Plant fossils in carbonaceous shale from the Carboniferous of Pennsylvania, USA. (10.3 cm across at its widest)

Plants are multicellular, photosynthetic eucaryotes. The oldest known land plant body fossils are Silurian in age. Fossil root traces of land plants are known back in the Ordovician. The Devonian was the key time interval during which land plants flourished and Earth experienced its first “greening” of the land. The earliest land plants were small and simple and probably remained close to bodies of water. By the Late Devonian, land plants had evolved large, tree-sized bodies and the first-ever forests appeared.

Shown above are some of the most attractive plant fossils around - whitish-colored fossil plants on black shale from near the town of St. Clair, Pennsylvania.

The rock is anchimetamorphosed fossiliferous carbonaceous shale (= very slightly metamorphosed). Most plant fossils in shale are preserved as black, carbonized compressions. Indeed, the St. Clair fossil plants are carbonized - below the whitish surface film of each plant is a silvery gray film of graphite (C - carbon). The graphite is covered with pyrophyllite (Al2Si4O10(OH)2 - aluminum hydroxy-silicate).

The St. Clair flora includes pteridosperms (Neuropteris ovata, Macroneuropteris scheuchzeri, Alethopteris), ferns (Pectopteris), and sphenophytes (Sphenophyllum).

Stratigraphy: bottom rocks below the Buck Mountain # 5 Coal, uppermost Pottsville Formation, Middle Pennsylvanian