Microfossils

Collecting and preparing microfossils brings out the engraver in some collectors, for they must learn to handle, mount, and identify fossil organisms smaller than grains of salt. The world of the ultra-small has been extended to the tiniest fossils of all, the coccoliths, which become visible only with scanning by the electron microscope. These little shell-like fossils are so small that many species are invisible under a good optical microscope and require a magnification of 40,000 power to make them clearly visible in detail. A collection of coccoliths could be mounted on the head of a pin with plenty of room remaining for a label, but nobody has yet solved the problem of how to mount them.

A microfossil is any fossil that requires magnification to be identified properly or to be seen clearly. For all practical purposes, this means a fossil less than i inch in the longest dimension. Few people collect these organic miniatures, probably because of lack of information about them, difficulty of identification, and the impossibility of persuading anyone else that a speck of dust is as interesting as a 6-inch trilobite.

Professional paleontologists have become greatly interested in certain mini-fossils in the past decade, as paleontology swings from the adventurous science of digging up dinosaurs to the laboratory drudgery of identifying conodonts and other microfossils.

Despite their size, microfossils have uses, particularly to the oil geologists, for they tell the driller when he has gone past where the oil should have been. In another hole, microfossils contained in the slender cores of stone removed from the hollow drill tell the geologist where he can expect a productive oil or coal horizon. Big fossils, the brachiopods and trilo-bites, are too rare to be conveniently centered in a drill core where the geologist happens to break it. So he must rely on fossils that are widespread, extremely common, and lived only during short periods of geologic time. These are the three qualities of an index fossil, which, when found, date and locate that rock layer. Thus, in the interest of higher profits to the mining and oil industries, micropaleontology has been highly developed.

Representatives of all major groups of the larger macrofossils are also found as microfossils. Many juvenile forms of larger invertebrates, such as crinoids and trilobites, are microscopic. Fascinating collections of these juvenile forms can be made and matched with their grown-up counterparts. Thousands of species of diminutive pelecypods and snails lived on the ocean floor. Brachiopods contain a few small members, as do the cephalopods and corals. Only the dinosaurs and large mammals have no real microfossils, except for thin sections of their bones, but the smaller mammals and fish have micro-parts, such as teeth, bones, and scales. There are several large groups of animals that have left no fossils except microfossils, notably the conodonts, ostracods, and foraminifera.

CONODONTS

Conodont fossils resemble a strange jawbone set with modernistic teeth. The largest run a dozen to the inch; most are half that size, still visible enough to the naked eye so that they can be picked up on the tip of a needle and mounted on a slide or can be seen on the surface of a slab or loose in the field. They are composed of solid calcium phosphate, making them a bit heavier than the quartz, dolomite, or calcite sediments in which they are found.

For quite a while conodonts were in a never-never land of classification, believed to be tooth structures of fish, worms, cephalopods, crustaceans, or snails, depending on whom you talked to. Because conodonts were often found along with vertebrate marine fossils, such as those of fish, many paleontologists tried to place them in some vertebrate order. But the elusive conodonts never came attached to any animal or even part of an animal, although they exist by the millions in almost every Paleozoic sedimentary rock.

At the 1969 meeting of North American paleontologists at the Field Museum in Chicago, the real conodont appears to have stepped forward to reveal its identity. In a surprise program presented by William G. Melton of the University of Montana, the fossil of a strange wormlike creature two to four inches long with conodont structures inside it was shown to the assembled scientists. The conodont animal, if it is such, was described as a soft-bodied, bottom-dwelling organism found with other coal-age organisms, mostly fish, in the Little Snowy Mountains of Montana. Whether it is the conodont animal, with a food-grinding mechanism in its midparts, or just a worm that swallowed a conodont assembly, has provided the paleontologists with a challenging new problem that may solve the old problem of the conodont's identity.

Conodont fossils are found in rocks ranging from Ordovician to Trias-sic, primarily in shales rich in organic matter, but are also found in some sandstones and limestones, particularly those interbedded with thin layers of shale. Some gray shales contain as many as 500 conodonts a pound, but the average for shale or limestone is more like 10 to 50 a pound.

Scolecodonts somewhat resemble conodonts but are composed of chiti-nous or horny material, similar to a fingernail. These are definitely worm jaws, found on occasion in association with fossil worms. They are not nearly as common and widespread as the conodonts, but some Paleozoic rocks do contain a fair number of them.

