Plant Fossils

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<p>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.</p>
<p>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 macro-fossils and are very useful for correlating rocks and determining their relative ages.</p><p>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.&nbsp; </p>
<p><strong>ALGAE PLANT FOSSILS</strong></p>
<p>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 <em>Protosalvinia </em>(formerly called <em>Foerstia</em>) 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.</p>