Lycopod Plant Fossils
By the Devonian Period, vascular plants had colonized the land and large, tree-like forms appeared along with smaller plants. A large tree present in the Late Devonian was Archaeopteris. Large logs, representing the trunks of Archaeopteris trees are not uncommon in the Ohio Shale. These dark, coalified logs were originally given the name Callixylon. Remains of other plants, sometimes well preserved, are occasionally found in the Cleveland Member of the Ohio Shale in the Cleveland area. These were plants that grew on the Catskill delta, to the east, in western Pennsylvania, and drifted out into the sea in which the Ohio Shale was deposited.
Remains of lycopods are sometimes found in marine rocks of Mississippian age but they are not common. These remains represent terrestrial vegetation that drifted to sea from the Catskill delta to the east.
In contrast, rocks of Pennsylvanian age are rich in lycopod remains. These trees, which reproduced by means of spores, dominated the coal swamps and were a major contributor to the organic material that eventually would become coal. Many of these trees were large, with heights of nearly 100 feet and trunks of more than three feet in diameter. Lycopods bore leaves near the top of the tree. As fossils, they are most easily identified by diamond-shaped leaf scars that cover the trunk in a spiral fashion. Generally, the fossils are preserved as sandstone or siltstone casts of the trunk. People sometimes mistake these trunks for “fossil snakes” as the leaf scars bear a superficial resemblance to the scales of snake skin.
The roots of lycopod trees are commonly preserved as casts in clay beds (underclays) beneath coal beds. These structures bear a series of circular pits that represent the attachment points of rootlets. These root structures are called “Stigmaria.” The two most common genera of lycopods found in Pennsylvanian rocks are Lepidodendron and Sigillaria. Lepidodendron logs have diamond-shaped leaf scars that are in rows that spiral around the tree trunk. Sigillaria logs have somewhat more rounded leaf scars that are arranged spirally, but vertical ridges between the scars give the appearance that the scars are in vertical rows. The long, grasslike leaves of these lycopods are known from fossils, as are reproductive cones. Lycopods were trees of moist, swampy areas and many species became reduced in abundance or extinct as the climate became drier in the Late Pennsylvanian and Permian.
- Cross, A. T., Gillespie, W. H., and Taggart, R. E., 1996. "Upper Paleozoic Vascular Plants," in Fossils of Ohio, edited by R. M. Feldmann and Merrianne Hackathorn. Ohio Division of Geological Survey Bulletin 70, p. 396-479.