Identifying Fossils

Excerpt from Introduction to Fossil Collecting

(C) 1994-2004, Glen Kuban,

Part of Kuban's K-Paleo Place home page

The first question to ask about a specimen is whether it really is a fossil. Certain markings on a rock may resemble fossils even though they are only inorganic or geologic features. These are known as pseudofossils (false fossils). Examples include dendrites (frilly, fern-like patterns caused by chemical activity in rocks), odd shaped nodules or concretions which may resemble some organism, and erosional markings (sometimes mistaken for fossil footprints). Some pseudofossils have even fooled experts, so don't be overly embarrassed if you accidentally collect one.

Next, try to determine the general group of animals or plants to which your specimen belongs. Most fossils fall into one of several common groups briefly summarized below. Further identification will be a matter of narrowing the search to a more specific level, such as the genus or species (discussed later). When collecting with other people, chances are that someone will have a good idea what the fossil is. If no one does, all the better--you may have found an unknown creature!

In many cases you will find only part of a fossil organism. Try to envision what part it may represent. Many books on fossils describe the detailed anatomy of various fossil organisms. Don't be intimidated by anatomic jargon; with time you will pick it up. Some of the terms are just technical versions of common words. For example, a trilobite's cephalon is its head; it's pygidium is the rear end or "tail."

The majority of fossils are aquatic forms--plants, animals, and microorganisms that lived in ancient seas, lakes, or other bodies of water. This should not be surprising, since most of the earth's surface is covered with water, and water creatures are more likely to be buried than land creatures. Of the marine fossils, most are invertebrates (animals lacking backbones).

Of invertebrate fossils, the vast majority belong to the following major groups: mollusks (clams, oysters, snails, and cephalopods); brachiopods (superficially clam-like creatures, most of which are extinct); echinoderms (spiny-skinned sea animals such as starfish, sea urchins, and crinoids or "sea lilies"); corals (tentacled sea animals that live in hard skeletons, often in a colonial fashion); sponges (relatively simple, porous-bodied sea animals with many living and extinct species); bryozoans (small, coral-like animals); and arthropods--a large group of animals with jointed legs and external skeletons. Major arthropod groups include crustaceans (such as shrimp, lobsters, and crabs), insects, and trilobites--an extinct group of marine arthropods which superficially resemble horseshoe crabs, and which once roamed the seas in large numbers and many varieties.

Less common invertebrate fossils include soft-bodied animals such as worms and jellyfish. However, it is not uncommon to find burrows and trails of ancient worms and other soft-bodied creatures, as well as the tracks and trails of various animals that squirmed, crawled, or walked on ancient sediments, or made burrows, nests, or dens in them. In some localities such trace fossils or ichnites are more common than body fossils. Moreover, many animals are known only from their tracks.

Fossils of vertebrates (animals with backbones) are generally less common than invertebrates, since there were fewer of them, and many vertebrates lived on land, reducing the chances of burial. Vertebrates include fish (many extinct and living forms), amphibians (salamanders, frogs, and various extinct types), reptiles (scaly animals such as lizards, snakes, and dinosaurs), mammals (animals that suckle their young with milk, and possess fur or hair, such as horses, cats, elephants), and birds. Despite their general rarity, vertebrate fossils are fairly common in some localities. For example, in the Green River Formation of Wyoming, large numbers of Eocene fish fossils are found.

Plant fossils are commonly found as natural casts, replacements, impressions, or compressions. Because a whole plant is rarely found, form genera names have been applied to the parts themselves (which may include branches, stems, leaves, roots, or seeds). Coal is largely made of the compressed parts of large ferns and other plants that thrived in lush prehistoric forests. Reproductive structures such as seeds and pollen are often important to the identification and classification of fossil plants.

Scientists use a formal method called systematics or taxonomy to classify both ancient and modern organisms. This system was first used by Swedish naturalist Carl Linnaeus in the mid-1700's, and a modern version is used by scientists the world over. In this system, organisms are classified to a number of different levels. The highest (broadest) taxonomic category (or taxon) is Kingdom, followed by phylum,1 class, order, family, genus, and species. Each level can have intermediate divisions, such as subphylum. Humans belong to the Phylum Chordata, class Mammalia, order Primates, family Hominidae, genus Homo, and species Homo sapiens. An individual specimen may be identified even more precisely to the levels of subspecies, race, and gender.

