Microfossils

Micropaleontology is the study of plant and animal fossils that are too small to be seen without magnification. These microscopic objects provide valuable information about the evolution of life on the earth and about changes that have occurred on the earth's surface through time. They also have great value as index fossils and as indicators of ancient environments, valuable data in the search for oil and natural gas.

Micropaleontology

When people think of fossils, they envision items such as the bones and teeth of dinosaurs, the petrified bones of Neanderthals, or the shells of oysters. Micropaleontologists study the preserved remains of organisms too small to be seen with the unaided eye. It is convenient to divide micropaleontology into two subfields: animal micropaleontology and plant micropaleontology. Animal micropaleontology encompasses the study of a wide variety of fossils. In most cases, the material studied is the shell the once-living animal constructed. Shells are commonly composed of calcium carbonate (the common mineral calcite), calcium phosphate (the mineral apatite, a constituent of human teeth and bones), and opaline silica (quartz with water molecules in its crystal structure). Other microscopic animals build shells from mixes of sand grains, shell fragments, and volcanic ash, all glued together with organic or mineral cement. Thus, shells are called calcareous, phosphatic, siliceous, or agglutinated.

Microfossils vary in size as well as in shell composition. As a rule, few are smaller than 0.05 millimeters in diameter; most are in the range of 0.75 to 2.0 millimeters, and very few are as large as 5 centimeters. Among the larger microfossils are nummulites, the major constituents of the limestone used in the Egyptian pyramids. In about 450 Before the Common Era (BCE), Herodotus described them, erroneously, as mummified lentils (the food of pyramid construction crews).

Systematic study of microfossils began until the early nineteenth century. Alcide Dessalines d'Orbigny published a paper describing some microfossils that he thought to be microscopic cephalopods related to the chambered nautilus. Subsequently, it was realized that d'Orbigny's microfossils were types of protozoans (single-celled animals) called foraminifera. Another pioneer was Christian Gottfried, the first to treat micropaleontology as a field of study. As is the case for larger fossils, microfossils can be used to determine the age of sedimentary rocks. European micropaleontologists used them for this purpose as early as 1874 and by 1930, micropaleontology was aiding in petroleum exploration worldwide.

Foraminifera and Radiolarians

Many animals can produce microscopic shells or other hard parts that may be preserved in sedimentary rocks. Among these are protozoans, gastropods (snails), worms, crustaceans (crabs, lobsters), sponges, echinoderms (starfish, sea urchins), and fish. Protozoans contribute significantly to the microfossil record. Two are particularly useful indicators of geologic age and ancient environments: the foraminifera and the radiolarians. Most living foraminifera are found in the ocean, where their distribution is controlled by water temperature, salinity, depth, turbulence, light intensity, bottom conditions, food availability, predators, parasites, and other biological factors. Foraminifera, or forams, usually have calcareous or agglutinated shells. The most significant number of species are bottom dwellers (benthic), but one group, the globigerinids, evolved to live as passive floaters (or plankton) in the surface waters of the oceans. Benthic forams have a long geologic history extending back 500 to 600 million years. For the last 300 million years, their shells have been significant rock formers. The rock that forms the White Cliffs of Dover is composed mainly of the shells of foraminifera. Planktonic forams evolved more recently, first appearing in rocks about 175 million years old. Since planktonic species live in surface waters, ocean currents may carry them thousands of miles. This broad geographic distribution and rapid morphologic evolution make planktonic foraminifera especially valuable as index fossils.

Radiolarians are also planktonic protozoans, but they differ from foraminifera in their soft-body-part anatomy and their shell construction. Radiolarian shells are composed of opaline silica, are usually in the 0.05 to 0.5 millimeter size range, and display a bewildering array of shapes. Most are variations of three shapes: spheres, cones, or disks. Some radiolaria secrete their shells as spongy masses, others build shells of perforated sheets of silica, and others construct lattice-work shells of great delicacy and beauty. Radiolaria are found in rocks almost 600 million years old, giving them the longest geologic range of planktonic microfossils. Mesozoic and Cenozoic radiolarians are better known than Paleozoic ones.

Ostracods

Microscopic crustaceans called ostracods (or ostracodes) have a 500-million-year-long fossil record and are abundant in many sedimentary rocks. Unlike the familiar macroscopic crustaceans, ostracods encase their minute, shrimplike bodies in a pair of tiny, calcareous, bean-shaped shells 1 to 5 millimeters long. Outer surfaces of these shells may be smooth, or they may bear spines, wartlike bumps, grooves, ridges, flanges, and pores. Hinge structures and muscle scars, found on the inner surfaces of the shells, are also useful features in distinguishing different ostracods. Although some species are planktonic, most ostracods are benthic. Living ostracods can be found from the deepest ocean floor to the shoreline and landward into lakes and streams. A few species have adapted to live on land in moist ground litter. Playa lakes in desert regions often are inhabited by ostracods. Species living in these harsh environments are often parthenogenic: Unfertilized females lay fertile eggs, which lie dormant in the muddy bottom of a dried-up lake; the next time the lake fills, the eggs hatch into a new generation of females, and the cycle continues. Dormant periods of a decade or more have been reported.

