Glossary of Paleontological Terms

Charles Salcido (left) and Patrick Wilson (right) documenting a fossil locality in Theodore Roosevelt National Park, North Dakota.

NPS photo by Jenna Streffon.

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Paleontology Glossary

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Paleontology Glossary

Work	Definition

Work	Definition
Acanthodian	A member of the class Acanthodii, an extinct group of jawed fish with shark-like bodies and fins supported by bony spines, also known as “spiny sharks”, known from the Silurian to the end of the Permian.
Acritarch	A “wastebasket” term for organic microfossils that cannot be assigned to another group.
Adhesive meniscate burrow	A fossil burrow containing millimeter-scale crescents representing compressed backfill.
Aeolian or eolian	Wind-blown transport or an environment where wind transport dominates.
Aetosaur	A member of the order Aetosauria, an extinct group of quadrupedal herbivorous armored reptiles related to crocodilians, known from the Triassic.
Aglaspidid	A member of the order Aglaspidida, an extinct group of arthropods resembling a cross between a trilobite and a horseshoe crab, known from the Cambrian and Ordovician.
Alluvial (adjective), alluvium (noun)	A general term for unconsolidated terrestrial sediment moved by water and not attributed to a more specific process (i.e., not fluvial, glacial, or lacustrine).
Amberat	Crystallized urine of the rodent Neotoma (packrats, woodrats, trade rats); may also sometimes be used in a general sense for undefined Neotoma midden contents.
Ammonoid	A member of the subclass Ammonoidea, an extinct group of shelled cephalopods known from the Devonian to the end of the Cretaceous. Most but not all ammonoids had planispiral, tightly coiled shells. Ammonoids are also sometimes called “ammonites”, although that term more strictly applies to a subgroup of ammonoids.
Amphicyonid	A member of the family Amphicyonidae, an extinct group of carnivoran mammals also known as a “bear dog”, known from the Eocene to the end of the Miocene.
Angiosperm	A member of the clade Angiospermae, the flowering plants, known from the Cretaceous to the present.
Anhydrite	A calcium sulfate mineral similar to gypsum but lacking water in its structure.
Ankylosaur	A member of the suborder Ankylosauria, an extinct group of heavily armored herbivorous dinosaurs known from the Jurassic and Cretaceous. The armor of ankylosaurs varied, with large spines, shields, and tail clubs being seen in a number of species.
Annelid	A member of the phylum Annelida. Also known as “segmented worms,” annelids are known from the Cambrian to the present and are incredibly diverse in the modern day (common examples are earthworms and leeches).
Annular or annual rings (biology)	Circular, concentric growth rings indicating the growth of an organism (visible rings corresponding to changes in growth rate). These can be found in various taxa including trees, fish, and bivalves.
Anthozoan	A member of the subphylum Anthozoa, a group of marine invertebrates including corals and sea anemones, known from the Ediacaran to the present.
Anthracothere	A member of the family Anthracotheriidae, an extinct group of hippo-like artiodactyl mammals known from the Eocene into the Pliocene.
Arachnid	A member of the class Arachnida, a group of arthropods including mites, scorpions, spiders, ticks, and relatives, known from the Silurian to the present.
Archaeocyath	A member of the clade Archaeocyatha, an extinct group of coral-like sponges known from the Cambrian.
Archosaur	A member of the clade Archosauria, which contains the most recent common ancestor of crocodilians and birds and all of its descendants. Archosaurs appeared in the Triassic and include crocodilians, birds, dinosaurs, pterosaurs, and many more unique reptile groups.
Archosauromorph	A member of the clade Archosauromorpha, which includes archosaurs (crocodilians and birds) and all reptiles more closely related to them than to lepidosaurs (lizards, snakes, and rhynchocephalians). Archosauromorphs appeared in the Permian.
Arenicolites	U-shaped invertebrate burrows, oriented vertically.
Arkose	Sandstone rich in grains of feldspar minerals.
Arthropod	A member of the phylum Arthropoda, a diverse group of invertebrates with exoskeletons, segmented bodies, and jointed appendages. Insects, arachnids, and crustaceans are modern examples. Arthropods appeared by the early Cambrian.
Articulate brachiopod	A brachiopod with a shell hinge featuring hard “teeth” and “sockets.”
Artiodactyl	A member of the order Artiodactyla, the even-toed ungulate mammals (e.g., bison, camels, deer, pigs, sheep). Artiodactyls are known from the Eocene to the present.
Asteroid	A member of the class Asteroidea, a group of echinoderms familiar for its five radiating arms; also known as “sea stars” or “starfish”, asteroids are known from the Ordovician to the present.
Azendohsaurid	A member of the family Azendohsauridae, an extinct group of Triassic reptiles resembling large, long-necked lizards (sometimes with horns), closely related to trilophosaurids.
Basal (geology)	Located near or at the bottom of a specific geologic unit.
Basal (phylogeny)	A taxon closer to the base/root of a phylogenetic tree or cladogram.
Basement (geology)	Crystalline rocks that underlie all other rocks and sediment and are above the mantle.
Basin (geology)	A geologic downwarp formed by folding and faulting; geologic basins can often be identified by rock strata dipping to a central point, a bullseye pattern of rocks in a geologic map with the youngest rocks at the center, or an anomalous area of young rocks at the same elevation as older rocks in the vicinity. An eroded geologic basin does not necessarily form a depression.
Bedrock (geology)	A general term for the rock that underlies soil or other unconsolidated, surficial material.
Belemnite	A member of the order Belemnoidea, an extinct group of squid-like cephalopods noted for possessing a bullet-shaped mineralized internal guard, known from the Triassic to the end of the Cretaceous.
Bellerophontid	A member of the order Bellerophontida, an extinct group of mollusks frequently with a planispiral shell and a flaring aperture, known from the Cambrian to the end of the Triassic. Bellerophontids are usually classified with snails but at least some may be monoplacophorans.
Bennettitale or cycadeoid	A member of the plant order Bennettitales, an extinct group of plants known from the Permian to the end of the Cretaceous that combined cycad-like or palm-like foliage, a stout trunk, and flower-like reproductive organs.
Bentonite	A clay produced by weathering of the volcanic glass particles that make up volcanic ash.
Bilobate	Having two lobes.
Bioclast	A clast of biological origin; biological remains functioning as sedimentary particles. Any fossil can technically be considered a bioclast, but the term is most often used for skeletal fragments of marine organisms.
Bioclastic	Describes rock or sediment made up of fragments of biologic materials (e.g., shell fragments).
Bioclaustration	A trace fossil of an organism embedded in another organism.
Biostratigraphy	Using fossils to identify the relative positions and relative ages of rocks.
Bioturbation	Disturbance of sediment by organisms to the extent that sedimentary features such as bedding and stratification are destroyed.
Bivalve	A member of the class Bivalvia, a group of mollusks with two shells; bivalves are also sometimes known as pelecypods, to reduce confusion (the term “bivalve” in a general sense also applies to other groups, such as brachiopods and some arthropods). Examples include clams, oysters, mussels, and scallops. Bivalves are known from the Cambrian to the present.
Blastoid	A member of the class Blastoidea, an extinct group of stalked echinoderm resembling crinoids but with nut-shaped bodies bearing numerous small filtering appendages, rather than cup-shaped bodies with a smaller number of large appendages. Blastoids are sometimes identified informally as “sea buds”, and are known from the Ordovician to the end of the Permian.
Body fossil	Fossilized remains of the body of past organisms, including bones, teeth, skin, shells, wood, and leaves.
