Microbial fossils
Single-celled life forms have been the most abundant life forms on Earth since life began. Because most single-celled life is soft and decays easily, fossils of single-celled organisms are usually rare. Some forms of single-celled life secreted shells (called “tests”), however, which can be fossilized. Microscopic and macroscopic (visible without a microscope) single-celled fossils with tests, including several types of foraminifera, have been found in Kentucky. Some other types of microscopic single-celled life amalgamated into microbial mats, which are macroscopic. The most common microbial fossils are fossil algae. Many people think of algae as plants, but algae are a diverse group of life, and much of what is termed “algae” are actually colonies of photosynthetic single-celled life, rather than true plants. Forms of green, blue-green, and red algae have been preserved as fossils in Kentucky.
Charophytes (green algae). Charophytes are one of several types of green algae that secreted calcareous skeletons. Microscopic fossil charophyte algae cysts have been reported from the Devonian Jeffersonville Limestone in Kentucky.
Dasyclades (green algae). Dasyclades are one of several types of green algae that secreted a calcareous skeleton. Some are microscopic, and others are macroscopic. They range from Cambrian to the Recent.. One group of macroscopic fossil dasyclades found in Kentucky are receptaculitids. Receptaculitids formed low-lying mound shapes with mesh-like patterns. They have historically been confused with sponges and bryozoa, and have been reported from Silurian strata in Kentucky.
Foraminifera are single-celled, protist organisms. Protists are single-celled life that are not animals, plants, or fungi. Forams are mostly microscopic, but some grew to lengths of more than a centimeter. Forams had shells, called “tests.” Two types of tests are found as fossils; one type formed from calcium carbonate, and another formed from sediment grains that were cemented together by the organism, into what are termed “agglutinated” tests. Even the large, macroscopically visible tests are from single-celled organisms. They range from the Cambrian to Recent, but are most common in Kentucky from the Carboniferous.
Fusulinids are a specific class of foraminifera. Fusulinids had elongate to lenticular tests, which were microscopic to macroscopic, and look like tiny seeds. Fusulinids range from the Silurian to the Permian, but are most common in Kentucky in the Middle to Late Pennsylvanian. The Middle Pennsylvanian Curlew Limestone was historically informally termed the Fusulina chert because it contained many macroscopic fusulinids.
Rhodophytes (red algae). Some red algae secrete calcareous filaments that form coral-like mound and branching shapes. Although the mound shapes can be large, they are still formed from single-celled algae filaments. Several types of red algae mound shapes are known from the Late Ordovician in Kentucky; the most common form was historically called Tetradium, but was recently renamed Prismostylus.
Stromatolites (blue-green algae). Colonies of blue-green cyanobacteria (formerly called blue-green algae) living in shallow tropical seas sometimes formed mats on the sea bottom. In certain cases, fine sediment layers (laminae) periodically covered the mats or layers of calcium carbonate that were deposited by the bacteria. Alternating layers of bacterial mats and sediment formed macroscopic, mound-shaped buildups called stromatolites. Stromatolites are among the earliest fossils known and range to the Recent. Fossil stromatolites have been found in Kentucky, but are not common.
Thrombolites (blue-green algae). Blue-green cyanobacteria can form sediment layers and buildups that lack lamination but contain a clotted fabric, termed thrombolites. Thrombolites are most common in the Cambrian, which is preserved in the subsurface of Kentucky, but have been encountered in Kentucky rock cores gathered from the subsurface.