Table 2. Microfossils. Some of these microfossils (indicated with a double asterisk) are easily prepared and studied in the classroom by the teacher and students, others (single asterisk) may be prepared and observed with special techniques, special samples, or high-powered microscopes, while others (no asterisk) will be suitable for students only by examination of previously prepared or purchased slides. MICROFOSSIL DESCRIPTION OF FOSSILS SIZE RANGE IN MILLIMETER COMPOSITION OF FOSSILS ENVIRONMENT WHERE THEY LIVE OR LIVED GEOLOGIC AGE RANGE Prokaryotes: Single-celled organisms with DNA loosely organized in the cell Cyanobacteria Single cells <0.05 Some with CaCO3 sheaths Shallow saline and normal marine waters Archean-Recent Eukaryotes: Single or multicellular organisms with DNA organized in the nucleus of the cell Algae: Single-celled, photosynthetic eukaryotes Chlorophytes Single cells 0.1-0.4 Silica and CaCO3 All Proterozoic-Recent Charophytes** Spheres, ovoids 0.5-3 CaCO3 Lakes, rivers Silurian-Recent Dinoflagellates* Lattice of rods 0.005-0.15 Silica Marine planktonic Silurian-Recent Ebridians Mesh of solid rods <0.2 Silica Marine planktonic Paleocene-Recent Diatoms** Two-part shell called a frustule <0.2 Silica Non-marine, marine, soil, ice Jurassic-Recent Chrysomonads Spherical cysts <0.1 Silica Aquatic Proterozoic-Recent Silicoflagellates* Lattice of hollow rods <0.1 Silica Marine planktonic Cretaceous-Recent Calcareous nannofossils* Plates, stars, rods <0.05 CaCO3 Marine Silurian(?), Pennsylvanian(?), Triassic-Recent Protozoans: Single-celled heterotrophic eukaryotes Arcellinids Single-chambered shell called a test <0.15 Agglutinated Lakes, slow rivers, damp litter Mississippian, Tertiary-Recent Foraminifera** Single or multi-chambered shells called tests .01-100 Agglutinated, CaCO3 Marine bethonic and planktonic Cambrian-Recent Radiolaria** Spherical or conical mesh of solid rods .03-1.5 Silica Marine planktonic Cambrian-Recent Acantharians Spicules <0.05 SrSO4 Marine planktonic Eocene-Recent Heliozoans Spines, scales <0.05 Silica Marine, fresh-water planktonic Pleistocene-Recent Tintinnids Single-chambered shell called a lorica <0.300 Agglutinated, CaCO3 Marine planktonic Ordovician-Recent Fungi: Mostly multicellular eukaryotes that absorb digested food Fungi spherical, filaments <0.01 Some CaCO3 All environments Precambrian-Recent Animals: Multicellular eukaryotes that ingest their food Sponges* Spicules <0.1 Silica, CaCO3 Marine, lakes, rivers Cambrian -Recent Octocorallians Spicules 1-3 CaCO3 Marine Silurian-Recent Brachiopods** Tiny shells >0.1 Ca5(OH)(PO4)3, CaCO3 Marine Cambrian -Recent Bryozoans** Tiny rods or branches with holes where the animal lived <0.5 CaCO3 Marine, lakes, rivers Ordovician-Recent Ostracods** Bivalved shell called a carapace .1-5 CaCO3 Marine, lakes, rivers Cambrian-Recent Echinoderms** Spicules, plates. .05-1 CaCO3. Marine, usually shallow water. Cambrian -Recent Conodonts* Toothed or flattened plates 0.1-3 CaCO3, Ca5(OH)(PO4)3 Marine Precambrian-Triassic Mollusks** Tiny snails, mostly coiled, and clams >0.1 CaCO3 Marine, terrestrial, lakes, rivers, streams Cambrian-Recent "Fish"** Bones, teeth & scales >0.01 CaCO3, Ca5(OH)(PO4)3 Marine, lakes, rivers, streams Cambrian-Recent "Higher vertebrates"* Bones, teeth, sometimes plates from the skin of certain forms >1 Ca5(OH)(PO4)3 Mostly terrestrial, marine Devonian-Recent