Brachial valve	The upper shell of a brachiopod.
Brachidium	A mineralized structure supporting a brachiopod’s lophophore.
Brachiopod	A member of the class Brachiopoda, a group of marine filter-feeding animals with two shells, resembling bivalve mollusks but more closely related to bryozoans. Brachiopods are sometimes known as “lamp shells” because some of them resemble ancient oil lamps, and are known from the Cambrian to the present.
Bradoriid	A member of the order Bradoriida, an extinct group of small bivalved arthropods resembling modern ostracodes and known from the Cambrian into the Ordovician.
Breccia	A rock composed of large angular fragments in a fine-grained matrix.
Brontothere	A member of the family Brontotheriidae, an extinct group of horned perissodactyls that resembled rhinoceroses and lived only during the Eocene.
Bryozoan	A member of the phylum Bryozoa, a group of filter-feeding aquatic colonial animals. Bryozoans are sometimes called “moss animals” because they often encrust objects, and are known from the Ordovician to the present.
Caecilian	A member of the order Gymnophiona, a group of limbless, worm-like or snake-like amphibians that live in soil or streambeds, known from the Triassic to the present.
Calcarenite	a limestone made up of sand-sized calcareous fragments, a “limestone sandstone”.
Calcareous	Describes rock or sediment that contains the mineral calcium carbonate (CaCO3), often as a cement; “lime-rich”.
Calcisphere	An enigmatic microfossil, possibly an algal cyst.
Calcite	A mineral made of calcium carbonate. Found in limestone and some fossils.
Calcium carbonate	CaCO3, a mineral-forming compound. Calcite is a common form.
Cambrian	The first geologic period of the Paleozoic Era, noted for the abrupt diversification of life (“Cambrian explosion”); approximately 539 to 485 Ma.
Camelid	A member of the family Camelidae, a group of mammals including camels and relatives, known from the Eocene to the present.
Canidae	A member of the family Canidae, a group of mammals including dogs and relatives, known from the Eocene to the present.
Carbonate (geology)	A general term for minerals containing carbonate ions (CO3-2), such as calcite, and rocks containing these minerals, such as limestone.
Carboniferous	Internationally, the fifth geologic period of the Paleozoic Era, often divided into the Mississippian and Pennsylvanian in North America; approximately 359 to 299 Ma.
Carnivoran	A member of the order Carnivora, a group of mammals including most of the living flesh-eating forms (e.g., felines, canines, bears, raccoons, weasels, seals, etc.), known from the Eocene to the present.
(Natural) Cast	An impression of an organism that has been filled in by sediment.
Cementation	Chemical precipitation of material into pores between grains that bind the grains into rock.
Cenozoic	A geologic era, dated approximately 66 Ma to the present, noted for the diversification of mammals; the term means “new life”.
Cephalon	The head shield of a trilobite.
Cephalopod	A member of the class Cephalopoda, a group of mollusks with a prominent head fringed by tentacles. Examples include squids, octopuses, Nautilus, and many extinct forms, especially ammonoids. Cephalopods are known from the Cambrian to the present.
Charophyte	A member of the clade Charophyta, a group of freshwater green algae from which terrestrial plants evolved.
Chert	A rock made of silica lacking obvious macroscopic crystals. Flint is a variety of chert found in chalk and limestone.
Chiropteran	A member of the order Chiroptera, the bats. They are known from the Eocene to the present.
Chitinozoan	A flask-shaped organic-walled microfossil that probably represents some kind of resting cyst or invertebrate egg case.
Chlorophyte	A member of the clade Chlorophyta, a type of green algae.
Choristodere	A member of the order Choristodera, an extinct group of semiaquatic reptiles with crocodile-like, lizard-like, and long-necked forms, known from the Jurassic into the Miocene.
Chronostratigraphy	A branch of stratigraphy that examines the relationship between strata and time. Isotope geology and geochronology are heavily utilized to determine the precise age of certain rocks.
Chronozone or chron	A unit in chronostratigraphy defined by the presence of certain fossil taxa (biochronozone) or geomagnetic reversal events (magnetozones).
Cladistics	A biological classification method grouping organisms in groups (clades) based on the proportion of shared measurable derived characteristics (synapomorphies). Cladistics is the most commonly utilized method for classifying organisms.
Cladogram	In cladistics, a diagram used to show the relationships of organisms.
Clast	A rock fragment of any size. A rock made up of clasts, such as sandstone or shale, is called clastic.
Clastic	Describes rock or sediment made of fragments of pre-existing rocks (clasts).
Clay	Can be used to refer to clay minerals or as a sedimentary fragment size classification (less than 1/256 mm [0.00015 in]).
Cloudinid (or cloudiniid)	A member of the family Cloudinidae, an extinct group represented by enigmatic conical tubular fossils known from the Ediacaran into the Cambrian.
Clubmoss	A type of diminutive evergreen plant that reproduces using spores and resembles a tiny pine tree.
Cnidarian	A member of the phylum Cnidaria, a group of invertebrates noted for capturing prey using stinging cells, known from the Ediacaran to the present. Most fossil cnidarians are corals.
Coelacanth	A member of the class Actinistia, a group of lobe-finned fish more closely related to tetrapods than to ray-finned fish, known from the Devonian to the present.
Coelophysid	A member of the family Coelophysidae, an extinct group of relatively small, bipedal, carnivorous theropod dinosaurs, known from the Triassic into the Jurassic.
Coeval	Occurring or existing at the same time; contemporary.
Coleopteran	A member of the order Coleoptera, a group of insects including the beetles, known from the Permian to the present.
Colluvial (adjective), colluvium (noun)	A general term for unconsolidated terrestrial sediment deposited at the base of a slope through various means (e.g., mass wasting).
Columnal	Individual segments of an echinoderm’s stalk, particularly crinoids. They resemble doughnuts, sprockets, or gears.
Conchostracan	A member of the paraphyletic clade Conchostraca, a group of bivalved crustaceans also known as “clam shrimp”, known from the Devonian to the present.
Conformable (geology)	A successive, continuous sequence of strata deposited without interruption or disturbance.
Conglomeratic (adjective), conglomerate (noun)	A clastic sedimentary rock comprised of a significant proportion of gravel-sized clasts (>2 mm [0.08 in]) and a finer grained matrix.
Conifer	A member of the division Pinophyta, including gymnosperm trees such as pines, firs, cypresses, cedars, hemlocks, larches, spruces, and more; non-flowering plants that produce seeds and pollen from cones, known from the Pennsylvanian to the present.
Conodont	A member of the class Conodonta, an extinct group of eel-like chordates known primarily from jaw elements, known from the Cambrian into the Jurassic.
Consolidant (paleontology)	A dilute adhesive used to penetrate a fossil specimen and adhere internally in order to prevent damage.
Contact (geology)	The boundary that separates two bodies of rock; the horizon or plane where two formations meet.
Conulariid	A member of the clade Conulariida, an extinct group of distant relative of corals that formed flat-sided conical structures from small mineralized rods, known from the Ediacaran into the Triassic.
Coprolite	Fossil feces.
Coquina	A bed consisting primarily of shells and/or other invertebrate body fragments. “Coquinoid” may also be used.
Coralline algae	A member of the order Corallinales, a group within the “red algae” that secretes calcareous structures. Modern forms contribute to reefs. They are known from the Ordovician to the present.
Corallite	The dwelling cell of an individual coral animal.
Cornulitid	A member of the order Cornulitida, an extinct group of enigmatic invertebrates that formed millimeter-scale elongate ribbed conical shells, known from the Ordovician into the Pennsylvanian. Cornulitids may have been mollusks or “worms” related to bryozoans, brachiopods, and horseshoe worms.
