Fossil Collecting Glossary

A reference guide to the terminology you’ll encounter in fossil collecting, paleontology, and geology. Terms are organized alphabetically for quick lookup.

A

Amber Fossilized tree resin that can preserve insects, plant material, and other small organisms in remarkable detail. Most commercial amber comes from the Baltic region or the Dominican Republic, and ranges in age from about 20 to 50 million years old.

Ammonite An extinct group of marine cephalopod mollusks with coiled, chambered shells. Ammonites are among the most recognizable and widely collected fossils, ranging from the Devonian to the end of the Cretaceous period. Their rapid evolution makes them excellent index fossils for dating rock layers.

Articulated Describes a fossil skeleton or multi-part organism found with its bones, shells, or segments still connected in their natural arrangement. An articulated fossil is much rarer and more scientifically valuable than a disarticulated one.

B

Bedding plane The surface separating two layers (beds) of sedimentary rock. Fossils are often found along bedding planes because organisms were buried on ancient sea floors or land surfaces that became these boundaries. Splitting rock along bedding planes is a common collecting technique.

Biostrome A sheet-like accumulation of fossil organisms, such as a coral or crinoid bed, that does not form a mound or reef shape. Biostromes are often rich collecting targets because they represent concentrated accumulations of marine life.

Body fossil The preserved remains of an actual organism, as opposed to traces of its activity. Shells, bones, teeth, leaves, and petrified wood are all body fossils. Compare with trace fossil.

Brachiopod A marine invertebrate with two shells (valves) that superficially resembles a clam but belongs to an entirely different phylum. Brachiopods are among the most common fossils found in Paleozoic-age rocks across the US, particularly in the Midwest and Appalachian regions.

Bryozoan Tiny colonial marine animals that build branching, lacy, or encrusting structures from calcium carbonate. Bryozoan fossils are extremely common in Paleozoic limestones and often look like small mesh-like fragments, tiny branching twigs, or honeycomb-textured rock surfaces.

C

Calcite A calcium carbonate mineral (CaCO3) that makes up limestone and is the primary material in many fossil shells and skeletons. Calcite dissolves in acid, which is why you should never use vinegar on calcite fossils.

Calyx The cup-shaped main body of a crinoid, composed of interlocking calcite plates. Complete calyxes are much rarer and more prized than the commonly found crinoid stem segments.

Cambrian The first period of the Paleozoic Era, spanning roughly 539 to 485 million years ago. The Cambrian is famous for the “Cambrian Explosion,” a rapid diversification of animal life that produced the ancestors of most modern phyla, including the first trilobites.

Carboniferous A geologic period spanning roughly 359 to 299 million years ago, divided in North America into the Mississippian and Pennsylvanian sub-periods. Carboniferous rocks are famous for coal deposits and abundant marine invertebrate fossils, including crinoids, brachiopods, and bryozoans.

Cast A fossil formed when a mold (cavity left by a dissolved organism) is filled with sediment or mineral material, creating a three-dimensional replica of the original organism’s shape. Compare with mold.

Cephalopod A class of marine mollusks including modern octopuses, squids, and nautiluses, as well as extinct ammonites and nautiloids. Fossil cephalopods are common in many formations and range from small straight-shelled forms to coiled ammonites several feet in diameter.

Chisel A flat-bladed hand tool used in fossil collecting to split rock along bedding planes or carefully remove matrix from around specimens. Cold chisels (designed for stone) in various widths are a staple of any collector’s toolkit.

Concretion A hard, rounded mass of mineral matter that forms within sedimentary rock, often around a nucleus like a fossil, shell, or organic fragment. Concretions are frequently mistaken for fossils or dinosaur eggs but are purely geological in origin — though they sometimes contain fossils inside.

Consolidant A liquid adhesive or hardener applied to fragile fossils to strengthen them and prevent crumbling. Paraloid B-72 dissolved in acetone is the most widely recommended consolidant because it is reversible, chemically stable, and museum-approved.

Coprolite Fossilized dung. Coprolites are classified as trace fossils and can provide valuable information about the diet and digestive systems of ancient animals. They range from tiny pellets to large masses and are found in both marine and terrestrial sediments.

Cretaceous The last period of the Mesozoic Era, spanning roughly 145 to 66 million years ago. The Cretaceous ended with the mass extinction event (K-Pg boundary) that wiped out the non-avian dinosaurs. Cretaceous fossils include ammonites, mosasaurs, and many shark species.