Taxonomic naming or nomenclature follows certain rules and conventions. For example, a species name should be underlined or italicized, and always includes two parts, starting with a capitalized genus name. Taking for example the trilobite Phacops rana, Phacops is the genus or generic name, and Phacops rana the full species name. Phacops sp. refers to an unidentified species of the genus Phacops. Most taxonomic names are derived from Latin or Greek roots. At the level of species, the assigned name often describes or refers to some aspect of the organism, the fossil site, or the discoverer. For example, Homo means "man"; sapiens means "wise" (in some cases the names are more honorary than precise). In technical publications a species name is often followed by parentheses enclosing a person's name and a year. The name is the person who first described the species; the year indicates when the description was first published.

After determining the general group (normally a class or order) to which your fossil belongs, try to narrow the identification further, with the ultimate goal being to determine the exact species, or at least the genus. However, even the genus may be difficult to pin down if the fossil is not a common one. You may need to compare many illustrations and descriptions in books and articles on fossils. If you know the geologic period of the site your specimen came from, it may help focus the search, since many books list the known time span of the illustrated fossils. Look for any unique or unusual features (however small) which may tend to separate similar species. A close resemblance between your fossil and an illustration will not guarantee an exact match at the species level, but will suggest you have found the correct genus or family.

Try to use the most thorough and up-to-date literature available, since the fossil classifications and names are frequently revised and expanded.2 Some books include identification keys--a series of branching choices designed to progressively narrow down the identification. Even with such aids, an identification may be only tentative, due to any number of complicating factors, such as similar appearances among different species, distorted or incomplete fossils, ambiguous descriptions and illustrations, classification or nomenclature revisions, and even disagreements and mistakes among different workers.3 However, this is all part of the challenge of fossil collecting. Veteran collectors and paleontologists are usually glad to assist you in identifying a fossil, although even they are sometimes stumped!

The following section further describes the major groups commonly found as fossils. For each group several examples are illustrated, although many more actually exist. Those presented often include very common forms (especially in the Eastern U.S.), along with some unusual forms indicating the diversity of size and shape within each group.

Although terms relating to the specific groups are explained as they arise in the next section, it may be useful to quickly review some common terms applied to many different groups (most of which refer to the organism's lifestyle or anatomy). Depending on the animal involved, the hard parts are referred to as a skeleton, shell, or a test (the latter often used in connection with small organisms). When describing the composition of these hard parts, the term calcareous or limey means primarily composed of calcium carbonate. A siliceous shell is made of silica--the major component of sand or quartz, whereas a chitinous or horny shell is composed of chitin--a hard, organic substance similar to the material that makes up your finger nails. Appendages are any extensions of the body, such as arms, legs, or antennae. Articulated means connected with hinges or joints. Bivalved means having a two-part shell. Aquatic creatures live in water (whether fresh- or saltwater); marine specifically refers to life in the oceans. A benthonic organism lives on the ocean floor, whereas planktonic organisms drift or float in the water. A sessile animal is anchored in one place. A carnivorous animal (carnivore) eats meat; herbivores feed on plants. A filter-feeder eats by straining small organisms or other particles from the water. Extinct means a group that has entirely died out; extant refers to a group that is still living.

Fossil measurements are usually given in the metric system, since that is the system used worldwide by scientists. It is a very efficient system, since all metric units are multiples of ten. It may help to remember that one meter (m) is roughly 39 inches (a little over 3 feet), that a centimeter (cm) is one tenth of a meter (it takes 2.54 cm to make an inch), and that a millimeter (mm) is a tenth of a centimeter (it takes 254 mm to make an inch).


1. For plants, some workers use the term "Division" for a taxonomic level roughly equivalent to Phylum.

2. Unfortunately, some of the most detailed treatises on fossils are not often updated (due to the effort and time needed to produce new versions), but they are still very useful due to the sheer numbers of fossils described. Two such works are Index Fossils of North America by Shimmer and Shrock, and Treatise on Invertebrate Paleontology, R. C. Moore, ed., published in a series of volumes, each covering a different invertebrate group. (See the bibliography for full citations). 3. The formal rules governing taxonomic nomenclature are established in the International Code of Zoological Nomenclature. Nevertheless, it is sometimes difficult to sort out complex naming issues, and code has been revised a number of times.