Archaeopteris fossil land plant from the Devonian of Pennsylvania, USA. (public display, FMNH UP 2054, Field Museum of Natural History, Chicago, Illinois, USA)

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.

This fossil branch with leaves is from a progymnosperm, an extinct group of vascular plants. Progymnosperms have characteristics of seed plants (they have lots of gymnosperm-like wood - secondary xylem) and characteristics of ferns (fern-like reproductive structures that produce spores).

In the 1960s, the leaf/frond genus Archaeopteris was recognized to be part of the same plant as the wood genus Callixylon. Archaeopteris had 3 to 4 feet diameter trunks (axes) and formed shrubby to tree-sized plants, possibly similar to modern conifers. The leaves (they are recognized as true leaves in this form) are flattened and range in shape from spatulate to fan-shaped to dissected. Species of Archaeopteris have been reported from the Devonian of North America, Eurasia, and Australia.