Plants in the form of algae and phytoplankton appeared early in the Precambrian. Blue-green algae (cyanobacteria) were the dominant life form for much of the Precambrian and built sediment-trapping mound-like structures, some quite large, known as stromatolites. Marine plant remains are found in Ohio' s oldest exposed rocks, of Ordovician age, but it was not until the Silurian that plants finally made the leap from aquatic environments to the land. Ohio's record of land plants dates to the Devonian. Ohio's record of Late Paleozoic plants, which represent the lush, vast coal swamps of the Pennsylvanian Period, is exceptional and fossils are abundant and easily collected from these rocks. Some sediments deposited in association with the glaciers of the Pleistocene Ice Age commonly contain only slightly altered logs, branches, leaves, and reproductive structures of plants that lived in Ohio during these cool times.
Terrestrial plants, like vertebrates, consist of many parts that commonly become separated and dispersed after death. Consequently, leaves, branches, trunks, reproductive structures, and roots have in many cases been given individual scientific names. Discovery of more specimens and intensive study has led to many of the parts being associated with a single biological species. Of much importance to scientists are microscopic remains of reproductive structures such as spores and pollen. These remains provide a record of plant species that may not be preserved as macrofossils and are very useful for correlating rocks and determining their relative ages.
Plant fossils are preserved as impressions in the rock, as compressions where some organic material is preserved on a bedding plane, casts and molds, or as permineralized (petrified) remains where ground water has replaced the original organic material, preserving the original cell structure. Plant fossils from the Pleistocene Ice Age are only slightly altered. An important means of preservation of plant remains associated with Pennsylvanian coal swamps is in structures called coal balls. The structure of most plant material is destroyed during the process of conversion to peat and eventually coal. However, in rare circumstances calcium carbonate or silica impregnated the plant material before destruction and preserved detailed remains in concretionary masses called coal balls.
ALGAE PLANT FOSSILS
Remains of aquatic, marine algae are found in Ohio's Paleozoic rocks but often these fossils are difficult to recognize and identify. Mound-shaped algal-sediment structures known as stromatolites or rounded ones known as oncolites are found in some rocks in Ohio. They have been recognized from Silurian and Pennsylvanian rocks. Other, larger algae, 'seaweeds,' have been found in Ordovician, Silurian, and Devonian rocks. Of particular note is a probable algal fossil known as Protosalvinia (formerly called Foerstia) that occurs in some abundance in a narrow zone in the Huron Member of the Ohio Shale. These fossils consist of shiny black, branched, lobed, or discoid structures preserved on bedding planes in fresh, unweathered shale.
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, grass-like 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.
HORSETAIL PLANT FOSSILS
Horsetails are tall, comparatively thin spore-producing plants that are characterized by a stem that is divided into segments by periodic constrictions from which whorls of leaves emerged. They appeared in the Devonian and survive to the modern day but became greatly reduced after the Late Paleozoic. Many people may be familiar with the modern horsetail, Equisetum, which is sometimes called a scouring rush. Some Paleozoic species grew as large as 60 feet high. Portions of the stem, with several segments, are not uncommon in Pennsylvanian rocks. These segments have a series of closely spaced vertical ribs and grooves that are divided by a horizontal groove. Horsetails lived in the luxuriant coal swamps. Calamites is a common genus.
TRUE FERN PLANT FOSSILS
True ferns first appeared in the Devonian but looked very different from the familiar ferns of today. True ferns reproduce by means of spores. One of the common forms in Pennsylvanian coal swamps was Psaronius, which was treelike in appearance. These plants stood as high as 30 feet with a crown of large fronds. The trunk was thick near the base because of adventitious roots that grew on the surface of the trunk to the ground. Pecopteris is a name that is applied to the pinnules on the fronds. True ferns such as Psaronius became more common in the Late Paleozoic as conditions became drier and the lycopods declined. Silicified (petrified) wood of Psaronius trunks have been found at several localities in Ohio.
SEED FERN PLANT FOSSILS
Seed ferns were a now extinct group of plants (gymnosperms) that reproduced by seeds. However, the seeds were unprotected by a hard covering, as are the seeds of modern angiosperm plants. Most seed ferns were small plants and important during the Late Paleozoic, but never dominant. Carbonized compressions of the leaves known as Neuropteris and Alethopteris are commonly found in shales associated with Upper Paleozoic rocks.
CORDAITES PLANT FOSSILS
These seed plants (gymnosperms) appeared in the Late Mississippian and persisted into the Triassic, when they became extinct. Cordaites of the Pennsylvanian were small trees or shrubs in most cases but some grew to heights of perhaps 120 feet. Some forms were like modern mangroves and lived along shorelines, with long, tangled roots above the water. The leaves were long and bladelike.
CONIFER PLANT FOSSILS
Conifers are gynosperms that bear seeds in cones ('pine cones') and have leaves that are short and needlelike. Everyone is familiar with a variety of species living today. Conifers first appeared in the Late Pennsylvanian, although their fossils are rare. Specimens of branches with attached needles have been found in Ohio. Most of these remains are placed in the genus Walchia.
ANGIOSPERM PLANT FOSSILS
Angiosperms are flowering plants that have protected seeds and are the dominant and familiar plants of today. They first appeared in the Cretaceous but Ohio's fossil record dates to the Pleistocene Ice Age, as rocks from the Mesozoic and most of the Cenozoic are absent from the state. Many sediments deposited in association with the glaciers have a rich record of vegetation living in the area at the time of deposition. These plants and trees in most cases are from species still living, although during colder glacial times many species do not live in Ohio today in abundance but do live in the boreal forests of Canada.
The most recognizable plant remains are logs and branches of spruce trees and other conifers. Some of them represent living forests that were destroyed as ice advanced and overrode the trees. Bogs and glacial kettle lakes may have many specimens of logs and branches entombed in the sediment along with seeds and a rich record of microscopic pollen. The larger logs and branches look like modern wood and will burn when dried out. Many people are surprised to learn that these 'sticks' are more than ten thousand years old. These specimens have been used extensively for radiocarbon dates at many sites in the state.
Glacial ice had retreated northward and was gone from Ohio by about 14,000 years ago. The open spruce forests that characterized Ohio during glacial episodes persisted until about 10,000 years ago when the vegetation changed in a relatively short period of time from boreal forest to northern hardwood forests. This was in response to a warming climate. Perhaps not coincidentally, many of the large mammals, such as mastodon and mammoth, became extinct at this time.