Cotype	An outdated term for one of a group of specimens designated a type series for an organism. The modern term is syntype.
Creodont	A member of an informal group of extinct carnivorous mammals, known from the Paleocene into the Miocene.
Cretaceous	The third and last geologic period of the Mesozoic Era, noted for the rise of flowering plants and the extinction of many groups at the end of the period; approximately 145 to 66 Ma.
Crinoid	A member of the order Crinoidea, a group of echinoderms featuring a cup-like body with feathery tentacular arms, usually but not always attached to a surface with a stalk. Crinoids are also known as sea “lilies”, and are known from the Ordovician to the present.
Crocodylomorph	A member of the clade Crocodylomorpha, containing modern crocodilians and their extinct relatives. Early crocodylomorphs, appearing in the Triassic, were small and active terrestrial hunters, with the modern crocodilian body-plan not appearing until the Jurassic.
Cross-bedding or cross-stratification	Uniform to highly varied sets of inclined sedimentary beds deposited by wind or water that indicate flow conditions such as deposition direction and depth/height.
Cyanobacteria	Photosynthetic bacteria, also known informally as “blue-green algae”.
Cycad	A member of the division Cycadophyta, a group of seed plants with stout, woody trunks and large, hard, evergreen leaves, sometimes mistaken for palms or ferns. They are known from the Permian to the present.
Cynodont	A member of the clade Cynodontia, including modern mammals and small, egg-laying early relatives that lived from the Permian into the Jurassic.
Cyst	A resting or dormant stage of some microbes and other organisms; during this stage the cell wall thickens or a protective structure is constructed. Cysts are common microfossils.
Cystoid	A member of the class Cystoidea, an extinct group of stalked echinoderms similar to crinoids, known from the Ordovician to the Devonian.
Czekanowskiale	A member of the order Czekanowskiales, an extinct group of tree-/shrub-like seed plants with scale-like leaves, known from the Permian to the end of the Cretaceous.
Dasycladaceans	A member of the family Dasycladaceae, a group of chlorophyte algae that construct mineralized walls, leading to large stony colonies. They are known from the Ordovician to the present.
Death assemblage	A mass collection of fossil organisms that forms after a mass die-off, often caused by storm events.
Decapod	A member of the order Decapoda, a group of crustaceans that include prawns, shrimp, lobsters, crayfish, and crabs, known from the Devonian to the present.
Devonian	The fourth geologic period of the Paleozoic Era; approximately 419 to 359 Ma.
Diagenesis	Physical and chemical changes produced in sediments after deposition.
Diatom	A kind of algae that secretes cell walls of silica, known from the Jurassic to the present. Diatoms are common microfossils.
Dicynodont	A member of the clade Dicynodontia, an extinct group of tusked beaked herbivores related to mammals, known from the Permian into the Triassic. They ranged in size from rodent-like to elephant-like.
Dinosaur	A member of the clade Dinosauria, a diverse group of reptiles that arose during the Triassic and continue to the present in birds.
Dinosauromorph	A member of the clade Dinosauromorpha, which contains dinosaurs and all organisms more closely related to them than to pterosaurs, known from the Triassic to the present.
Docodont	A member of the order Docodonta, an extinct group of cynodonts closely related to modern mammals, known from the Jurassic and Cretaceous. A number of different forms were taken by docodonts, including mole-like, squirrel-like, and otter-like taxa.
Dolomite	A mineral made of calcium-magnesium carbonate, and a rock (alternatively dolostone) composed primarily of this mineral. Most dolomite is formed by the replacement of some of the calcium ions in calcium carbonate by magnesium, a process called dolomitization.
Dolomitization	The conversion of calcium carbonate to dolomite by the replacement of calcium ions with magnesium. This process involves recrystallization and is destructive to fossils made of calcium carbonate, although not all such fossils are affected the same way: mollusk shells are more susceptible than brachiopod and bryozoan specimens, which in turn are more susceptible than echinoderm columnals and plates.
Doswelliid	A member of the family Doswelliidae, an extinct group of semi-aquatic armored archosauromorph reptiles resembling crocodilians although distantly related, known from the Triassic.
Drepanosaur	A member of the clade Drepanosauromorpha, an extinct group of enigmatic reptiles resembling chameleons with bird-like heads, known from the Triassic. Different drepanosaurs display evidence of arboreal and burrowing lifestyles.
Drift (geology)	A general term for any deposit of glacial origin.
Echinoderm	A member of the phylum Echinodermata, a group of invertebrates noted for their five-fold symmetry. Sea stars, sea urchins, sea lilies, and sea cucumbers are some examples. Echinoderms are known from the Cambrian to the present.
Echinoid	A member of the class Echinoidea, a group of echinoderms commonly having a globose body covered in spines, also called “sea urchins”. They are known from the Ordovician to the present.
Ediacaran	(Geochronology) The last geologic period of the Neoproterozoic Era; approximately 635 to 539 Ma. It marks the end of the Proterozoic Eon, and the beginning of the Phanerozoic Eon. (Paleontology) A general adjective for soft-bodied, often frond-like organisms that flourished during this period.
Edrioasteroid	A member of the class Edrioasteroidea, an extinct group of echinoderms that resemble a sea star attached to a bulb attached to the sea floor (reflected in the informal name “seated star”), known from the Cambrian into the Permian.
Encrinite	A grain-supported sedimentary rock in which the majority of the grains are crinoid fragments.
Entelodont	A member of the family Entelodontidae, an extinct group of pig-like artiodactyls known from the Eocene into the Miocene.
Eocene	The second epoch of the Paleogene Period; approximately 56 to 34 Ma.
Eocrinoid	A member of the class Eocrinoidea, an extinct group of stalked echinoderms known from the Cambrian to the end of the Silurian, somewhat similar to crinoids but not closely related.
Epifauna	Aquatic animals that live on the seafloor or other surface under a body of water.
Erg	A sand-covered desert at least 125 km2 (48 mi2) in area; may also be called a dune sea if dunes are abundant.
Eurypterid	A member of the order Eurypterida, an extinct group of arthropods often known informally as “sea scorpions” because of their large scorpion-like appendages and general body shape; known from the Ordovician to the end of the Permian.
Eutriconodont	A member of the clade Eutriconodonta, an extinct group of early mammals that possessed a diverse variety of body plans and occupied various niches including small shrew-like insectivores, large badger-like carnivores, and even gliding flying squirrel-like taxa; known from the Jurassic and Cretaceous.
Extant	Still in existence, not extinct.
Facies	A body of rock that is distinct from the surrounding rock, due to specific aspects of deposition and setting. Facies are generally invoked when there are differences within a formation or member that are not suitable for mapping (i.e., not laterally persistent).
Fanglomerate	A gravel-rich conglomerate that originated as a debris flow on an alluvial fan.
Faunal zone	An assemblage of specific fossils with a limited vertical extent in a formation or other geologic unit, representing a specific interval in time.
Feldspar	One of a number of minerals composed of crystals including various metal ions, aluminum, silicon, and oxygen.
Felid	A member of Felidae, a group of mammals including cats and relatives known from the Oligocene to the present.
Fissile (geology)	A rock that has a tendency to split along flat planes of weakness.
Float (geology)	Eroded material resting on the surface, not attached to an outcrop.
Fluvial (geology)	A river as depositional environment or mode of transport.