Leperditia

Left to right: Leperditia sp., a crustacean about 3 mm, Ordovician through Pennsylvanian; scolecodont, worm jaw about 1/2 mm, ranging from Cambrian to Recent; conodont, toothlike microfossil about 1/2 mm, ranging from Ordovician to Triassic. (This and the following drawings by Betty Crawford)

OSTRACODS

Ostracods are tiny fossils that look exactly like an ornamented clam, complete with both valves. The largest members of this widespread and rather common group of fossils border on macrofossil size, a few being the size of a small bean, though most are in the size range of the conodonts, 1/10 inch or less in length. Unlike the conodonts, which disappeared sometime in the Triassic, the ostracods first left their fossil record in the Ordovician and are still swimming about today.

Ostracods are members of the class Crustacea, subclass Ostracods. This makes them small brothers and sisters to lobsters, shrimp, and trilobites and no relation at all to clams or brachiopods, which they closely resemble. If one thinks of the ostracod shell as being like the chitinous covering of the crab, lobster, or shrimp, though divided strongly down the middle to form a top and bottom, with most of the organs tucked inside, it is easier to recognize ostracods as relatives of the other crustaceans. They are not bottom crawlers like crabs or lobsters but swim in both fresh and salt water.

Actually, the sea is full of tiny crustaceans, but few of these leave a fossil record because their protective shell disintegrates easily. It is made of organic material rather than the stony calcium carbonate of the brachio-pods and clams. Ostracods deposit a layer of calcium carbonate beneath their more typical crustacean outer shell, and it is this part that becomes the typical ostracod fossil. The fossil shells are clamlike in shape, ornamented with bumps, ridges, or projections, generally light in color, and are found in vast numbers in some rock layers.

Any fossil-bearing layer of limestone or shale, but particularly limestone, is likely to contain numbers of these little creatures. Since they dissolve in acid, they cannot, like the conodonts, be removed from a calcareous matrix. Sometimes, by sheer luck, they will be found exposed on top of a limestone slab or weathered free in the soil. Shales containing ostracods can be disintegrated to release the specimens. The fossil record contains a vast number of species of these little things, which are widely used in rock dating, as are the conodonts.

FORAMINIFERA

Foraminifera are also found in the washings of shales, or more commonly, lying loose by the thousands near weathering limestones and chalks. They are generally in the category of microfossils, though a few very large specimens are the size of a coin. One even reaches a diameter of four inches, but most are about the size of a grain of rice or less. Forams, as they are usually called, are members of the phylum Protozoa and are found in prodigious numbers in present-day seas. Modern descendants live only in salt water, though some are found in brackish water that contains only a trace of salt. A few adventurous types live far underground in the slightly saline groundwater of wells in central Asia and northern Africa. It appears that all forams found as fossils were ocean dwellers.

The first fossil forams are found in the Ordovician, but they are not particularly common until they burst forth with tremendous prodigality in the Pennsylvanian and Permian periods. They are not rare as fossils in any rocks since the late Paleozoic but are again found in explosive numbers in Cretaceous rocks, sometimes making up the bulk of thick layers of limestone. In general, the post-Paleozoic forams are larger than their early counterparts.

Forams built outer skeletons (called "tests") of several hard materials. These are ideally suited to becoming fossilized. Most of these little animals built tests of calcium carbonate, but a few, particularly the early ones, built a test of carefully selected sand grains, sponge spines, or even tests of smaller forams. Each species was fussy about its building blocks, selecting only one type of sand (quartz grains, calcite grains, or even mica grains) or one particular shape of sponge spicule or foram skeleton.

The forams that constructed their houses of such carefully selected building blocks look like a rather baggy lump, usually rounded but some-

Three foraminifera. Left to right: Hantkenina, 1/2 mm, ranging from Eocene to Miocene; typical fusulinid, side view and cross section, about the size of a wheat grain, ranging from Mississippian to Permian; Nodosaria, 2 mm, ranging from Triassic to Recent.

times irregular or elongated. The calcareous ones are far more interesting to the micro-collector because they have definite shape. The typical calcareous foram looks like a miniature ammonite, especially in cross section, with a spiral arrangement of chambers. Some look like a rattlesnake rattle or a rose. They cannot be mistaken for any other microfossil.

