MICROFOSSILS

INTRODUCTION

Microfossils are perhaps the most important group of all fossils — they are extremely useful in age-dating, correlation and paleoenvironmental reconstruction, all important in the oil, mining, engineering, and environmental industries, as well as in general geology. Billions of dollars have been made on the basis of microfossil studies. Because they usually occur in huge numbers in all kinds of sedimentary rocks, they are the most abundant and most easily accessible fossils. Indeed, some very thick rock layers are made entirely of microfossils. The pyramids of Egypt are made of sedimentary rocks, for example, that consist of the shells of foraminifera, a major microfossil group.

Microfossils can also be very useful in teaching science at all levels. Students are commonly fascinated by things they cannot see with their naked eyes, especially when the objects are beautiful or interesting in their own right. Furthermore, collection of microfossils is usually possible close to many schools — in fact, some schools are built right on top of microfossil-bearing sedimentary rocks! Processing the rock samples is usually easy and safe enough for children to do themselves, or at least to watch. Prepared samples can be purchased or obtained from museums and some universities. Because so many microfossils are usually found in any sample, the students can even keep their own finds!

MICROFOSSILS

Prokaryotes and protists are very well represented in the fossil record. Prokaryotes are the oldest known fossils, and they were the only life on Earth for most of its history — from 3.5 to 1.5 billion years ago. Protists joined them at least 1.5 bya, and animals and plants were latecomers at less than .55 bya. All these microfossils provide insights to Earth and life history, and so are important to study in paleontology. The single-celled forms help to develop and test evolutionary models using organisms that are not multicellular or sexual in all cases, and with greater ecological variety.

Generally prokaryotes and protists are single-celled. Yet the most significant contrast among life forms separates them. Prokaryotes have their DNA loosely organized within the cell and not in the cell nucleus, and chromosomes are absent. All protists, fungi, animals and plants are eukaryotes and have chromosomes made of DNA, RNA, and proteins in a nucleus. Many other very important differences occur too (Table 1). Animals, plants and fungi are multicellular; protists are generally unicellular and include all other eukaryotes.

Prokaryotes and protists are often called "simple", but this is just not true. Each one must do everything with just a single cell that higher plants or animals do with millions of cells. Single-celled organisms have many different kinds of specialized organelles within their cells that function in extraordinary ways. Prokaryotes, protists, fungi, animals, and plants are all very successful at making a living, and that is all that evolution requires. Although prokaryotes and protists seem simpler, they arose much earlier than their multicellular descendants and so might be considered more primitive, but some have also existed for at least 3.5 billion years and must therefore be considered very successful indeed.

Fungi, plants, and animals contribute a vast multitude of small parts to the microfossil record. Fungi are found as isolated microscopic filaments and spherical spores, usually associated with larger fossil plant material. As such, they have largely been ignored by paleontologists. Many plants have small pieces and parts that can be found as microfossils. Most important of these are pollen and spores which can be very abundant in terrestrial and nearshore marine deposits. Just about any animal with skeletal parts also contributes to the microfossil record.
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THE MICROFOSSIL RECORD

SYSTEMATICS AND RELATIONSHIPS AMONG MICROFOSSILS

The relationships among life forms are complex. A satisfactory overview of what kinds of life there are on Earth is still ephemeral. Six domains of life have been proposed: (1) Archaebacteria; (2) Eubacteria; (3) Protista; (4) Animalia; (5) Plantae; and (6) Fungi.

All these groups make microfossils (Table 2). However, not all are easily studied as microfossils. Many require special techniques of removing them from rocks or making them visible, others require very specialized equipment for their study. But a good number of very important ones are easily studied by any students with a pot to boil the rock sample, a simple microscope, and curiosity about the world near them.

IMPORTANT AND EASILY STUDIED MICROFOSSILS

Protists include several groups that have shells or skeletons of hard materials that preserve well, are easy to extract from rocks, and can be seen with a classroom dissecting microscope (Table 2). These are ideal for students of most any age to study. They include foraminifera, radiolaria and diatoms. Of course, other kinds may also occur in the same rocks, but they will be hard to find and difficult to see.

FORAMINIFERA — Foraminifera, usually called "forams" or "bugs", comprise a large group. Some 60-80,000 species have been described from Cambrian through Recent age sediments. The species generally have shells made of silt and sand particles or secreted calcium carbonate that range in size from less than .1 mm to 10 cm. Most are about the size of a pin head. Each shell has one or more chambers in which the protoplasm of the living protist resided.

Forams live today from the shallowest intertidal zones to the deepest trenches of the oceans. A very few live in salty lakes and springs, where they have been transported by birds or other means. About 4,000 species are alive today. Of these, only 40 float in the water and are planktonic, the rest are benthic and live on the bottom of the ocean or on plants or animals. Some even live on other forams! Forams eat a wide variety of food, from bacteria through algae to various kinds of animals and other protists.