Foraminifera	A member of the subphylum Foraminifera, “shelled amoebas”; “foraminifera” is often used for both singular and plural, although some authorities prefer “foraminiferan” for singular and “foraminiferans” or “foraminifers” for plural, and others use the more informal “foram” and “forams.” Foraminifera are well-known as microfossils from their tests, which are usually composed of calcium carbonate or tiny sediment grains (agglutinated). They are known from the Ediacaran to the present.
Formation (geology)	Fundamental rock-stratigraphic unit that is mappable, lithologically distinct from adjoining strata, and has definable upper and lower contacts; the basic unit of stratigraphy.
Fossiliferous	A rock or deposit that yields fossils.
Friable	Rocks and deposits that are easily crumbled.
Fucoid (paleontology)	A term often used in the 19th and early 20th centuries for fossils thought to be seaweed impressions (after Fucales, the brown algae), but now known to be invertebrate burrows and other trace fossils.
Fusulinid	A member of Fusulinida, an extinct group of foraminifera noted for their chambered, relatively enormous tests (as much as several cm long). In the field, they may resemble rice or other similar grains, and can be abundant in rocks of Pennsylvanian or Permian age.
Ga	An abbreviation for “gigaannum” or “giga-annum” meaning “billion years”, often informally used to mean “billion years ago”.
Ganoid scales	A type of fish scale seen today in sturgeons, paddlefish, bichirs, bowfin, and gar, as well as in a number of extinct taxa. These thick, diamond-shaped scales provide exceptional protection against predators.
Gastropod	A member of Gastropoda, a group of mollusks including snails and slugs (snails without shells), known from the Cambrian to the present.
Geochronology	A branch of geology dedicated to determining the age of rocks, sediments, fossils, and geologic events. A number of different types of geochronology exist based on the use of varying dating methods.
Ginkgo	A member of the genus Ginkgo; may also be used to refer to relatives within the order Ginkgoales. The single extant species is a tree most closely related to cycads and possesses unique fan-shaped leaves. The order Ginkgoales is known since the Permian, whereas the genus Ginkgo is known from the Jurassic to the present.
Gnetophyte	A member of the division Gnetophyta, a group of gymnosperm plants with a diverse fossil record yet only a handful of modern genera (including the widespread genus Ephedra), known from the Jurassic to the present.
Goniopholidid	A member of the family Goniopholididae, a group of semi-aquatic crocodyliforms that very closely resemble extant crocodilians, known from the Jurassic and Cretaceous.
Gradational (geology)	A conformable contact between rock layers in which the transition between depositional environments is prolonged.
Graptolite	A member of the subclass Graptolithina, a group of colonial invertebrates that live in bottom-attached or free-floating colonies, known from fossils of Cambrian to Mississippian age and found in the present as pterobranch “worms”.
Graywacke (also spelled greywacke)	Dark poorly sorted sandstone with larger grains in a clay matrix.
Group (geology)	A lithostratigraphic unit comprising several related formations classified together.
Gymnosperm	A member of the clade Gymnospermae, a group of seed plants including conifers, cycads, gnetophytes, and ginkgophytes, known from the Pennsylvanian to the present.
Gypsiferous	Containing gypsum.
Gypsum	A mineral made of calcium sulfate dihydrate, CaSO4 2H2O. Often deposited through evaporation of lake/sea water, as well as in veins.
Gyttja	Decayed peat.
Hardground	Naturally cemented seafloor; hardening occurred before the seafloor was buried. Hardgrounds were inhabited by assemblages specialized to live on or in the hard surface.
Helcionelloid	A member of the class Helcionelloida, an extinct group of superficially snail-like extinct mollusks known from the Cambrian and Ordovician.
Helicoplacoid	A member of the class Helicoplacoidea, an extinct group of sac-shaped echinoderms with bodies made up of small mineralized plates, known from the Cambrian.
Hemipteran	A member of the order Hemiptera, a large and diverse group of insects commonly referred to as “true bugs” and including cicadas, aphids, bed bugs, water bugs, and many more, known from the Pennsylvanian to the present.
Herrerasaur	A member of the clade Herrerasauria (or Herrerasauridae), an extinct group of early bipedal carnivorous dinosaurs, known from the Triassic.
Heterodontosaur	A member of the family Heterodontosauridae, an extinct group of fox-sized herbivorous or omnivorous ornithischian dinosaurs with large tusks, known from the Jurassic and Cretaceous.
Holocene	The second and most recent epoch of the Quaternary Period, following the Pleistocene and the last glacial maximum; approximately 11,700 years ago to the present.
Holothuroid	A member of the class Holothuroidea, a group of sac-shaped echinoderms also known as “sea cucumbers”, with sac-like bodies generally supported by an internal skeletal framework of microscopic mineralized elements known as ossicles or sclerites, known from the Ordovician to the present.
Holotype	A specimen that serves as the basis for an organism’s scientific name and is ideally representative of the given taxon, although this is not necessarily the case. Other “types” exist for specific situations, such as a paratype or a neotype. Some species are based on multiple specimens, which are called cotypes (outdated; sometimes “co-types”) or syntypes.
Horn coral	A solitary rugose coral. The name refers to the shape of the coral’s skeleton, resembling an animal horn.
Hymenopteran	A member of the order Hymenoptera, a large and diverse group of insects including ants, wasps, and bees, known from the Triassic to the present.
Hyolith	A member of the class Hyolitha, an extinct group of animals with elongate triangular shell, apparently close to the ancestry of brachiopods, mollusks, or both, known from the Cambrian to the end of the Permian.
Hyolithellus	A conical worm-tube-like fossil from the Cambrian.
Hypertragulid	A member of the family Hypertragulidae, a group of small extinct deer-like artiodactyl ungulates known from the Eocene and Oligocene.
Hyphae	Fungal filaments.
Ichnofossil (also trace fossil)	Fossilized remains of biological activity of an organism, including root traces, footprints, tracks, burrows, trails, coprolites, and other biogenically produced features.
Ichnogenus, ichnospecies, ichnofamily, ichnotaxon	A scientific name used for a trace fossil, existing in parallel to the names used for body fossils.
Ichnology	The study of traces or biological activity beyond an organism’s remains. Sometimes divided into paleoichnology (the study of trace fossils or ichnofossils) and neoichnology (the study of modern traces).
Ichthyosaur	A member of the order Ichthyosauria, an extinct group of marine reptiles resembling dolphins in form, known from the Triassic into the Cretaceous.
In situ (geology)	When describing fossils, this means found in place at an outcrop.
Inarticulate brachiopod	A brachiopod with a shell hinge lacking hard structures to keep it closed, instead relying on muscles.
Infauna	Aquatic animals that live within the sediment under a body of water.
Interbedding (geology)	Rock layers of a specific lithology lying between or alternating with those of a different lithology.
Interdune	A flat or gently sloping surface enclosed or partially enclosed by dunes, taking a variety of different forms (such as erosional, dry, damp/wet, evaporitic, and vegetated).
Interfingering, intertonguing, interdigitating, interlocking (geology)	A conformable contact between rock layers in which an alternating series of wedge-shaped “tongues” represent the fluctuating boundary between different contemporary depositional environments.
Intertidal	Within the range from low tide to high tide.
Intraclast	A clast representing material that was redeposited after being partially lithified.
Involute	In descriptions of coiled shells, a coil that is tightly curled or spiraled.
Isopod	A member of the order Isopoda, a group of crustaceans including woodlice and relatives, known from the Pennsylvanian to the present.