Fusilinids

One special group of forams —the fusilinids— were abundant during the Pennsylvanian and Permian and then became extinct. The fusilinids, members of the family Fusilinidae, look like grains of rice on the outside and have a series of tiny chambers in a spiral on the inside. Most are about the size of a grain of rice. Chert filled with these tiny fossils is known as "rice agate" in Iowa and is used as a gemstone. Some species are larger in size, up to half an inch in length, but retain the rice-grain shape. These fusilinids are common in some rock layers lying in a belt from Iowa to Texas, where they weather out by the millions. They are particularly at home in limestones but are not common in shales.

RADIOLARIANS

The radiolarians are not widespread as fossils in the United States, but in some areas they are abundant and particularly show microfossils. Like the forams, they are members of the phylum Protozoa, and the order Radio-laria. Of all microfossils, they have the widest range in time, from the pre-Cambrian to recent times, though in this country they are found commonly only in Devonian cherts of Texas and California and in Jurassic rocks of California. The radiolarian skeleton is made of silica and, like the snowflake, often has threefold or sixfold symmetry, with projections also much like a snowflake. Others looks like a symmetrical wishbone. A few look like a cross or the head of a medieval mace. These creatures can be found in silica rocks, particularly quartzites and sandstones.

DIATOMS

Rather similar to the radiolarians in shape, but far more intricate, are the dazzling beauty queens of the microfossil world, the diatoms. They, too, have a skeleton of silica, but of opal rather than quartz. Opal is an unstable form of silica, and their skeletons were often damaged or destroyed as the opal changed to chert. Where they are well preserved, as in thick beds in California, the diatomaceous earth is mined for use as filters in such industries as brewing.

The diatoms make excellent filters because their incredibly intricate lacy skeletons are full of microscopic holes. Typically rounded if from marine deposits and elongated if from fresh water, the tiny skeletons have in places piled up into a solid layer of rock many feet thick. No fossil diatoms have been found in rocks older than Upper Cretaceous, but because they were quite intricate and highly evolved by this time it is suspected that their origin goes back much further in time.

Diatoms are plants, not animals: strange plants that still live in both fresh and salt water. The lacy opal skeleton is the support of the tiny plant, which is visible only under high magnification. The fossils are symmetrical and have thousands of tiny openings, always in matched pairs, piercing the clear opaline matrix. They seem to try to outdo each other in complexity, and many are somewhat reminiscent of snowflakes.

Because they are so tiny, diatoms are not likely to be picked up in the field except where a layer of diatomaceous earth is being mined. It is a simple matter to take a chunk of this white substance, crumble it, and examine the dust for desirable specimens. A piece the size of a fist may contain not hundreds, or thousands, but millions of diatoms. They are too small for an amateur collector to mount singly, but a little sifting of the dust on a slide should provide hours of fun at the microscope.

Quaternary Plant Microfossils

Left to right: a radiolarian, ranging from Cambrian to Recent; a diatom, ranging from Jurassic to Recent; a coccolith. These microfossils are all greatly magnified.

OTHER MICRO FOSSILS

Other fossils are found solely as microfossils. Some are valuable tools for the professional paleontologist, telling him not only the age of the rock but what the climate, or sea temperature, or oxygen level of the water was at that time. Fossil spores and pollen are now widely studied, but this field is of little interest to the amateur collector.

Although every major group of invertebrates and plants has some species that can be considered microfossils, there are strange fossil occurrences of normally large-sized creatures found instead as small, but apparently mature, miniature versions of themselves. A Lilliputian community of this kind is called a depauperate fauna. The reason for the communal disregard for normal size is not at all clear. Such depauperate areas are not large but are widely scattered in time and space. Famous depauperate zones are found in the Ordovician of Iowa and the Mississippian of Indiana. In both places microfossils of cephalopods, snails, and several other types can be collected, many only a quarter normal size.

COLLECTING AND PREPARING MICROFOSSILS

Except for these depauperate zones in otherwise normal fossil-bearing localities, there is no particular place to look specifically for microfossils. They can be found in most rocks that carry bigger fossils, since the small

Collecting microfossils is best done horizontally. Dr. Dwayne Stone demonstrates the proper position. A small plastic bucket is an ideal container for this kind of collecting. (Photo by Betty Crawford)

creatures lived among the larger ones. There are some rock layers that have only microfossils, particularly those of such organisms as diatoms or protozoa. In fact, there are few marine sedimentary rocks that do not contain some microfossils.

