Abelisauridae
A family of mainly large Gondwanan predatory theropods with deep skulls, very short forelimbs and powerful hind limbs.
Palaeontology reference
Palaeontology glossary
Look up the language used to describe dinosaurs, fossils, ancient environments, geological time and evolutionary relationships. Every definition is available on this page and can be filtered instantly.
249defined terms
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A to Z reference
Terms and definitions
249 terms shown
Acritarchs
An informal group of organic microfossils of uncertain affinity, commonly the walls of single-celled organisms and especially useful in Precambrian and Palaeozoic research.
Adaptation
An inherited feature that increases fitness in particular conditions. Not every useful feature originally evolved as an adaptation.
Adaptive radiation
A comparatively rapid divergence of one evolutionary lineage into many forms occupying different ecological niches.
Air sac
A thin-walled extension of the avian respiratory system. Pneumatic bones suggest comparable air-sac systems in many theropods and sauropods.
Allometry
A change in relative body proportions during growth or among animals of different sizes.
Allosauroidea
A clade of large predatory theropods containing allosaurids, carcharodontosaurids and their closest relatives.
Ammonites
An extinct group of marine cephalopods with coiled chambered shells. Rapid evolution and wide distribution make many species useful index fossils.
Amniotic egg
An egg with extraembryonic membranes that allowed the ancestors of reptiles, birds and mammals to reproduce away from open water.
Anapsid skull
A conventional term for a skull without temporal openings behind the orbit. The absence of openings alone does not prove close relationship.
Ankylosaurs
Quadrupedal herbivorous ornithischians with bony plates in the skin. Some later ankylosaurids evolved a bony tail club.
Anthropocene
A proposed interval of conspicuous human influence on Earth. It is not a ratified geological epoch in the current international time scale.
Antorbital fenestra
An opening in the archosaur skull in front of the orbit, varying in size and surrounding depressions among lineages.
Apomorphy
An evolutionarily derived character state compared with the ancestral condition.
Aquatic lifestyle
Permanent life in water with a corresponding adaptation complex. A few swimming features do not prove that a dinosaur was fully aquatic.
Archaeocyaths
Extinct Early Cambrian marine organisms with calcareous cup-shaped skeletons that helped build the first large Phanerozoic reef systems.
Archean Eon
The eon from 4,031 to 2,500 million years ago, when ancient continental nuclei formed and microbial life existed.
Archosaurs
The diapsid clade containing crocodilians, pterosaurs, dinosaurs and birds. Birds and crocodilians are its living members.
Beak
A toothless keratin covering over all or part of the jaws. Fossils usually preserve its bony base, so the covering's limits require caution.
Bed
A comparatively uniform sedimentary rock body bounded by bedding surfaces.
Belemnites
Extinct marine cephalopods related to modern squid. Their bullet-shaped internal rostrum is the part most often preserved.
Benthos
Organisms living on the bottom of a body of water or within its sediment.
Binocular vision
Overlap between the visual fields of both eyes, improving distance judgement. Orbit orientation constrains overlap, but soft tissues are usually unknown.
Binomial name
A two-word scientific species name consisting of genus and specific epithet, such as Tyrannosaurus rex.
Binomial nomenclature
The system of two-part species names consisting of a genus name followed by a specific epithet.
Biogeography
The study of organism distribution through space and time. In palaeontology it connects dispersal with continental drift and climate change.
Biomarker
A chemical compound or isotope signal that may indicate particular organisms. Its origin must be tested because similar signals can form without life.
Biome
A large ecological region united by broadly similar climate, vegetation and biological communities.
Biomechanics
The study of forces and movement in organisms. Fossil studies use bone and joint form, trackways and computer models.
Biostratigraphy
Correlation and relative dating of rock layers using the fossils they contain.
Biota
The organisms of a particular region or geological interval.
Bioturbation
Mixing of sediment by burrowing or crawling organisms. It can erase earlier structures or create trace fossils.
Bipedality
Locomotion mainly on two hind limbs. Every known theropod was bipedal, although posture and gait varied.
Birds
Living feathered theropod dinosaurs, supported by shared evidence from skeletons, feathers, eggs, breathing anatomy and behaviour.
Bone cells
Cells of bone tissue and the spaces they occupied. Palaeohistology studies their form and density, but intact cells rarely survive.
Bone histology
Study of microscopic bone structure. Thin sections constrain growth, remodelling and approximate individual age.