Jurassic	The second geologic period of the Mesozoic Era; approximately 201 to 145 Ma.
Ka	An abbreviation for “kiloannum” or “kilo-annum” meaning “thousand years.”
Karst	Topography produced by the dissolution of bedrock, such as caves and sinkholes, usually associated with limestone.
Lacustrine	A lake as a depositional environment or mode of transport.
Lagerpetid	A member of the family Lagerpetidae, an extinct group of small bipedal pterosaur relatives, known from the Triassic.
Lagomorph	A member of the order Lagomorpha, a group of mammals including pikas, rabbits, and relatives, known from the Paleocene to the present.
Laurentia	The ancient core of the North American continent.
Lepidosaur	A member of the superorder Lepidosauria, the group including lizards, snakes, and rhynchocephalians, known from the Triassic to the present.
Leptictid	A member of the order Leptictida, an extinct group of small mammals with long hind legs and tails, traditionally classified as insectivores, known from the Cretaceous into the Oligocene.
Leptochoerid	A member of the family Leptochoeridae, an extinct group of small artiodactyl ungulates, known from the Eocene and Oligocene.
Leptomerycid	A member of the family Leptomerycidae, an extinct group of small deer-like artiodactyl ungulates, known from the Eocene into the Miocene.
Lignite	Low-grade coal.
Limestone	A sedimentary rock consisting mainly of calcium carbonate, primarily in the form of the mineral calcite.
Lithified	A sedimentary deposit that has become rock.
Lithology	The variety or varieties of rock in a formation, member, bed, or other division.
Locality	A distinct site, especially one known to field fossils or other unique features.
Loess	A deposit of generally silt-sized wind-blown dust.
Lophophore	The tentacle-bearing feeding organ of brachiopods, bryozoans, and some other groups.
Lycopod	A member of the class Lycopodiopsida, a group of plants containing clubmosses and firmosses, quillworts, and spikemosses, known from the Devonian to the present. While modern examples are relatively small, giant lycopods were abundant during the Carboniferous.
Ma	An abbreviation for “megaannum” or “mega-annum” meaning “million years”, often informally used to mean “million years ago”.
Macrofossil	A fossil that can be studied with the naked eye or a hand lens.
Mammaliamorph	A member of the clade Mammaliamorpha, including tritylodontids, some other close relatives of mammals, and true mammals, known from the Triassic to the present.
Manus (ichnology)	The forefoot track (“hand”) of a tetrapod.
Marl	Calcium carbonate mud or mudstone that commonly forms in lakes.
Mass wasting	A general term for the downslope movement of soil and rock material under the direct influence of gravity.
Massive (geology)	Rocks without visible stratification or other bedding features.
Matrix (geology)	A fine-grained mass of rock in which larger grains, clasts, crystals, and/or fossils are embedded.
Meganeurid	A member of the family Meganeuridae, an extinct group of relatives of modern dragonflies, known from the Pennsylvanian and Permian.
Member (geology)	A lithostratigraphic unit with definable contacts; a subdivision of a formation.
Merostomate	A member of the class Merostomata, a group including horseshoe crabs and eurypterids, known from the Ordovician to the present.
Mesaxonic (adjective), mesaxony (noun)	A foot structure in which an enlarged middle digit forms the axis of the foot.
Mesoproterozoic	The second geologic era of the Proterozoic Eon; 1600 to 1000 Ma.
Mesozoic	A geologic era, dated approximately 252 to 66 Ma, noted for the dominance of dinosaurs on land and the appearance of birds, mammals, and flowering plants. The term means “middle life.”
Metagraywacke	Metamorphosed graywacke, a poorly sorted sandstone with clay matrix.
Metamorphic	Refers to a rock that formed through metamorphism. One of the three main classes of rocks, alongside igneous and sedimentary.
Metamorphism (geology)	The transformation of an existing rock’s composition/structure into a different form through heat, pressure, and/or chemical processes. The rock remains mostly solid throughout this transformation.
Metasedimentary	A variety of metamorphic rock in which the original material was sedimentary rock, produced through sedimentary processes.
Metazoan	Formerly used to refer to multicellular animals, now synonymous with Animalia following the reassignment of single-celled forms.
Metoposaur	A member of the family Metoposauridae, an extinct group of amphibians that reached 1.5 to 3 m (4.9 to 9.8 ft) in length with large flat heads, known from the Triassic.
Microbially induced sedimentary structure	Sedimentary features formed by the interaction of microbes with sediment. They are at the intersection of trace fossils and sedimentary structures.
Microconchid	A member of the order Microconchida, an extinct group of enigmatic extinct invertebrates known from small spiral-coiled shells resembling certain fossil “worm” tubes, found encrusting other fossils, known from the Ordovician into the Jurassic.
Microfossil	A fossil, typically a millimeter (0.04 inches) or less in size, that must be studied with a microscope.
Microvertebrate	A vertebrate microfossil, requiring a microscope for study.
(Packrat) Midden	A collection of debris by a packrat or woodrat (Neotoma spp.) including a variety of material (such as plants, bones, rocks, artifacts, etc.) that can provide important ecological/environmental data. Middens may preserve debris from the late Pleistocene through the Holocene.
Miocene	The first epoch of the Neogene Period; approximately 23 to 5.33 Ma.
Mississippian	In North America, the de facto fifth geologic period of the Paleozoic Era (internationally a subperiod of the Carboniferous Period); approximately 359 to 323 Ma.
(Natural) Mold	In paleontology, an internal or external impression of an organism left in sediment.
Mollusk	A member of the phylum Mollusca, a diverse group of invertebrates noted for their combination of a muscular foot, a shell, and a mantle that covers the innards and secretes the shell. Examples include bivalves, cephalopods, and gastropods (snails). They are known from the Cambrian to the present.
Monophyletic (adjective), monophyly (noun)	A taxon that includes a common ancestor and all of its descendants (e.g., mammals within Mammalia).
Monoplacophoran	A member of the class Monoplacophora, a group of shelled mollusks known from the Cambrian to the present. Most monoplacophorans have a cap-like or limpet-like shell, but some extinct forms had planispiral shells.
Multituberculate	A member of the order Multituberculata, an extinct group of mammals resembling rodents, ranging from burrowers, arboreal-dwellers, and hoppers, and coming in a variety of sizes from mouse-sized to beaver-sized. They are known from the Jurassic into the Eocene.
Mustelid	A member of the family Mustelidae, a group of mammals including badgers, otters, weasels, and relatives, known from the Oligocene to the present.
Nautiloid	A member of the subclass Nautiloidea, a group of shelled cephalopod mollusks represented today by species of chambered nautilus, but in the past including various straight (orthoconic) and coiled forms, known from the Cambrian to the present.
Neogene	The second geologic period of the Cenozoic Era; approximately 23 to 2.58 Ma.
Neoproterozoic	The third and final geologic era of the Proterozoic Eon; approximately 1000 to 539 Ma.
Nepomorph	A member of the infraorder Nepomorpha, a group of insects including water bugs, known from the Triassic to the present.
Nimravid	A member of the Nimravidae, an extinct group of saber-toothed carnivoran mammals also known as “false saber-toothed cats”, known from the Eocene into the Miocene.
Oligocene	The third and final epoch of the Paleogene Period; approximately 34 to 23 Ma.
Oncoid	A concentrically laminated structure formed by microbes; rocks made of oncoids are called oncolites.
Oolite	An inorganic spherical concentrically layered carbonate grain, which can be confused for an organic microbial fossil.
Ophiuroid	A member of the class Ophiuroidea, a group of echinoderm with five slender radiating arms, also known as “brittle stars”, known from the Ordovician to the present.
Ordovician	The second geologic period of the Paleozoic Era, following the Cambrian; approximately 485 to 443 Ma.