Crinoid A marine animal related to starfish and sea urchins, also known as “sea lilies.” Crinoid stem segments — small, round, disc-shaped fossils with a hole in the center — are among the most abundant fossils in Paleozoic-age limestones throughout the central and eastern US.

D

Devonian A period of the Paleozoic Era spanning roughly 419 to 359 million years ago, sometimes called the “Age of Fishes.” Devonian rocks are rich in brachiopods, corals, crinoids, trilobites, and early fish fossils, and are common collecting targets in New York, Ohio, and the Great Lakes region.

Disarticulated Describes a fossil skeleton or multi-part organism found with its component parts separated and scattered. Most fossils are disarticulated because decay and scavenging typically break organisms apart before burial.

Dolomite A carbonate mineral and rock type similar to limestone but containing magnesium. Dolomitization can alter or destroy fossils originally preserved in limestone, which is why some carbonate outcrops are fossil-rich and others nearby are barren.

E

Echinoid A class of echinoderms that includes sea urchins, sand dollars, and heart urchins. Echinoid fossils are common in Cretaceous and younger sedimentary rocks and are recognized by their round shape and characteristic five-fold symmetry pattern.

Eon The largest division of geologic time. Earth’s history is divided into four eons: the Hadean, Archean, Proterozoic, and Phanerozoic. Nearly all fossils come from the Phanerozoic Eon, which began about 539 million years ago.

Eocene An epoch of the Paleogene Period, spanning roughly 56 to 34 million years ago. Eocene deposits yield important mammal fossils and well-preserved plant and insect specimens, especially in formations like the Green River in Wyoming.

Epoch A subdivision of a geologic period. For example, the Miocene, Pliocene, and Pleistocene are epochs within the Neogene and Quaternary periods. Epochs are commonly used when discussing younger fossils like shark teeth and mammal bones.

Era A major division of geologic time, smaller than an eon. The three eras of the Phanerozoic Eon — Paleozoic, Mesozoic, and Cenozoic — are defined by major shifts in the fossil record and bounded by mass extinction events.

Exoskeleton A hard external covering that provides support and protection for invertebrate animals such as trilobites, insects, and crustaceans. Trilobite exoskeletons are composed of calcite and preserve exceptionally well, making them some of the most detailed invertebrate fossils.

F

Formation A formally named, mappable unit of rock with distinct characteristics. Formations are the basic unit of geologic mapping and are essential for fossil collectors because they tell you what age the rocks are and what fossils to expect. Examples include the Morrison Formation (Jurassic dinosaurs) and the Cincinnatian Series (Ordovician invertebrates).

Fossil The preserved remains or traces of ancient life, typically older than about 10,000 years. Fossils form through various processes including mineralization, compression, impression, and amber encapsulation. They are found almost exclusively in sedimentary rock.

Fossil park A publicly accessible site, often managed by a local government or educational institution, specifically designated for fossil collecting by the general public. Examples include Fossil Park in Sylvania, Ohio, and Devonian Fossil Gorge in Iowa.

G

Gastropod A class of mollusks that includes snails and slugs. Fossil gastropods are recognized by their coiled, often spiraling shells and are common in marine, freshwater, and terrestrial sedimentary rocks of all ages.

Geologic map A map showing the distribution of rock formations, faults, and other geologic features at the earth’s surface. Geologic maps are invaluable for fossil collectors because they reveal the age and type of rock in any given area, which predicts what fossils you might find.

Geologic time scale The system used to divide Earth’s 4.6-billion-year history into hierarchical units: eons, eras, periods, epochs, and ages. Understanding the geologic time scale helps collectors date their finds and understand the environments that produced them.

Graptolite An extinct colonial marine organism that built organic skeletons, often preserved as dark, saw-blade-like impressions on shale surfaces. Graptolites are common in Ordovician and Silurian rocks and are important index fossils.

H

Hand lens A small magnifying glass, typically 10x magnification, used in the field to examine fossils, mineral grains, and rock textures up close. A hand lens is one of the most useful and inexpensive tools in a fossil collector’s kit.

Horn coral An informal name for rugose corals, a group of solitary or colonial corals common in Paleozoic rocks. Solitary horn corals have a distinctive cone or horn shape and are very common in Ordovician, Silurian, and Devonian limestones. They are among the easiest fossils for beginners to recognize.

