From Ohio History Central
no edit summary
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.
'''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'''
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'''
'''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.
]] [[Category:Geology]] [[Category:Fossils]]