Oreodont	A member of the superfamily Merycoidodontoidea, an extinct group of artiodactyl ungulates with low-slung pig-like bodies and short stocky skulls, known from the Eocene to the end of the Miocene.
Organic	Made of organic compounds (i.e., molecules of carbon, hydrogen, nitrogen, and oxygen); pollen, spores, and cysts, all of which can be fossilized, have organic walls.
Ornithischian	A member of the clade Ornithischia, also known as “bird-hipped” dinosaurs, which despite the name were not closely related to birds. Ornithischians included the ankylosaurs, stegosaurs, pachycephalosaurs, ceratopsians, and ornithopods, and are known from the Triassic to the end of the Cretaceous. All had a bony beak at the tip of the lower jaw.
Ornithomimosaur	A member of the clade Ornithomimosauria, an extinct group of fast-running omnivorous/herbivorous theropod dinosaurs resembling ostriches, known from the Cretaceous.
Ornithopod	A member of the clade Ornithopoda, an extinct group of beaked herbivorous dinosaurs that were bipedal or quadrupedal, known from the Jurassic and Cretaceous. Hadrosaurs, or “duck-billed dinosaurs”, are the best-known ornithopods.
Orthocone, orthoconic	A type of nautiloid shell that is straight and gently tapering.
Orthopteran	A member of the order Orthoptera, a group of insects containing grasshoppers, crickets, and locusts, known from the Mississippian to the present.
Ostracode or ostracod	A member of the class Ostracoda, a group of shelled and generally microscopic crustaceans known informally as “seed shrimp”, known from the Ordovician to the present.
Palaeoscolecidan	A member of the class Palaeoscolecida, an extinct group of armored worms bearing phosphatic plates, known from the Cambrian into the Silurian.
Paleocene	The first epoch of the Paleogene Period; approximately 66 to 56 Ma.
Paleoclimatology	The study of ancient climates through the use of various proxies (e.g., rock/sediment, ice, tree rings, fossils, etc.).
Paleocurrent	The direction of flowing water at the time of sediment deposition, typically determined through the use of sedimentary structures and useful in reconstructing depositional environments.
Paleoecology	The study of interactions among organisms and between organisms and their environments on a geologic timescale.
Paleogene	The first geologic period of the Cenozoic Era; approximately 66 to 23 Ma.
Paleogeography	The study, description, and reconstruction of the physical landscape from past geologic periods.
Paleokarst	Karst features formed in the past and now part of the geologic record.
Paleoproterozoic	The first geologic era of the Proterozoic Eon; 2500 to 1600 Ma.
Paleosol (geology)	A preserved ancient soil buried and sometimes lithified. Can be useful for interpretation of the depositional environment and/or climate. Divided into distinct classes similar to those used in modern soil science.
Paleozoic	A geologic era, dated approximately 539 to 252 Ma, noted for the diversification of invertebrates and the appearance of vertebrates and land plants; the term means “early life”.
Palynomorph	An organic-walled microfossil. Pollen and spores are common examples; other types include tests of some microorganisms and jaw elements of worms.
Paralic (geology)	A system of interfingered marine and nonmarine sediments recording a mix of terrestrial/aquatic coastal environments and various marine transgressions/regressions.
Paraphyletic (adjective), paraphyly (noun)	A taxon that includes some descendants of a common ancestor but excludes others. For example, birds evolved from reptiles, but Reptilia as traditionally used does not include birds and is paraphyletic.
Paratype	A specimen of an organism that helps define it, but which is not a name-bearing specimen.
Parting	A thin layer of softer rock between thicker beds of harder rock.
Peat	partially decayed vegetation.
Pedicle valve	the lower shell of a brachiopod; it supports the pedicle.
Pedicle	in brachiopod anatomy, a stalk used to attach them to objects.
Pelite	a fine-grained sedimentary rock such as siltstone and mudstone.
Pelmatozoan	A general term for any echinoderm with a “stalk” or “stem”, made up of columnals; the majority are crinoids, but several other groups of pelmatozoans existed during the Paleozoic.
Pennsylvanian	In North America, the de facto sixth geologic period of the Paleozoic Era (internationally a subperiod of the Carboniferous Period); approximately 323 to 299 Ma.
Perissodactyl	A member of the order Perissodactyla, the odd-toed ungulates mammals (e.g., horses, rhinos), known from the Paleocene to the present.
Peritidal	From just below to just above tidal range.
Permian	The seventh and final geologic period of the Paleozoic Era, ended by the largest mass extinction event in Earth’s history; approximately 299 to 252 Ma.
Pes (ichnology)	A hindfoot track (“foot”) of a tetrapod.
Phanerozoic	The fourth and final geologic eon of Earth’s history; approximately 539 Ma to the present.
Phosphate	In geology, a general term for minerals containing phosphate (PO4?3), such as apatite, found in bones and some shells (the phosphatic shells of some inarticulate brachiopods, for example).
Phylogenetics	The study of the evolutionary relationships and history of organisms, determined through a variety of methods (such as DNA sequences and morphology). A phylogenetic tree is utilized to show hypothesized relationships and the evolutionary history of a group of organisms.
Phytolith	A microscopic silica structure that forms within the cells of some plants.
Phytosaur	A member of the order Phytosauria, an extinct group of semi-aquatic carnivorous reptiles known from the Triassic, distantly related to modern crocodilians but very similar in appearance and lifestyle.
Planispiral	A shell coiled in a flat plane, like a Nautilus shell.
Planktonic	An organism that floats freely but cannot swim against a current.
Planolites	Simple meandering to straight horizontal cylindrical burrows.
Playa	A dry lakebed.
Pleistocene	The older of two epochs in the Quaternary Period, noted for ice ages; approximately 2.58 Ma to 11,700 years ago.
Plesiosaur	A member of the order Plesiosauria, an extinct group of marine reptiles with four large flippers and often a small head on a long neck; forms with large heads and short necks are often identified by other terms, such as pliosaur. Plesiosaurs are known from the Triassic to the end of the Cretaceous.
Pliocene	The second and final epoch of the Neogene Period; approximately 5.33 to 2.58 Ma.
Pliosaur	A plesiosaur with a large head and short neck.
Polyphyletic (adjective), polyphyly (noun)	A taxon that includes multiple distinct groups, but not a common ancestor or all of its descendants (e.g., algae).
Polyplacophoran	A member of the class Polyplacophora, a group of mollusks with ovoid bodies protected by a series of plate-like shells, known from the Cambrian to the present. Chitons are modern polyplacophorans.
Poposaurid	A member of the family Poposauridae, an extinct group of large carnivorous pseudosuchians that filled similar niches to theropod dinosaurs, known from the Triassic.
Precambrian	Geologic time before the Cambrian, before approximately 539 Ma.
Proboscidean	A member of the order Proboscidea, a group of large herbivorous mammals with tusks and trunks such as gomphotheres, elephants, mammoths, and mastodons, known from the Paleocene to the present.
Procolophonid	A member of the family Procolophonidae, an extinct group of stocky lizard-like herbivorous or insectivorous parareptiles with large heads, known from the Permian and Triassic.
Proterozoic	The third and final geologic eon of the Precambrian; approximately 2500 to 539 Ma.
Protist	Informally, either a single-celled organism with a nucleus, or an organism that has one or more nucleated cells that is not an animal, fungus, or plant.
Protoceratid	A member of the family Protoceratidae, an extinct group of artiodactyl ungulates resembling deer, known from the Eocene into the Pliocene; males had large bony horns behind the eyes and on the snout.
PRPA	Paleontological Resources Preservation Act (2009).
Pseudofossil	An object or feature that resembles a fossil but is not of organic origin.