Forams are among the most abundant fossils. They first occur as simple tubes of sand in the Cambrian. Later more complex tubes and coils appear, even developing chambers in the Ordovician. In the Silurian, secreted calcium carbonate types appear; these diversify into many different shapes and kinds. Like animals and plants, they undergo extinctions at particular times in the geologic past, but radiate again into new, but similar shaped, forms. They remain abundant and diverse. Because of this, forams are the principal microfossil used to age-date and correlate marine sedimentary rocks — they are particularly useful in the oil industry. A single oil company, during the boom days of exploration for new deposits, might have employed 40 or 50 micropaleontologists to study forams. They are used also to decipher ancient environments, climates and oceanography. They are probably the most important fossils of any, simply because they are so useful.

RADIOLARIA — "Rads", as they are commonly known in paleontology, are marine protists with siliceous skeletons of rods and lattices arranged in many complex ways. Thousands of species of fossil forms have been described in the scientific literature, and hundreds are alive today. They first appear in the Cambrian as spherical lattices with a few rods sticking beyond the sphere. In the later Paleozoic, they also radiated into many species, and except for periods of extinction continued to modern times. They too have been useful in age-dating and correlation, but mostly of deep-sea sedimentary rocks. Rads have always been marine and all, as far as we know, are planktonic. They are especially abundant in regions of upwelling where phytoplankton that they feed on are common. Rads are generally smaller than forams and look glassy.

DIATOMS — Diatoms are common marine and freshwater algae. They photosynthesize, hence only need certain nutrients and sunlight to survive and to make their siliceous skeletons. These fossilized skeletons, known as frustules, extend back only to the Jurassic or Cretaceous, although molecular biological evidence indicates that they arose in the Precambrian. Perhaps the missing record results from a lack of skeletons or a failure to preserve. In some places, thousands of feet of rock may be composed principally of diatom remains. These rocks, called diatomites, are usually light gray or white, fairly soft, and commonly well-bedded. Diatomites are mined for use as fine abrasives, filter materials for swimming pools and beer filtration, color carrier in cosmetics, toothpaste, and over a hundred other things.
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Diatoms range in size from quite small to specks large enough to see with the naked eye. Their frustules are in two parts, one fitting inside the other like a petri dish. Diatoms live both in the water column and on a solid substrate — some even live on the skin of whales or between sand grains on beaches! The planktonic diatoms usually are circular or trigonal, while the benthic ones are elongated with a groove down one side. The groove or raphe allows protoplasm to extrude and move the entire diatom along the substrate. Diatoms, throughout their geologic history, have been most common in areas of shallow water or upwelling where nutrients are carried into the euphotic zone.

Most diatoms are generally easy to study. The methods of freeing them from rocks are the same in a classroom as for other microfossils. Once disaggregated from their host rock, they can be spread on a glass slide or dark piece of cardboard and viewed in transmitted or reflected light. They can be moved around with a very small, damp paintbrush. In the 1800's, people used to make designs and pictures on glass slides by placing different kinds of diatoms in particular places.

ANIMALS — Nearly all sedimentary rocks have microfossils of animals and plants in them. Many times they are unidentifiable. Others can be readily attributed to a known group. For examples, fish scales, teeth, and bones are common in marine fine grained rocks, and small teeth and bones of terrestrial vertebrates can be found in nonmarine rocks. Most common, however, are small parts of invertebrates — shells, crinoid columnals, echinoid spines and plates, sponge spicules, ostracodes, and others. Some of these are not too useful in paleontology, but the tiny tooth-like structures known as conodonts are used for precise age-dating and correlation of rocks ranging in age from Cambrian to Triassic and ostracodes are useful in all respects from the Paleozoic to the Recent. Both conodonts and ostracodes make good classroom materials.

PLANTS — Bits and pieces of various plants are fairly common in sedimentary rocks. These are usually cuticle and other small parts. Seeds may be encountered in microfossil samples taken from rocks deposited in terrestrial environments. Pollen and spores are very common microfossils but they require difficult extraction techniques using dangerous acids and high powered microscopes to see them. These are best demonstrated with prepared slides purchased from supply.

USEFUL REFERENCES

Boardman, R. S., Cheetham, A. H. and Rowell, A. J. (Editors). 1987. Fossil Invertebrates. Blackwell Science, 238 Main St., Cambridge, MA 02142. 713 p. An advanced college-level text on fossil invertebrates with exellent descriptions and good illustrations.

Brasier, M. D. 1980. Microfossils. George Allen & Unwin, London. 193 p. Brief overview of major groups of microfossils. Simple drawings of each kind of microfossil.

Haq, B. U., and Boersma, A. (Editors). 1978. Introduction to Marine Microfossils. Elsevier, New York. 376 p. A college text describing important microfossils found in marine rocks, excluding fish parts. Good illustrations.

Lipps, J. H. 1981. What, if anything, is micropaleontology? Paleobiology, vol. 7, p. 167-199. An essay on the history, use, and potential of microfossils in paleontology.

Lipps, J. H. (Editor). 1992. Fossil Prokaryotes and Protists. Blackwell Science, 238 Main St., Cambridge, MA 02142. 342 p. A college-level text on microfossils of single- celled prokaryotes and protists. Good illustrations showing the variety of forms and their terminology, as well as detailed descriptions of their paleobiology, biostratigraphy, and evolutionary history.

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