Bone pneumaticity
Air-filled cavities in bones connected to the respiratory system, especially conspicuous in sauropod and many theropod vertebrae.
Bonebed
An accumulation of bones from several individuals or species formed by mass death, water transport or prolonged concentration.
Brontosaurus
A genus of large Late Jurassic North American sauropods. After long synonymy with Apatosaurus, several modern studies again recognise Brontosaurus as distinct.
Calymmian Period
The first Mesoproterozoic period, from 1,600 to 1,400 million years ago, marked by widespread platform sedimentary covers.
Cambrian Period
The first Palaeozoic and Phanerozoic period, from 538.8 to 486.85 million years ago, with major animal diversification and widespread mineralised skeletons.
Carboniferous Period
The Palaeozoic period from 358.86 to 298.9 million years ago, known for extensive coal swamps and diverse early terrestrial vertebrates.
Cast
A copy of a fossil surface or volume produced by filling a natural or manufactured mould. It is not the original bone.
Cenozoic Era
The present Phanerozoic era, beginning 66 million years ago and containing the Palaeogene, Neogene and Quaternary periods.
Read the full guideCeratopsians
Herbivorous ornithischians with beaks and a distinctive rostral bone; later ceratopsids carried large frills and horns.
Cervical vertebrae
Vertebrae between skull and trunk; their number, elongation and pneumaticity are especially important in reconstructing sauropod necks.
Chemostratigraphy
Correlation of layers using changes in the chemical or isotopic composition of rocks.
Chicxulub impactor
The extraterrestrial body that formed the Chicxulub crater about 66 million years ago and was the principal driver of the end-Cretaceous mass extinction.
Choanae
Internal nostrils connecting the nasal cavity with the mouth or pharyngeal region.
Chron
An interval of one geomagnetic polarity recorded in rock; chron sequences are used in magnetostratigraphy.
Clade
A group containing a common ancestor and all its descendants; such a group is monophyletic.
Cladistics
A method for reconstructing relationships from the distribution of shared derived characters.
Read the full guideCladogram
A branching diagram of a relationship hypothesis. Branch length usually does not show time or amount of change unless explicitly scaled.
Climate proxy
Indirect evidence for ancient climate, such as isotope composition, soil type, pollen or organism distribution.
Coelurosauria
The large theropod clade containing tyrannosauroids, ornithomimosaurs, maniraptorans and birds.
Collagen
The main protein of connective tissue and bone. Ancient fragments may survive, but claims require strict exclusion of contamination.
Collection number
A unique museum identifier linking a specimen to its catalogue, locality, documentation and custody history.
Computed tomography
A non-destructive method producing sectional X-ray images of internal anatomy and objects hidden within rock.
Concretion
A dense mineral body within sedimentary rock. Concretions may preserve fossils, but many form without an organism at their centre.
Consensus tree
A summary phylogenetic tree showing relationships shared among several equally well-supported trees.
Continental drift
Movement of continents through geological time, now explained by plate tectonics.
Convergence
Independent evolution of similar traits in distantly related groups under similar functional or environmental pressures.
Coprolite
Fossilised faeces. Contents and structure may reveal diet, although the exact producer is often uncertain.
Cranial suture
A joint between skull bones. Fusion may relate to age but differs among groups and is not a universal maturity clock.
Cranium
The braincase and facial skeleton without the lower jaw. It is often used loosely for the whole skull, although the strict meaning is narrower.
Cretaceous Period
The final Mesozoic period, from 143.1 to 66 million years ago, ending with the Cretaceous–Palaeogene mass extinction.
Read the full guideCretaceous–Palaeogene boundary
The boundary about 66 million years old that coincides with the extinction of non-avian dinosaurs and many marine groups.
Crown group
The last common ancestor of all living members of a group and all its descendants, including extinct forms.
Cryogenian Period
The middle Neoproterozoic period, roughly 720 to 635 million years ago, including the Sturtian and Marinoan global glaciations.
Cursoriality
Adaptation for efficient running, reflected in limb proportions, light distal segments and joint construction.
Cynodonts
The therapsid clade containing mammals and their closest extinct relatives. Many non-mammalian forms combine reptile-like and mammal-like traits.
Deinonychosauria
A historically used group combining dromaeosaurids and troodontids. Its exact membership and monophyly depend on phylogenetic analysis.