Two different kinds of toothlike structures are frequently found in assemblages of microfossils. Annelid worm jaws, called scolecodonts, are composed of a resistant organic material (chitin) and have been found in rocks up to 600 million years old. The second type are called conodonts (“cone-toothed” animals). These fossils are 0.2 to 6.0 millimeters in greatest dimension, are composed of calcium phosphate, and occur as isolated specimens or clustered assemblages. The composition suggests that an animal produced them, and their distribution in sedimentary rocks indicates that these “conodont animals” (ordinal name Conodontophorida) were marine and planktonic. They first appeared in Cambrian time, flourished through the rest of the Paleozoic, and became extinct during the Triassic period. Specimens of conodont animals are known from the Carboniferous of Scotland and the Ordovician of southern Africa. They appear to have been eel-like animals in which the conodonts themselves formed a grasping structure in the throat. Although the affinities of this animal remain obscure, it is generally thought to have been a chordate of some kind.

Study of Microfossils

While microfossils can be recovered from many types of marine sedimentary rocks, they are most abundant in fine-grained rocks such as shale. Samples of these rocks can be obtained from surface outcrops or subsurface boreholes in the form of cores or cuttings. Care must be taken to eliminate any contamination when collecting samples for study, and accurate records of sampling localities must be maintained.

In the laboratory, various techniques are used to extract the microfossils from the host rock and clean them. The composition of the fossils, the composition of the host rock, and the kind of study to be done dictate the separation methods used. Some foraminifera, for example, can be studied profitably only in thin section. To prepare them, fossiliferous samples are glued to microscope slides and are ground and polished until the rock is paper thin and the internal features of the shell can be seen. Acids are used to dissolve sedimentary rocks effectively to release insoluble siliceous or phosphatic microfossils. Acetic acid will dissolve limestone without damaging conodonts, and radiolarians can be freed best by dissolving the host rock in hydrochloric acid.

Calcareous-shelled ostracods and foraminifera are usually extracted from shales in the following manner. A clean, dry, crushed (to pea-size) sample is soaked in kerosene for thirty to sixty minutes. The kerosene is poured off, filtered, and saved for reuse. The sample container is filled with water, and a wetting agent is added. This mixture is boiled for twenty to thirty minutes and then poured through a 200-mesh screen. Fine clay particles pass through, leaving the microfossil residue on the screen. Usually, it is necessary to repeat the boiling and screening process several times to yield a good, clean residue. After drying, the residue is ready for “picking.” A one-grain-thick layer of residue is spread on a picking tray and then placed on a binocular microscope's stage. Magnification of thirty to forty times is required. A picking brush is also needed. The micropaleontologist slides the picking tray back and forth so that all the residue is scanned. Once a microfossil is found, the picking brush and a steady hand come into play: The brush is moistened and guided to the fossil, which sticks to the brush while being transferred to a microscope slide for future study. Routinely, several hundred specimens will be picked from each sample; each specimen will be identified and tallied in the sample census. Specimens can be repicked for further study using the greater magnification of the scanning electron microscope (SEM). Thin coatings of a conductive metal are vacuum-plated on the specimens, which are then ready for SEM analysis. In a vacuum chamber, a beam of electrons is focused on the specimen; reflected electrons are collected and converted electronically into an image of the specimen. Magnification of fifty thousand times or more is possible, and the electronic image has a three-dimensional appearance.

The most critical application of microfossils is in biostratigraphy, or the use of fossils to determine the age of rocks. Fundamental to biostratigraphy has been the establishment of reference sections. These are sequences of rock that have been precisely dated and whose fossils have been described in considerable detail. Fossils from a rock sequence of unknown age are compared with fossil sequences from reference sections, and the best match is obtained. Because of their rapid evolution and broad geographic distribution, fossils of planktonic organisms are particularly well suited for this work. For example, radiolarian assemblages from Japan, western Texas, and the Ural Mountains of Eurasia can be compared directly with one another.

