of the plant fossil were found. One part is the positive and the other is the negative impression of the plant. The darker half illustrated here is microscopically thicker, and thus is the positive.