Dental battery
A dense system of many replacement teeth forming one working surface, especially developed in hadrosaurids and ceratopsids.
Dentary
The principal tooth-bearing bone of the lower jaw in most vertebrates and a commonly preserved fossil element.
Devonian Period
The Palaeozoic period from 419.62 to 358.86 million years ago, noted for diverse fishes, early forests and the first tetrapods.
Diapsids
A major reptile branch ancestrally bearing two temporal openings on each side of the skull; it includes lizards, snakes, crocodilians, pterosaurs, dinosaurs and birds.
Dinosaurs
The archosaur clade containing the common ancestor of ornithischian and saurischian dinosaurs and all its descendants, including birds.
Read the full guideDromaeosauridae
A family of feathered maniraptoran theropods with an enlarged sickle-shaped claw on the second toe.
Ectasian Period
The middle Mesoproterozoic period, from 1,400 to 1,200 million years ago, named for continued expansion of sedimentary basins.
Ectothermy
A strategy in which body temperature depends substantially on external heat sources, without implying complete absence of physiological regulation.
Ediacaran Period
The final Neoproterozoic period, from 635 to 538.8 million years ago, preserving the first widespread large complex organisms.
Endemic taxon
A taxon known only from a restricted region. Fossil endemism must be separated from apparent restriction caused by poor sampling.
Endothermy
Maintenance of body temperature mainly through internal metabolic heat, inferred in extinct animals from several indirect evidence lines.
Eoarchean Era
The first Archean era, from 4,031 to 3,600 million years ago, containing some of the oldest rocks and possible early-life evidence.
Eocene Epoch
The middle Palaeogene epoch, from 56 to 33.9 million years ago, including one of the warmest Cenozoic intervals.
Eon
The largest formal unit of geological time, divided into eras.
Epoch
A subdivision of a geological period; its chronostratigraphic counterpart is a series of rocks.
Era
A subdivision of a geological eon made of periods; its chronostratigraphic counterpart is an erathem.
Erosion
Breakdown and transport of rock by water, wind, ice or gravity, exposing fossil beds while also destroying material.
Evolution
Change in inherited population traits over generations and the historical divergence of organism lineages.
Extinction
The complete disappearance of a species or larger lineage, inferred from its last secure records while allowing for an incomplete rock record.
Facultative bipedality
Ability to alternate between two-legged and four-legged locomotion according to speed or behaviour.
Feather
A keratinous covering structure that evolved in dinosaurs before powered flight. Fossils range from simple filaments to complex vaned feathers.
Femur
The proximal hind-limb bone between pelvis and knee. Its dimensions and proportions are often used to estimate fossil animal length and mass.
Formation
See Geological formation: the principal named lithostratigraphic unit distinguished by rock properties.
Fossil
A preserved body remain, impression, trace of activity or chemical signal from the geological past.
Fossil preparation
Removal of surrounding rock, stabilisation and preparation for study. Poor technique can permanently destroy anatomical evidence.
Fossil record
All known fossils and their geological contexts. It is incomplete and uneven because burial, preservation, exposure and study differ among environments.
Fossilisation
The processes by which an organism's remains or activity traces enter and persist in the geological record.
Read the full guideGastralia
Slender bones in the abdominal wall of some vertebrates that do not attach to the spine and are not true ribs.
Gastroliths
Stones retained in an animal's digestive tract, possibly for grinding food or ballast. A rounded stone beside a skeleton is not automatically a gastrolith.
Genus
A rank above species. It is the capitalised first word in a binomial species name.
Geochronology
The branch of geology that establishes the sequence and ages of events in Earth history.
Geological formation
A named, mappable body of rock. A formation is not a unit of time and may span unequal durations in different places.
Geological time scale
The hierarchy of time units and corresponding rock bodies: eons, eras, periods, epochs and ages.
Read the full guideGondwana
The southern part of Pangaea and an older continental assembly including future South America, Africa, Antarctica, Australia, India and Arabia.
Graviportality
Limb adaptations for supporting very great mass, including massive bones, stable joints and columnar legs.
Growth plate
A cartilage zone near the end of a growing bone. Fossil maturity is usually assessed from several fusion and histological indicators.
Growth trajectory
Change in growth rate through life, recorded in bone microstructure and fusion of skeletal elements.
GSSP
A ratified point in a particular rock section defining the lower boundary of an international time-scale unit, informally called a golden spike.