Collecting these little fellows can be done by hands-and-knees scrutiny of weathered shales and limestones in the field, popping the tiny fossils into a pill bottle. This will do for some larger microfossils; but to make a real collection, chunks of unweathered rock must be disintegrated to release their tiny captives. This may be done by crushing the rocks, washing the residue, and inspecting it for unbroken individuals. Some shales fall apart easily when alternately wet and dried or chemically treated. A few species of microfossils not composed of calcium carbonate can be released by dissolving blocks of limestone with acid. The resulting sludge is washed, dried, and spread thin on a microscope slide for sorting.

Shales of dark color are likely to contain conodonts and scolecodonts, particularly if there is much organic material in the shale. Gray shales are often the home of small invertebrates or juvenile forms of larger animals. Many sheety black shales cannot be disintegrated, but most shales will crumble into their original silt and clay if properly treated.

Cleaning

Boiling with the Quaternary-O, as described in Chapter IX under cleaning techniques, will release fossils of conodonts and other mini-creatures as well. If this chemical is not available, satisfactory results can be obtained by gently boiling the shales in water in which trisodium phosphate or sodium hydroxide has been added. The shales will turn into mud, releasing the microfossils.

The disaggregated shale is bulky compared to its fossils. Before pouring off the water used to boil the shale, examine its surface carefully for floating fossils. Some forams are hollow and bob about like corks. They could easily wind up in the sewer instead of the cabinet. Then the chemicals can be poured off and the muddy residue flushed several times with fresh water. The water should be allowed to remain still for at least a few seconds before it is poured off, in order to allow the fossils to settle to the bottom. If this washing is done carefully, much of the clay still in suspension can be poured away with the water without losing too many fossils. Pouring this clay-laden water down the sink may stop up the drains or sewer. It is better to pour the muddy water into a bucket and then on the ground.

When it seems inadvisable to pour off any more water, the residue should be washed with distilled water. Most water contains some dissolved minerals which would be deposited around the tiny clay grains and microfossils when the water evaporates. For a tiny fossil this mineral water acts as an effective glue, cementing it to fellow fossils, mud grains, or the side of the container. Fragile fossils so fastened will break in pieces before coming loose. But distilled water removes most of the chemical glue. For faster drying, the residue can then be soaked with alcohol or acetone after the washing with distilled water.

The final drying can be done on a sheet of metal such as a cookie sheet, or on a piece of filter paper or smooth white paper. Do not force-dry the mud. If it is heated it will not only splatter, but tiny fossils may explode when the water included with them turns to steam.

When the sediment has dried thoroughly, it can be spread thin, a small amount at a time, on a microscope slide and examined under low power for fossils. These can then be removed for mounting.

Limestones can be dissolved to release conodonts and scolecodonts, both of which are impervious to gentle acids such as 10 to 15 percent acetic acid. Pyritized or silicified microfossils obtained by dissolving the host rock are much better specimens than those obtained from the surface of a natural exposure. The pieces of limestone are placed in plastic or glass bowls and covered with the acid. The acid is refreshed when it appears

Disintegrated shale spread on white paper is examined for microfossils under a low-power microscope. Pointer touches a Pennsylvanian brachiopod from Danville, Illinois.

that its action has stopped after some hours or days, depending on the quantity of stone and acid. Be careful not to move the stone when pouring in new acid; tiny fossils protruding from the matrix at that point can easily be broken. Do not cover the bowl tightly. Carbon dioxide gas is produced during the destruction of the limestone and can build up dangerous pressure.

The sediment remaining after acid treatment of limestone must be carefully washed with many changes of water. There is usually not much excess sediment, such as there is with shales; so the water should settle for at least fifteen seconds between changes to allow all fossils to sink to the bottom. A pinch of sodium bicarbonate tossed in the water of one of the last changes and allowed to remain for a few minutes will neutralize any acid remaining in the fossils. From this point, the residue is treated like the shale residues, washed in acetone or alcohol, and spread out to dry.

Sorting Microfossils

Commercial sorting techniques have developed among those who spend most of their lives looking for microfossils in drill cores. But they are beyond the means of the amateur collector. Conodonts are composed of calcium phosphate, which makes them a bit heavier than clay or limestone particles, and they can be separated by using heavy liquids to float off the lighter sediment, which is useless. Conodonts are also weakly magnetic and have been successfully sorted out by jiggling the residues down a slight slope; a strong magnet on one side pulls the conodonts toward it and feeds them into a separate chute.

