JERE H. LIPPS
INTRODUCTIONMICROFOSSILS ARE the tiny remains of bacteria, protists, fungi, animals, and plants. Microfossils are a heterogeneous bunch of fossil remains studied as a single discipline because rock samples must be processed in certain ways to remove them and microscopes must be used to study them. Thus, microfossils, unlike other kinds of fossils, are not grouped according to their relationships to one another, but only because of their generally small size and methods of study. For example, fossils of bacteria, foraminifera, diatoms, very small invertebrate shells or skeletons, pollen, and tiny bones and teeth of large vertebrates, among others, can be called microfossils. But it is an unnatural grouping. Nevertheless, this utilitarian subdivision of paleontology, first recognized in 1883, is very significant in geology, paleontology, and biology.
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!
MICROFOSSILSAlthough plants and animals are the most obvious life around us today, they are not the most numerous nor the most important contributors of microfossils. Bacteria (prokaryotes) and protists far outnumber them, live in more diverse habitats, and leave a greater diversity of microfossils. Today these organisms live from Antarctic ice deserts to steaming volcanic hot springs, and from the highest mountains to the deepest sea. Some cause diseases, such as malaria which infects 350-400 million people today; others are useful to humans. Most simply live their lives unknown to us but contributing enormously to our well being through the production of oxygen, the degradation of waste materials, recycling of nutrients, production of food, and a multitude of other functions, some of which take place in our own bodies. Fungi, another group in modern environments that both benefit and plague humans, have a long, but mostly unstudied, microfossil record.
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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.
THE MICROFOSSIL RECORDMost organisms lack substantial hard parts and rarely fossilize. The fossil record in its entirety is estimated to include only from 3% to 13.6% of all species that ever lived. The vast majority of described species are living only 8.7% of those described formally are fossils. No one has ever calculated how many of those are microfossils, but probably between 1/4 and 1/2, largely because they are so useful in geologic applications. Most prokaryotes and protists do not have skeletons of any sort, and have no fossil record whatsoever. Nevertheless, remarkable prokaryotes are known from very old Precambrian rocks, and dome-shaped stromatolites (built by photosynthesizing cyanobacteria) are very common from 3.5 to about .4 billion years ago and less common right up to the Recent. Among the described species of protists, about 50% are fossil, again because they are so useful in age dating, correlation and paleoenvironmental analysis. However, entire groups like the ciliates (with the exception of the small group of tintinnids which do have shells) are virtually absent from the fossil record. Parasitic prokaryotes, protists, and animals, have surely inhabited other organisms for billions of years, but have no fossil record of any import whatsoever. Indeed, even among these larger, multicellular groups, the fossil record is nowhere near complete. In spite of these problems, microfossils provide one of the best records of evolution's course because they are generally rather abundant and complete.
SYSTEMATICS AND RELATIONSHIPS AMONG MICROFOSSILSBecause microfossils are an arbitrary grouping based on methods of study, no single classification of them will suffice except at the highest levels. The systematics of prokaryotes and protists has long been confused and multifaceted at all levels. For example, among the prokaryotes, the cyanobacteria were thought to be plants called blue- green algae. We now know that they are really bacteria. Protist classification is very complex. They have traditionally been classified along with animals and plants. "Animal-like" protists, or those that can move on their own, were claimed by protozoologists; "plant-like" protists, or those that generally photosynthesize, were studied by phycologists. This dichotomy produced considerable confusion and impeded understanding of the relationships among the various kinds. The different investigators soon formed their own exclusive classifications, all of which are now under reconsideration based on molecular studies. Likewise, bacteria have not been clearly deciphered, although molecular techniques are helping enormously here as well. Animals, plants and fungi have mostly held their own as inclusive groups, although modern paleontological and molecular studies are changing views within each group as well.
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 MICROFOSSILSNot all microfossils can be easily used in classrooms. Bacterial and fungal microfossils are usually difficult to study, unless a prepared thin-section of rock containing them is available. Such thin sections can sometimes be purchased from scientific supply houses. Likewise, pollen and spores can be had in prepared slides. These microfossils are generally very small and require a fairly powerful microscope for viewing. This kind of exercise, then, becomes observational but can be used successfully for interpretation.
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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.
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 REFERENCESBlome, C. E., Whalen, P. M., and Reed, K. M. (Convenors). 1995. Siliceous Microfossils. Short Courses in Paleontology Number 8. Paleontological Society. 185 p.
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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