Gut contents
Food remains preserved within the trunk region. They are direct dietary evidence, but position and association must be verified.
Hadean Eon
Earth's earliest eon, from planetary formation about 4,567.3 to 4,031 million years ago.
Hadrosauridae
A family of Late Cretaceous ornithopods with broad beaks and dental batteries. Some carried hollow or solid cranial crests.
Heterodonty
The presence of teeth with different forms and functions in one jaw.
Holocene
The present Quaternary epoch, beginning 11,700 years ago after the last cold stage of the Pleistocene.
Holotype
The single specimen fixed as the name-bearing type of a new species in its original description. It need not be the most complete specimen.
Read the full guideHomology
Similarity inherited from a common ancestor. A bird wing and crocodilian forelimb are homologous as tetrapod forelimbs.
Host rock
Rock directly surrounding a fossil and preserving evidence about its burial conditions.
Ichnology
The branch of palaeontology concerned with traces of organism activity.
Ichnotaxon
A taxon established for traces such as footprints or burrows. It classifies the trace, not necessarily the biological species that made it.
Impression
A two-dimensional or shallow relief record of a body or body part in sediment, sometimes preserving outline without original tissue.
Index fossil
A widespread, recognisable and comparatively short-lived taxon useful for correlating the ages of rock layers.
Intraspecific variation
Differences among members of one species caused by age, sex, individual variation or developmental conditions.
Jurassic Period
The middle Mesozoic period, from 201.4 to 143.1 million years ago, when large sauropods, stegosaurs and many theropod groups spread widely.
Read the full guideLagerstätte
A fossil locality with exceptional preservation or unusual richness. The term does not imply that every organism is equally complete.
Late Cretaceous
The second Cretaceous epoch, from 100.5 to 66 million years ago, when tyrannosaurids, ceratopsids and many hadrosaurids lived.
Lateral line
A sensory system in aquatic vertebrates that detects water movement. In fossils it may be inferred from canals in skull bones.
Laurasia
The northern part of fragmented Pangaea from which most of Eurasia and North America later formed.
Lectotype
One specimen from an original syntype series later designated as the single name-bearing type.
Line of arrested growth
A microscopic bone line associated with slower or stopped growth. Counting lines requires correction for remodelling of inner cortex.
Lithostratigraphy
Description and correlation of rock successions by composition, structure and position rather than numerical age.
Maniraptora
The coelurosaur clade containing birds, dromaeosaurids, troodontids, oviraptorosaurs and other feathered theropods.
Mass extinction
A geologically rapid global loss of diversity across many unrelated groups, exceeding background extinction in scale and rate.
Matrix
The rock surrounding a fossil. In phylogenetics, the same word means a table of taxa and character states.
Medullary bone
Temporary calcium-rich tissue in female birds before egg laying. Similar tissue in dinosaurs may indicate reproductive state but needs careful diagnosis.
Mesoarchean Era
The Archean era from 3,200 to 2,800 million years ago, when continental masses and diverse microbial ecosystems expanded.
Mesoproterozoic Era
The middle Proterozoic era from 1,600 to 1,000 million years ago, containing the Calymmian, Ectasian and Stenian periods.
Mesothermy
A proposed heat-exchange strategy intermediate between typical endothermy and ectothermy. Its application to dinosaurs remains debated.
Mesozoic Era
The Phanerozoic era from 251.902 to 66 million years ago, containing the Triassic, Jurassic and Cretaceous periods.
Read the full guideMicrofossil
A fossil normally studied under a microscope, such as a spore, pollen grain, single-celled organism, tiny tooth or tissue fragment.
Miocene Epoch
The first Neogene epoch, from 23.04 to 5.333 million years ago, when open grassland ecosystems expanded and many modern mammal groups developed.
Monophyly
The condition of a group containing a common ancestor and every descendant of that ancestor.
Morphology
The study of organism form and structure.
Morphometrics
Quantitative analysis of anatomical form and size, including statistical comparison of landmark coordinates.
Mosasaurs
An extinct group of large Late Cretaceous marine squamates. They were not dinosaurs.
Name-bearing type
The type specimen or specimen series to which the application of a zoological name is objectively attached.
Natural selection
Unequal contribution of heritably different organisms to later generations, which can change trait frequencies through time.
Neoarchean Era
The final Archean era, from 2,800 to 2,500 million years ago, ending before major early Proterozoic atmospheric change.