I

In situ Latin for “in place.” Describes a fossil that is still embedded in its original rock layer, as opposed to one that has been weathered out or transported. Recording whether a fossil was found in situ is important for scientific documentation.

Index fossil A fossil species used to define and identify a particular geologic time period. Good index fossils are widespread, abundant, easily recognized, and existed for only a short time span. Ammonites and graptolites are classic examples.

Invertebrate An animal without a backbone. Invertebrate fossils — including trilobites, brachiopods, crinoids, corals, mollusks, and echinoderms — make up the vast majority of fossil finds and are generally legal to collect on US federal public land under casual collecting rules.

J

Jurassic The middle period of the Mesozoic Era, spanning roughly 201 to 145 million years ago. The Jurassic is famous for large dinosaurs like Allosaurus and Stegosaurus, but Jurassic rocks also yield abundant marine invertebrates, fish, and plant fossils.

K

K-Pg boundary The geologic boundary (formerly called the K-T boundary) marking the end of the Cretaceous Period and the beginning of the Paleogene Period, approximately 66 million years ago. This boundary corresponds to the asteroid impact that caused the extinction of non-avian dinosaurs and roughly 75% of all species.

L

Lagerstatte (plural: Lagerstatten) A German term for a sedimentary deposit with exceptionally well-preserved fossils, often including soft tissues not normally fossilized. Famous examples include the Burgess Shale in Canada and the Solnhofen Limestone in Germany.

Limestone A sedimentary rock composed primarily of calcium carbonate, often formed from the accumulated shells and skeletons of marine organisms. Limestone is one of the most productive rock types for fossil collecting and is widespread across the US, especially in the Midwest and Southeast.

M

Matrix The rock material surrounding or enclosing a fossil. Removing matrix from around a fossil is called “preparation” or “prepping.” The type of matrix (soft shale vs. hard limestone) greatly affects how difficult preparation will be.

Mesozoic The geologic era spanning roughly 252 to 66 million years ago, comprising the Triassic, Jurassic, and Cretaceous periods. Often called the “Age of Reptiles,” the Mesozoic is defined by the dominance of dinosaurs on land and large marine reptiles in the oceans.

Miocene An epoch of the Neogene Period spanning roughly 23 to 5 million years ago. Miocene marine deposits along the US Atlantic and Gulf coasts are rich sources of shark teeth, whale bones, and other marine vertebrate and invertebrate fossils.

Mold A fossil formed as an impression or cavity in rock where an organism once sat. An external mold shows the outer surface; an internal mold (steinkern) shows the interior. Compare with cast.

Morrison Formation A Late Jurassic sedimentary formation spanning much of the western US, famous for its abundant dinosaur fossils including Allosaurus, Stegosaurus, Diplodocus, and Apatosaurus. The Morrison also yields invertebrate and plant fossils.

N

Nautiloid An ancient cephalopod with a straight or loosely coiled chambered shell, related to the modern nautilus. Nautiloid fossils are common in Ordovician through Devonian rocks and can range from a few centimeters to several meters in length.

O

Ordovician A period of the Paleozoic Era spanning roughly 485 to 444 million years ago, known for rich marine invertebrate faunas. Ordovician rocks in Ohio, Indiana, Kentucky, and New York are among the most productive fossil collecting targets in the eastern US.

Outcrop An area where bedrock is exposed at the earth’s surface, either naturally (cliff faces, creek banks) or through human activity (road cuts, quarries). Outcrops are where fossil collectors find fossils still in their original rock layers.

P

Paleozoic The geologic era spanning roughly 539 to 252 million years ago, comprising periods from the Cambrian through the Permian. Paleozoic fossils — trilobites, brachiopods, crinoids, corals — are the most commonly collected fossils in the central and eastern US.

Paraloid B-72 An acrylic resin widely used in paleontology and museum conservation as an adhesive and consolidant for fossils. Dissolved in acetone, it penetrates fragile bone and shell to strengthen them. It is preferred because it is chemically stable, non-yellowing, and reversible with acetone.

Period A division of geologic time smaller than an era. Familiar periods include the Cambrian, Devonian, Jurassic, and Cretaceous. Each period is defined by its characteristic fossil assemblages.

Permian The last period of the Paleozoic Era, spanning roughly 299 to 252 million years ago. The Permian ended with the largest mass extinction in Earth’s history, which eliminated roughly 90% of marine species. Permian fossils include late trilobites, brachiopods, and early reptiles.