Pseudosuchian	A member of the clade Pseudosuchia, one of the major branches of Archosauria, containing all animals more closely related to crocodilians than to birds, known from the Triassic to the present.
Pteridosperm	A member of the polyphyletic clade Pteridospermatophyta, an extinct group of seed-producing plants with fern-like fronds known from the Devonian into the Eocene, commonly referred to as “seed ferns”. Often used in an informal sense to classify or describe seed plants that are not cycads, cycadeoids, gingkoes, conifers, or angiosperms.
Pterosaur	A member of the order Pterosauria, an extinct group of flying reptiles known from the Triassic to the end of the Cretaceous.
Pygidium	The “tail” section of a trilobite. This section detached readily when the exoskeleton was shed, so pygidia are common fossils.
Quartz	A common mineral composed of silica.
Quartzite	1) Moderately metamorphosed quartz sandstone in which the quartz sand grains have recrystallized into an interlocking mosaic (also known as metaquartzite); 2) Unmetamorphosed quartz-cemented quartz sandstone (also known as orthoquartzite).
Quaternary	The most recent geologic period, third and last in the Cenozoic Era, noted for multiple glacial and interglacial episodes; approximately 2.58 Ma to the present.
Radiocarbon years (BP)	Uncalibrated radiocarbon dates are presented as radiocarbon years before present (defined as 1950).
Radiocarbon, carbon-14, 14C	A radioactive isotope of carbon that forms the basis of the radiocarbon dating technique of radiometric dating. The half-life (the time required for half of a sample to decay) of radiocarbon is 5,700 ± 30 years. Radiocarbon is used to date organic materials younger than approximately 50,000 years.
Radiodont	A member of the order Radiodonta, an extinct group of arthropods including forms such as Anomalocaris, known from the Cambrian into the Devonian.
Radiolarian	A member of the subphylum Radiolaria, a group of microscopic planktic organisms with silica tests, known from the Cambrian to the present.
Radiometric dating	A geochronologic method in which the age of a given material can be determined through measuring a radioactive isotope’s amount of decay. Different dating schemes may be employed given the age range of a sample (e.g., radiocarbon (14C) dating is reliable for samples up to roughly 50 ka, whereas uranium-lead (U-Pb) dating is reliable for samples ranging from 1 Ma to upwards of 4.5 Ga).
Rauisuchid	A member of the family Rauisuchidae, an extinct group of large carnivorous pseudosuchians that filled similar niches to theropod dinosaurs, known from the Triassic.
Receptaculitid	A member of the family Receptaculitidae or Receptaculitaceae, an extinct group of “sunflower corals,” known from the Ordovician to the end of the Permian. The evolutionary relationships of receptaculitids are unclear; they may be chlorophyte green algae.
Recrystallization	Reorganization of a mineral’s structure under the influence of temperature, pressure, and/or chemically active fluids.
(Marine) Regression (noun), regressive (adjective)	A marine retreat, exposing land.
Repository	A place, often an institution, where artifacts such as fossils are permanently held.
Reworked (geology)	A fossil or other object that has been eroded from an older rock unit and redeposited in a younger unit.
Rhizoconcretion	A concretion that formed around or in response to a root.
Rhizohalo	A zone of mottled strata representing mineral depletion developed around a root.
Rhizolith	A sedimentary structure formed by plant roots and found in paleosols; sometimes referred to as root traces.
Rhodophyte	A member of the division Rhodophyta, commonly known as “red algae”, known from the Mesoproterozoic to the present. Reef-dwelling coralline algae and many seaweeds are rhodophytes.
Rhynchocephalian	A member of the order Rhynchocephalia, a group of lizard-like reptiles with a single living representative, the tuatara. Rhynchocephalian diversity reached its height during the Triassic and Jurassic before being largely overtaken by lizards during the Cretaceous. Members may also be called sphenodontians.
Riparian	Pertaining to a riverbank setting.
Rostroconch	A member of the class Rostroconchia, an extinct group of mollusks that had “taco”-like single folded shells, known from the Cambrian to the end of the Permian.
Ruga	Concentric corrugation on brachiopod shells; plural rugae.
Rugose coral	A member of the subclass Rugosa, an extinct group of corals, known from the Ordovician to the end of the Permian. Rugose corals included both colonial and solitary forms, with the solitary forms known as horn corals.
Sabkha	A salt flat above the level of tides.
Sand	A clastic particle smaller than a granule and larger than a silt grain, having a diameter in the range of 1/16 mm (0.0025 in) to 2 mm (0.08 in).
Sandstone	Clastic sedimentary rock of predominantly sand-sized grains.
Sauropod	A member of the clade Sauropoda, an extinct group of quadrupedal herbivorous dinosaurs with proportionally long necks and tails and small heads, known from the Triassic to the end of the Cretaceous. Most known sauropods were at least 9 m (30 ft) long and weighed several tons.
Sauropodomorph	A member of the clade Sauropodomorpha, an extinct group of dinosaurs including sauropods and their close relatives (sometimes known informally as prosauropods), known from the Triassic to the end of the Cretaceous. The earliest sauropodomorphs differed from the sauropods by being small and bipedal.
Scaphopod	A member of the class Scaphopoda, a group of mollusks with elongate conical shells, also known as “tusk shells”. Scaphopods are known from the Mississippian to the present.
Scenellid	A member of the superfamily Scenelloidea, an extinct group of invertebrates with pyramidal conical shells, known from the Ordovician. Scenellids are probably mollusks, and possibly monoplacophorans or gastropods.
Scolecodont	A jaw element of a bristle worm, a common microfossil.
Screenwashing or wet sieving	A technique by which water is used in conjunction with screens of various sizes in order to separate fossils (particularly microfossils) from sediment and/or matrix.
Sediment	An eroded and deposited, unconsolidated accumulation of rock and mineral fragments.
Sedimentary	Refers to a consolidated and lithified rock consisting of clastic and/or chemical sediment(s). One of the three main classes of rocks, alongside igneous and metamorphic.
Septum	In corals, a septum (plural-- septa) is a vertical plate in a corallite; in shelled cephalopods, the wall between chambers.
Sessile	An organism that cannot move on its own, usually permanently attached to something.
Shale	A clastic sedimentary rock made of clay-sized particles that exhibit parallel splitting properties.
Shoreface	The sloping zone between a horizonal offshore bottom and low tide.
Shuvosaurid	A member of the family Shuvosauridae, an extinct group of toothless, herbivorous, theropod-like pseudosuchians with some taxa strongly resembling the later ornithomimosaurs, known from the Triassic.
Silesaurid	A member of the possibly paraphyletic family Silesauridae, an extinct group of quadrupedal reptiles from the Triassic that are traditionally considered one of the closest relatives of dinosaurs within Dinosauromorpha; recent research has suggested that they may be true dinosaurs.
Silica	The compound SiO2, the building block of the common mineral quartz.
Silicate mineral	A mineral with a framework of silica. Quartz and feldspar minerals are common silicates.
Silicification	The replacement of a non-silicate mineral with a silicate mineral.
Silt	Clastic sedimentary material intermediate in size between fine-grained sand and coarse clay (1/256 to 1/16 mm [0.00015 to 0.002 in]).
Siltstone	A clastic sedimentary rock composed of silt-sized grains.
Silurian	The third geologic period of the Paleozoic Era, following the Ordovician; approximately 443 to 419 Ma.
Siphuncle	The internal passage between chambers in a cephalopod shell.
Skolithos (frequently misspelled Scolithus)	Cylindrical vertical invertebrate burrows.
Slate	Slightly metamorphosed mudstone (such as shale), known for thin flat (“slaty”) cleavage.
Soil	Surface accumulation of weathered rock and organic matter capable of supporting plant growth and often overlying the parent material from which it formed.