Neogene Period
The Cenozoic period from 23.04 to 2.58 million years ago, containing the Miocene and Pliocene epochs.
Neoproterozoic Era
The final Proterozoic era, from 1,000 to 538.8 million years ago, containing the Tonian, Cryogenian and Ediacaran periods.
Neotype
A replacement name-bearing specimen designated when original type material is lost or absent and a new type is necessary for stable usage.
Nomen dubium
A name of doubtful application, usually because its type material cannot reliably distinguish the taxon from others.
Read the full guideNomen nudum
A published name that fails zoological nomenclature requirements and is therefore not an available scientific name.
Nomenclature
Rules governing the formation, publication and application of organism names. Nomenclature regulates names, not evolutionary relationships.
Non-avian dinosaurs
All dinosaurs outside birds, a phrase that makes explicit that birds are living dinosaurs.
Oligocene Epoch
The final Palaeogene epoch, from 33.9 to 23.04 million years ago, marked by general cooling and growth of a major Antarctic ice sheet.
Ontogeny
Development of an individual from early stages to adulthood. Age-related change can be mistaken for differences between species.
Opisthopubic pelvis
A pelvis in which the pubis points backwards, characteristic of ornithischians and independently evolved in some theropods.
Ordovician Period
The second Palaeozoic period, from 486.85 to 443.1 million years ago, including the Great Ordovician Biodiversification Event.
Ornithischians
A traditional major dinosaur branch containing ornithopods, ceratopsians, stegosaurs, ankylosaurs and pachycephalosaurs.
Ornithopods
Herbivorous ornithischians including iguanodontians and hadrosaurids; early members were generally small and bipedal.
Orosirian Period
The Palaeoproterozoic period from 2,050 to 1,800 million years ago, marked by major mountain building and impact structures.
Osteoderm
A bony plate formed in the skin, contributing to the armour of ankylosaurs, crocodylomorphs and many other vertebrates.
Outcrop
A place where rock is exposed at the surface, such as a slope, shore, quarry or road cutting.
Palaeoarchean Era
The Archean era from 3,600 to 3,200 million years ago, preserving ancient microbial structures and early continental crust.
Palaeocene Epoch
The first Palaeogene epoch, from 66 to 56 million years ago, immediately following the end-Cretaceous extinction.
Palaeoclimate
Climate of the geological past reconstructed from rocks, fossils, isotopes and models.
Palaeoecology
Reconstruction of interactions between ancient organisms, their environment and one another.
Palaeogene Period
The first Cenozoic period, from 66 to 23.04 million years ago, containing the Palaeocene, Eocene and Oligocene epochs.
Read the full guidePalaeogeography
Reconstruction of the positions of continents, seas, mountains and basins in the past.
Palaeontology
The science of past life using fossils, traces, chemical signals and their geological context.
Palaeoproterozoic Era
The first Proterozoic era, from 2,500 to 1,600 million years ago, containing the Great Oxidation Event and four official periods.
Palaeozoic Era
The first Phanerozoic era, from 538.8 to 251.902 million years ago, containing the Cambrian through Permian periods.
Read the full guidePangaea
The supercontinent containing almost all land by the late Palaeozoic. Its Mesozoic breakup changed oceans, climate and dispersal routes.
Paraphyly
The condition of a group containing a common ancestor but not all descendants; traditional reptiles excluding birds are paraphyletic.
Paratype
A specimen in the type series of a new species other than the holotype and cited in the original description.
Permian Period
The final Palaeozoic period, from 298.9 to 251.902 million years ago, ending with the largest Phanerozoic mass extinction.
Permineralisation
Filling of pores and cavities with minerals from water, sometimes preserving original microstructure in bone or wood.
Phanerozoic Eon
The eon from 538.8 million years ago to the present, containing the Palaeozoic, Mesozoic and Cenozoic eras.
Read the full guidePhotogrammetric model
A measurable digital three-dimensional model built from many overlapping photographs of an object or surface.
Photogrammetry
Creation of a measurable three-dimensional model from overlapping photographs, used for bones, tracks and whole excavations.
Phylogenetic matrix
A table of taxa and coded character states used to construct and test relationship hypotheses.
Phylogenetics
The study of evolutionary relationships and lineage divergence.
Plankton
Organisms drifting in the water column and unable to move fully against currents.