When thoroughly dry, the dusty remains of the amateur's washday are spread out thinly on a glass slide and examined under a low-power microscope or a powerful hand lens. When an outstanding or unusual specimen is found, it is transferred to a special microfossil mounting slide. This kind of slide is designed to hold a piece of black, ruled paper (white for dark-colored specimens) in a little well, with room above for a cover slide that will keep out dust and prevent stray fingers from dislodging the fossils. One such slide can hold several dozen specimens. A homemade mounting slide can also be used, but unless the specimens are protected they are likely to pop off and get lost. On ruled-paper mounting slides, a code number can be written next to each specimen for identification. It is also possible to mount several specimens atop a cork in a standard micromount box.

The microfossils are most easily removed from the debris on the sorting slide with the moistened tip of an artists' paintbrush. They can then be transferred to their permanent home and placed on a tiny spot of glue to hold them down. Several water-soluble glues are used professionally, but the amateur collector can cement the tiny things with the glue from gummed paper tape such as is used for sealing boxes. If an artists' brush is moistened, passed along the glue side of the tape, and then touched to the mounting spot of the fossil, the mucilage remaining on the mount will be sufficient to hold the fossil. This method works well for small, light microfossils. Heavier cements such as Duco or Elmer's are too bulky to work well. Professional paleontologists often attach the specimens with a solution of gum tragacanth.

Identifying Microfossils

Microfossils are identified just like their bigger relatives. There will be much difficulty in identifying juvenile forms of crinoids and trilobites, because these creatures are not commonly illustrated in texts available to the amateur. Professional journals and papers dealing with the locale where the fossils were collected may be of help. The easiest way to specialize in juvenile forms and identify them is to find a paper written about the juvenile fossils of a specific locale, then go to that locale and collect. Such articles appear occasionally in the Journal of Paleontology.

Young individuals rarely have more than a slight resemblance to their parents, but depauperate forms have the form and shape of their normal counterparts on a smaller scale. They can be identified with standard texts, as can those fossils that are normally found only as microfossils. Lately a wealth of material has been written about conodonts, ostracods, and other commercially useful microfossils. State Geological Survey bulletins and such magazines as the Journal of Paleontology are good sources of locales and identification for these tiny creatures.

Microscopes

As with micromount collections of minerals, the best viewing device for microfossils is the binocular microscope, which allows a three-dimensional view. A high-powered scope is not necessary; most microfossils need only about ten to sixty times magnification. Inexpensive (under $15) binocular scopes imported from Japan will do a satisfactory job, and a secondhand binocular dissecting microscope, such as is used in school biology classes, can sometimes be bought for $50. When purchased new, large binocular scopes (the Japanese ones are small) are rarely available for less than $100, and most cost more than $200.

A monocular scope is usually cheaper and gives satisfactory results. New imported small scopes cost $10, and a satisfactory used one of a larger type can be found for $25.

Most microfossils are not transparent. They have to be lighted from above rather than from underneath. Most microscopes have either a light under the stage that holds the slide or a mirror underneath that reflects light from an outside source. Since such devices are useless for most microfossils, a special light that gives an intense small light from above will be needed. Such a light can be purchased, but it is cheaper to buy a small high-intensity lamp that uses six or twelve volt bulbs. These little lamps usually are jointed so that they can be twisted to shine the light just where it is needed. The light should not be left burning too long above a specimen, or the heat it creates may pop the fossil from its mounting.

Storing

Prepared slides with microfossils can be stored in standard slide files obtainable at any store that sells microscopes. Since most slide files are designed to hold a slide vertically and since microfossil slides must be stored horizontally, the collector should either buy a horizontal holder or

Microfossils that are not too small can be mounted in a micromount box. This ray tooth is mounted on top of a blackened tooth pick. Identification is written on a label on the lid keyed by number to the box.

store a vertical one on edge. The tiny spot of glue holding a microfossil may break loose when the slide is vertical, and the fossil will be left hanging out in space. These slides should always be handled with utmost care.

Slightly larger microfossils can be mounted individually in a standard micromount box such as the kind used for minerals. These boxes are obtainable from most mineral supply dealers; some are made of black plastic, others of clear plastic that must be painted flat black to cut down light reflection. The fossil is mounted on a small cork or atop a toothpick or thin balsa rod, also painted black. A plastic cement such as Duco or sili-cone rubber can be used to cement the specimen to the mounting rod. The name of the specimen and locale are written on a label attached to the plastic cover of the box, and the boxes are stored in shallow drawers.

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Responses

  • hope
    What type of microfossils can be found in shale with small white dots on it?
    1 year ago
  • miley
    How to collect microfossils?
    10 months ago
  • Tammy
    How can we collect and make thin section of microfossils under microscope?
    9 months ago

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