Petrified Describes organic material (usually wood) in which the original structure has been replaced by minerals, typically silica, while preserving the original cellular detail. Petrified wood is one of the most recognizable and widely collected fossil types.

PRPA (Paleontological Resources Preservation Act) A 2009 US federal law that governs fossil collecting on federal land. The PRPA allows casual collection of common invertebrate and plant fossils for personal use but prohibits collecting vertebrate fossils without a permit. It also establishes penalties for illegal fossil theft.

Pyrite An iron sulfide mineral (FeS2) commonly known as “fool’s gold.” Pyrite frequently replaces or infills fossils, creating strikingly beautiful golden specimens. However, pyritized fossils are susceptible to pyrite disease if stored in humid conditions.

Pyrite disease A deterioration process in which pyrite (iron sulfide) within a fossil reacts with moisture and oxygen to form sulfuric acid and iron sulfate, causing the specimen to crack, crumble, and produce a sulfurous smell. Storing pyritized fossils in a dry environment and treating with Paraloid B-72 helps prevent it.

R

Road cut An excavation made through rock or soil during road construction, exposing cross-sections of underlying geology. Road cuts often reveal fossil-bearing layers that would otherwise be buried and are common collecting spots, though safety and legality vary by location.

Rugose coral See horn coral. An order of Paleozoic corals, both solitary (horn-shaped) and colonial, that are common fossils in Ordovician through Permian rocks.

S

Sedimentary Describes rock formed from accumulated layers of sediment (sand, mud, silt, organic material) compressed over time. Sedimentary rocks — limestone, shale, sandstone, mudstone — are where nearly all fossils are found.

Shale A fine-grained sedimentary rock formed from compressed mud and clay. Shale splits easily along bedding planes, making it a favorite among collectors because fossils are often revealed on freshly split surfaces. Many of the best-preserved leaf, fish, and trilobite fossils come from shale.

Silicified Describes a fossil in which the original material has been replaced by silica (quartz). Silicified fossils are extremely durable and can sometimes be freed from limestone matrix using acid, since silica does not dissolve in acid but the surrounding calcite does.

Silurian A period of the Paleozoic Era spanning roughly 444 to 419 million years ago. Silurian rocks yield corals, brachiopods, crinoids, trilobites, and eurypterids (sea scorpions), and are found in the Great Lakes region, Appalachians, and elsewhere.

Steinkern An internal mold — a fossil formed when sediment fills the interior of a shell or body cavity and hardens, then the original shell dissolves away. Steinkerns show the interior shape of the organism rather than external surface detail.

Stratigraphy The branch of geology concerned with the study of rock layers (strata) and their arrangement, composition, and age relationships. Understanding basic stratigraphy helps collectors predict where to find fossils and date their finds.

T

Tabulate coral An extinct group of colonial corals found in Paleozoic rocks, characterized by closely packed tubular corallites separated by flat platforms (tabulae). Common genera include Favosites (honeycomb coral) and Halysites (chain coral). They are abundant in Ordovician through Devonian limestones.

Trace fossil A fossil that records the activity of an organism rather than its body — footprints, trackways, burrows, borings, and coprolites (fossilized dung). Trace fossils provide information about animal behavior that body fossils cannot. Compare with body fossil.

Triassic The first period of the Mesozoic Era, spanning roughly 252 to 201 million years ago. The Triassic followed the Permian mass extinction and saw the rise of dinosaurs, early mammals, and modern coral groups. Triassic fossils are less commonly collected than Jurassic or Cretaceous ones but are found in parts of the western and eastern US.

Trilobite An extinct marine arthropod with a three-lobed body plan, ranging from the Cambrian to the end of the Permian period. Trilobites are among the most iconic and avidly collected fossils, found in sedimentary rocks worldwide. Complete specimens are highly prized; partial fragments and molts are far more common.

V

Vertebrate An animal with a backbone, including fish, amphibians, reptiles, birds, and mammals. Vertebrate fossils are subject to stricter collecting regulations on US public land than invertebrate fossils because of their rarity and scientific importance. Always check the legal guide before collecting anything that might be a vertebrate fossil.

W

Weathering The natural process by which rock and the fossils within it are broken down and exposed by wind, water, ice, and chemical action. Weathering is a fossil collector’s friend — it continuously exposes fresh specimens at the surface, which is why sites can be productive year after year.