Plate tectonics
The theory of moving and interacting lithospheric plates that explains continental drift, oceans, mountains, volcanic arcs and many earthquakes.
Pleistocene Epoch
The first Quaternary epoch, from 2.58 million to 11,700 years ago, marked by repeated glacial cycles and the spread of Homo.
Plesiomorphy
An ancestral character state inherited from a distant ancestor and therefore weak evidence of close relationship by itself.
Plesiosaurs
Extinct four-flippered marine reptiles from the Late Triassic to the end of the Cretaceous. They were not dinosaurs.
Pliocene Epoch
The final Neogene epoch, from 5.333 to 2.58 million years ago, preceding Pleistocene glacial cycles.
Polytomy
A phylogenetic node with three or more branches, reflecting either rapid divergence or insufficient evidence for one branching order.
Postcranial skeleton
Every skeletal element behind the skull, including vertebrae, ribs, girdles and limb bones.
Powered flight
Flight generating thrust through wing movements, unlike gliding in which an animal mainly loses height.
Precambrian
An informal supereon covering the Hadean, Archean and Proterozoic, from Earth's formation to the Cambrian boundary 538.8 million years ago.
Predation
An interaction in which one organism kills and consumes another, evidenced by gut contents, bite marks, coprolites and sometimes healed injuries.
Proterozoic Eon
The eon from 2,500 to 538.8 million years ago, spanning major oxygen changes, eukaryote diversification and the first secure large multicellular organisms.
Provenance
The documented origin and custody history of a specimen, including locality, layer, collection date and legal export.
Pterosaurs
An extinct flying archosaur clade closely related to but outside Dinosauria, with a wing supported by an elongated fourth finger.
Quaternary Period
The present Cenozoic period, beginning 2.58 million years ago and containing the Pleistocene and Holocene epochs.
Read the full guideRadiometric dating
Determining mineral age from radioactive isotopes and their decay products. Volcanic layers are usually dated, not sedimentary fossil bone itself.
Reconstruction
Evidence-based restoration of missing anatomy or appearance. Bones may be direct, soft tissues inferred by comparison and colour an artistic choice.
Regurgitalite
A fossilised mass of regurgitated, undigested food that may contain prey bones, scales or shells.
Rhyacian Period
The second Palaeoproterozoic period, from 2,300 to 2,050 million years ago, containing much of the Huronian glaciation.
Rock age
The time when a geological layer formed. Relative and numerical methods estimate it, but layer age need not equal the exact age of a reworked bone.
Saurischians
One of the two traditional major dinosaur branches, containing theropods and sauropodomorphs. The name describes ancestral pelvic form, not affinity with modern lizards.
Sauropodomorphs
The saurischian branch containing sauropods and their earlier bipedal or facultatively quadrupedal relatives.
Sauropods
Usually large quadrupedal sauropodomorphs with long necks, long tails and heads small relative to the body.
Sedimentary rock
Rock formed by accumulation, compaction and cementation of sediment or by chemical precipitation. Most fossils occur in sedimentary rocks.
Sedimentation rate
The rate at which sediment accumulates, affecting a layer's time resolution and the chance of rapid burial.
Sexual dimorphism
Consistent differences between males and females of one species. In dinosaurs it is difficult to demonstrate without large samples and independent sex evidence.
Siderian Period
The first Palaeoproterozoic period, from 2,500 to 2,300 million years ago, named for widespread banded iron formations.
Silurian Period
The third Palaeozoic period, from 443.1 to 419.62 million years ago, when marine ecosystems recovered and life expanded on land.
Sister groups
Two lineages that are each other's closest relatives on a phylogenetic tree.
Species
A basic unit of biological classification. Fossil species are usually separated by stable anatomical differences because interbreeding cannot be tested.
Specific epithet
The second part of a binomial species name. An epithet alone is not a complete scientific name.
Stage
The smallest principal international chronostratigraphic unit; the corresponding unit of geological time is an age.
Statherian Period
The final Palaeoproterozoic period, from 1,800 to 1,600 million years ago, named for relative stabilisation of continental platforms.
Stegosaurs
Quadrupedal herbivorous ornithischians with vertical plates or spikes along the back and spikes on the tail.
Stem group
Extinct forms closer to a particular living crown group than to any other, while lying outside that crown group.
Stenian Period
The final Mesoproterozoic period, from 1,200 to 1,000 million years ago, associated with Rodinia's assembly and complex eukaryotes.