Several other studies can be carried out using a picked collection of foraminifera. If the assemblage contains both planktonic and benthonic species, the planktonic-benthonic ratio gives a measure of water depth at the time of deposition. Another depth indicator is the ratio of calcareous-shelled benthonics to agglutinate-shelled benthonics. The diversity of the assemblage may also provide useful information. In a tropical assemblage, many species are usually present, but only a few individuals of each species occur (high diversity). Only a few species are present in a high-latitude assemblage, but each is represented by many individuals (low diversity). Diversity measures must be used carefully, as similar effects can be seen concerning water depth—low diversity in very shallow or very deep water and higher diversity at intermediate depths.

Calcareous-shelled forams can also be analyzed to measure the water temperature in which they lived directly. Oxygen atoms in two forms (isotopes) occur in seawater. The ratio of oxygen-18 to oxygen-16 is a function of water temperature. When forams extract calcium carbonate from seawater and build their shells with it, the oxygen isotopes are incorporated as well. The shells are converted to carbon dioxide in the laboratory, and the ratio of oxygen-18 to oxygen-16 is determined. The measured ratio is then plugged into a mathematical formula to calculate water temperature. A second temperature-measuring technique can be used with planktonic forams. As the animals grow, they add chambers to their shells in a spiraling pattern. The spiral is clockwise (right-handed); in cold water, it is counterclockwise (left-handed). The coiling ratio for planktonic forams in a sample thus indicates water temperature. Although this technique is less precise than isotopic measurements, it is faster and less expensive.

Studies of evolution can also be conducted using microfossils. Because planktonic animals seem to evolve faster than benthonic, plankton is often preferred in these studies. Ideally, one needs closely spaced samples from a core in an area where insignificant environmental change occurred while the deposition of the planktonic shells was continuous. Some lineages of Cenozoic radiolarians have been traced for several million years. During that time, small morphologic changes accrued so that the descendant species differed from their ancestors in morphology. In some of these studies, the fossils have recorded hybridization between distinct but related species.

In the twenty-first century, microfossils continue to play an integral role in science. In 2022, researchers in Japan discovered new types of microfossils in the 1.9-billion-year-old Gunflint Formation. In 2023, researchers discovered well-preserved spherical microfossils in 22.4-billion-year-old rocks from the Turee Creek Group in Western Australia. These microfossils linked the rapidly changing environment of the Great Oxidation Event with an increase in the complexity of life. That same year, the oldest known three-dimensional fossils dating back over 1.5 billion years were discovered by scientists in the Volyn quartz mine near Zhytomyr, Ukraine. These discoveries offer insights into the evolution of life on Earth and its increasing complexity.

Principal Terms

foraminifera: single-celled, amoeba-like animals

index fossils: indicators of geologic age; they have short geologic (time) ranges and broad geographic distribution

isotopes: atoms of an element that differ in weight because different numbers of neutrons are present in their atomic nuclei

morphologic evolution: changes in the body or skeleton shape of organisms through time

parthenogenic: organisms in which unfertilized females produce viable, fertile offspring without copulation with males of the species

Bibliography

Anderson, O. R. Radiolaria. New York: Springer-Verlag, 1983.

Armstrong, Howard, and Martin Brazier. Microfossils. 2d ed., Malden, Mass.: Blackwell Publishing, 2005.

Briggs, Derek E. G., and Peter R. Crowther, editors. Paleobiology: A Synthesis. Oxford: Blackwell Scientific Publications, 1990.

Feldmann, Rodney M., Ralph E. Chapman, and Joseph T. Hannibal, editors. Paleotechniques. Knoxville, Tenn.: Paleontological Society, 1989.

Hag, Bilal, and Anne Boersma, editors. Introduction to Marine Micropaleontology. 2d ed., New York: Elsevier, 1998.

Jenkins, J. M., editor. Applied Micropaleontology. New York: Springer, 2010.

Nield, David. “Scientists Discover The Oldest 3D Microfossils Ever Found.” Science Alert, 29 July 2023, www.sciencealert.com/scientists-discover-the-oldest-3d-microfossils-ever-found. Accessed 18 July 2024.

Palaeobiology II. New York: Wiley-Blackwell, 2001.

Pokorný, Vladimír. Principles of Zoological Micropaleontology. Translated by K. A. Allen. 2 vols. Elmsford, N.Y.: Pergamon Press, 1965.

Prothero, Donald R. Bringing Fossils to Life. 2d ed., Boston: McGraw-Hill, 2004.

Tohoku University. “Discovery of New Types of Microfossils May Answer Age-old Scientific Wuestion.” Phys.org, 5 Sept. 2022, phys.org/news/2022-09-discovery-microfossils-age-old-scientific.html. Accessed 18 July 2024.

"2.4-Billion-Year-Old Microfossils Suggest Earlier Rise in Complex Cellular Organization." Sci.News, 14 Nov. 2023, www.sci.news/paleontology/paleoproterozoic-microfossils-12448.html. Accessed 18 July 2024.