Microfossils and Microscopy
Microfossils are often abundant in marine limestones and shales. Marine transgressions during the Phanerozoic deposited enormous quantities of marine sedimentary rocks on the continents. These strata may be exposed at the surface in mountain cliffs, deserts, canyons, quarries, and roadcuts. A kilogram of rock may yield hundreds or thousands of specimens less than a millimeter in size, requiring a microscope or microscope camera to observe.
Recent sediments taken from the sea floor may contain unaltered material, but over time sediments are compressed or cemented together into rock by precipitating calcite or silica, and may be transformed by other types of diagenetic action. A variety of strategies are available to disaggregate rock and free the microfossils within, especially for shales and argillaceous marls. Even highly indurated limestones may be digested in acid to yield insoluble residues containing polycystine radiolaria or agglutinated foraminifera. Properly buffered solutions of acetic or formic acid also enable recovery of phosphatic fossils such as conodont elements. Palynologists often use stronger acids to eliminate all residues other than organic fossils. Since most acids dissolve calcareous fossils, solid fossiliferous limestones may be cut and ground into thin sections to reveal the calcareous fossils in transmitted light. Many variations on these techniques can be found in the literature.
Link to a book summarizing several of these techniques: A manual of practical laboratory and field techniques... - Google Scholar
Why are microfossils important?
Microfossils are the products of past life. As such, their structure and composition change over geological time scales as a consequence of genetic drift and the evolutionary pressures exerted by the environment. Throughout the past, a number of microfossil-producing organisms have been distributed globally throughout the world’s oceans. They continuously evolved distinguishable morphologies over timescales of roughly a million years or so. As a result, these fossils allow for global correlation of strata, even Paleozoic rocks heavily altered by diagenesis. Indeed, microfossils (and conodonts and graptolites especially) are the basis of most GSSPs which partition the geologic timescale. Macrofossils such as trilobites, ammonites, a couple of bivalves, and a brachiopod determine many of the remaining GSSPs.
Global Boundary Stratotype Section and Point - Wikipedia
International Commission on Stratigraphy
Geologic TimeScale Foundation - Stratigraphic Information
Geologic Time Scale 2020 - 1st Edition (elsevier.com)