Stratigraphic range
The interval between a taxon's earliest and latest known occurrences in rock, which may not equal its true duration.
Stratigraphy
The study of the sequence, composition, age and relationships of rock layers.
Stromatolites
Layered sedimentary structures often built by microbial communities. Not every similar layered structure is necessarily biological.
Subduction
Descent of one lithospheric plate beneath another, linked with volcanism, earthquakes and recycling of oceanic crust.
Synapomorphy
A shared derived character inherited by clade members from their most recent common ancestor.
Synapsids
The amniote branch containing mammals and extinct forms closer to them than to reptiles. Dimetrodon and therapsids were not dinosaurs.
Synonym
One of two or more scientific names judged to refer to the same taxon; normally the earliest available name has priority.
Syntype
Each specimen in a type series when the original description did not designate one holotype.
Systematics
The study of organism diversity, classification and evolutionary relationships.
Read the full guideTail club
The enlarged impact structure at the end of some ankylosaurid tails, built from fused vertebrae and large osteoderms.
Taphonomy
Study of how remains pass from the biosphere into the rock record through death, decay, transport, burial and later alteration.
Taxon diagnosis
A character or character combination that distinguishes a taxon from closely related groups.
Temporal fenestra
An opening behind the orbit associated with jaw-muscle arrangement. Its number and position help compare major amniote groups.
Tetrapods
Vertebrates with four limbs or descended from four-limbed ancestors, including amphibians, reptiles, birds and mammals.
Thermoregulation
Maintenance of body temperature through behavioural, physiological and anatomical mechanisms.
Theropods
Mainly bipedal saurischian dinosaurs. Most were carnivorous, some became omnivorous or herbivorous, and birds are theropods.
Thin section
A very thin slice of rock, bone or tooth prepared for microscopic study of minerals or tissues.
Tonian Period
The first Neoproterozoic period, from 1,000 to about 720 million years ago, when Rodinia fragmented and eukaryote diversity expanded.
Tooth wear
Marks produced by food, substrate and tooth contact. Macrowear and microscopic scratches help test dietary hypotheses.
Trace character
A feature of a fossil trace recording movement or interaction with substrate rather than necessarily revealing exact anatomy.
Trace fossil
A fossilised footprint, trackway, burrow, nest, feeding mark, coprolite or other evidence of activity.
Tracing
A precise graphic transfer of fossil or trace-surface outlines, now commonly supplemented by photogrammetry and three-dimensional scanning.
Trackway
A sequence of prints made by one moving animal, constraining direction, gait and approximate speed but not always exact species.
Triassic Period
The first Mesozoic period, from 251.902 to 201.4 million years ago, when the first dinosaurs, pterosaurs and true mammals appeared.
Read the full guideTrilobites
An extinct class of marine arthropods with a three-lobed longitudinal and segmented transverse exoskeleton, known from the Cambrian to end-Permian.
Troodontidae
Small feathered maniraptoran theropods with slender limbs and relatively large braincases; the group's membership is under revision.
Type locality
The place from which the type material of a species or subspecies originated.
Type species
The species to which a genus name is objectively attached. It need not be the oldest, commonest or best-known species.
Vertebra
One element of the vertebral column, usually consisting of a centrum, arch and processes that vary along the spine.
Volcanic ash
Fine material expelled by a volcano. Minerals within it can sometimes radiometrically date layers above or below a fossil occurrence.
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Frequently asked questions
What is the difference between a fossil and palaeontology?
A fossil is preserved evidence of past life, such as a bone, shell, footprint or burrow. Palaeontology is the science that studies fossils and uses them to reconstruct organisms, environments and the history of life.
Why do palaeontological terms and names change?
New specimens, improved dating and revised comparisons can change how researchers interpret a feature or relationship. Scientific names also follow formal nomenclatural rules, so a name may be corrected, replaced or treated as a synonym when evidence changes.
What is the difference between a body fossil and a trace fossil?
A body fossil preserves part of an organism, such as bone, tooth, shell or wood. A trace fossil records activity rather than the body itself, for example a footprint, trackway, burrow, bite mark or coprolite.
Why do geological dates sometimes contain decimals or change over time?
Boundaries are calibrated from radiometric dates, stratigraphic markers and correlated rock sections. More precise measurements can move a boundary slightly, while decimals preserve the agreed numerical estimate instead of rounding away useful precision.