Spicule	A structural element of many types of organisms. Many sponges have a skeleton of spicules; the spicules can be simple and toothpick-like or have multiple points, and the form of the tips also varies.
Spikemoss	A member of the order Selaginellales, a group of diminutive plants that reproduces using spores, known from the Pennsylvanian to the present.
Stegosaur	A member of the clade Stegosauria, an extinct group of herbivorous dinosaurs distinguished by large spikes, including at the end of their tail (thagomizer), and bony plates along their backs; known from the Jurassic into the Cretaceous.
Steinkern	A cast of the interior of a hollow organic structure, such as a snail shell, formed when sediments entered the structure and became rock, followed by dissolution of the outer structure.
Stem group (phylogenetics)	A paraphyletic group of organisms basal to and lacking certain traits of a derived clade (e.g., dinosaurs and pterosaurs are stem-birds).
Stratification	The accumulation or layering of sedimentary rocks in strata. Tabular, or planar, stratification refers to essentially parallel surfaces. Cross-stratification refers to strata inclined at an angle to the main stratification.
Stratigraphy	The geologic study of the origin, occurrence, distribution, classification, correlation, and age of rock layers, especially sedimentary rocks.
Stratum (singular), strata (plural)	A layer of rock/sediment distinguished by specific lithological characteristics.
Stromatolite	A layered microbial mat, typically with a columnar or domal structure.
Stromatoporoid	A member of the class Stromatoporoidea, a group of coral-like reef-building sponges. Stromatoporoids were abundant from the Ordovician into the Devonian. They are regarded as extinct, although some modern sponges are similar.
Strophomenid	A member of the order Strophomenida, an extinct group of articulate brachiopods with wide D-shaped shells, known from the Ordovician into the Pennsylvanian. They lacked pedicles and lived directly on the sea floor.
Structure (geology)	Three-dimensional features such as folds and faults.
Stylolite	surfaces that form within rocks, especially carbonate rocks, due to mineral dissolution, usually marked by thin accumulations of insoluble minerals; they generally form parallel to bedding, but at a small scale are extremely irregular. Fragments of stylolites may have a woody appearance, and can be mistaken for petrified wood.
Stylophoran	A member of the class Stylophora, an extinct group of enigmatic echinoderms featuring broad or ovoid bodies with a single appendage, known from the Cambrian into the Pennsylvanian. Stylophorans are also sometimes known as carpoids.
Subtidal	Below low tide.
Supratidal	Above high tide.
Symmetrodont	A member of the order Symmetrodonta, an extinct group of mammals resembling shrews, known from the Cretaceous.
Synapsid	A member of the clade Synapsida, the group containing mammals and all animals more closely related to them than to other vertebrates, known from the Pennsylvanian to the present. Early members are sometimes informally known as “mammal-like reptiles.”
Syntype	One of a group of specimens designated as a name-bearing series for an organism.
Tabula	In corals, a tabula (plural
Tabulate coral	A member of the class Tabulata, an extinct group of colonial corals, known from the Ordovician to the end of the Permian.
Talus or scree	Broken rock shed from cliffs through rock falls.
Tanystropheid	A member of the family Tanystropheidae, an extinct group of long-necked piscivorous reptiles, known from the Triassic.
Taxonomy	The study of naming, defining, and classifying organisms into groups. Taxonomic groups or taxa (singular
Tentaculitid	A member of the order Tentaculitida, an extinct group of “tube worms” of uncertain affinities that produced mm- to cm-scale annulated shells, known from the Ordovician into the Mississippian.
Test	The formal name for the shells of foraminifera, echinoids, and some other marine organisms.
Tetrapod	A member of the superclass Tetrapoda, meaning “four feet”, although some members have lost some or all four limbs (e.g., snakes). Tetrapods include amphibians, reptiles, birds, and mammals, and appeared in the Devonian.
Theropod	A member of the clade Theropoda, a group of bipedal dinosaurs that usually possessed a large head and three long functional toes per hind foot; generally carnivorous but sometimes omnivorous or herbivorous. Theropod dinosaurs include birds, and appeared in the Triassic.
Thrombolite	A clotted microbial structure.
Time-transgressive	The age of a formation varies from place to place because deposition began earlier in some areas than others, due to factors such as sediment input, sea level, and the depositional environment.
(Marine) Transgression (noun), transgressive (adjective)	A marine encroachment, submerging land.
Travertine	a carbonate rock formed by the precipitation of carbonate minerals from groundwater or surface water, or at hot springs.
Trematosaurian	A member of the clade Trematosauria, a diverse extinct group of amphibians with many medium- to large-sized and semi-aquatic to aquatic taxa, known from the Triassic into the Cretaceous.
Triassic	The first geologic period of the Mesozoic Era; approximately 252 to 201 Ma.
Tridactyl	A digit arrangement in which only three digits are present. In ichnology, a track bearing three toes.
Trilobite	A member of the class Trilobita, an extinct group of marine arthropods known from the Cambrian to the end of the Permian. A typical trilobite had a roughly oval body featuring an axial lobe and two lateral lobes (the three lobes of the name), also divided into three sections (cephalon for the head, thorax, and pygidium for the tail); trilobites vaguely resembled woodlice (roly-poly bugs or pillbugs).
Trilophosaurid	A member of the family Trilophosauridae, an extinct group of herbivorous archosauromorphs reptile that resembled large lizards, known from the Late Triassic.
Tritylodontid	A member of the family Tritylodontidae, an extinct group of specialized cynodonts with rodent-like teeth, known from the Triassic into the Cretaceous.
Tufa	In sedimentology, calcareous deposits formed by precipitation from a body of water.
Tuff	A rock that contains more than 75% volcanic ash. Rock with significant but lesser ash content (25% to 75%) is called tuffaceous.
Turbidite	A deposit of a submarine density current.
Unconformity	A general term for any erosional or non-depositional surface between two rock units.
Ungulate	In general usage, a hoofed mammal (Ungulata also includes whales and relatives). Ungulates are known from the Paleocene to the present.
Unionid	A member of the family Unionidae, a diverse group of freshwater bivalves sometimes referred to as “river mussels”, known from the Jurassic to the present. Unionids possess a parasitic larval stage that relies on fish.
Ursid	A member of the family Ursidae, a group of mammals including bears and relatives, known from the Eocene to the present.
Valve	A shell. Snails and monoplacophorans are univalved (one shell) and bivalves and brachiopods are bivalved (two shells).
Volborthella	An enigmatic Cambrian animal known from conical tubular fossils of agglutinated sediment.
Weathering	The physical, chemical, and biological processes by which rock is broken down.
Western Interior Seaway	A shallow continental sea that covered much of the midsection of North America from the Arctic to the Gulf of Mexico between approximately 100 and 70 Ma, during the Cretaceous. Lowering sea levels and the growth of the Rocky Mountains caused it to recede.

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Sources

These definitions were compiled from several park paleontological resource inventories. Sources used to develop and check definitions include:

• The Paleobiology Database

• Wikipedia

• NPS Geologic Resources Inventory—Glossary of Geologic Terms

• Neuendorf, K. K. E., J. P. Mehl Jr., and J. A. Jackson. 2005. Glossary of geology. Fifth edition. American Geological Institute, Alexandria, Virginia.

• Other professional papers and sources.

Related Links

• NPS—Glossary of Geologic Terms

• NPS—Glossary of Volcanic Terms

• The American Geosciences Institute