Glossary

A body fossil is the preserved remains of part or all of an organism's physical body, as opposed to trace fossils (ichnofossils) that record only the evidence of biological activity such as footprints, burrows, or coprolites. Body fossils encompass the full range of anatomical hard parts—bones, teeth, shells, exoskeletons, plates, and wood—as well as, more rarely, soft tissues including skin, organs, feathers, leaves, flowers, and seeds. The distinction between body fossils and trace fossils constitutes the most fundamental classification in paleontology: body fossils document the morphology and anatomy of organisms, while trace fossils document behavior. Body fossils form through a variety of taphonomic processes that begin immediately after the death of an organism. Rapid burial in sediment is the most critical factor enabling preservation, as it shields remains from scavenging, weathering, and aerobic decomposition. Once buried, hard parts may be preserved in their original mineralogy (unaltered remains), undergo permineralization as dissolved minerals fill pore spaces, experience replacement by secondary minerals such as pyrite or silica, recrystallize from one mineral polymorph to another, be carbonized into thin films of stable carbon, or be preserved as molds and casts after the original material dissolves. In exceptional cases, organisms may also be preserved in amber, glacial ice, tar pits, or desiccated cave environments. The body fossil record is inherently biased toward organisms possessing mineralized hard parts—such as the calcite shells of brachiopods, the hydroxyapatite bones and teeth of vertebrates, or the siliceous tests of radiolarians—because these structures are far more resistant to physical and chemical degradation than soft tissues. Consequently, entirely soft-bodied organisms such as jellyfish, worms, and most insects have extremely poor body fossil records outside of exceptional preservation deposits known as Konservat-Lagerstätten.

A **fossil** is any preserved evidence of past life, including physical remains, impressions, traces, and life-history artifacts such as nests or coprolites. Fossils are found almost exclusively in sedimentary rocks and typically refer to evidence of organisms that existed at least 10,000 years ago—before the end of the last ice age. The oldest widely accepted fossils are stromatolites from the Pilbara region of Western Australia, dated to approximately 3.48 billion years ago, indicating that life arose very early in Earth's history. Fossils form through a suite of taphonomic processes that remove organic material from the zone of aerobic decomposition and replace or stabilize it with minerals. Rapid burial in sediment is generally essential, as it shields remains from scavengers and oxygen-driven decay. Once buried, groundwater carrying dissolved minerals can infiltrate pore spaces in bone, shell, or wood (permineralization), or entirely replace original biological material with minerals such as silica, calcite, or pyrite (replacement). Because fossilization demands specific and relatively rare conditions, only a tiny fraction of all organisms that have ever lived have entered the fossil record. Fossils constitute the primary direct evidence for reconstructing the history of life on Earth. They are the foundational data of paleontology, enabling scientists to identify extinct species, trace evolutionary lineages, infer past behaviors through trace fossils, reconstruct ancient ecosystems and climates, and calibrate the geologic time scale through biostratigraphy. Without fossils, knowledge of the approximately 3.5-billion-year saga of biological evolution would remain almost entirely inferential.

The fossil record is the totality of all fossils that have ever existed throughout the history of life on Earth, whether discovered or not, as preserved in sedimentary rocks and other geological deposits. It encompasses body fossils (bones, shells, teeth, leaves, and other physical remains), trace fossils (tracks, burrows, coprolites, and other evidence of biological activity), and chemical fossils (molecular biomarkers and isotopic signatures). The fossil record accumulates through the process of fossilization, in which the remains or traces of organisms are buried in sediment and subsequently lithified over geological time. Because fossilization requires specific conditions—rapid burial, the presence of hard tissues, and favorable geochemical environments—the record is inherently incomplete and subject to multiple biases, including taphonomic, preservational, geographic, and sampling biases. Only a small fraction of all species that have ever lived, commonly estimated at less than one percent, are represented by known fossils. Despite this incompleteness, the fossil record serves as the primary empirical source for reconstructing the history of biodiversity, documenting evolutionary transitions, calibrating molecular clocks, establishing biostratigraphic correlations, and understanding the dynamics of origination, extinction, and ecological change across geological time. It provides the only direct observational evidence for the temporal sequence of life's major evolutionary innovations and the timing and magnitude of mass extinction events.

A holotype is the single physical specimen upon which a new nominal species-group taxon is based in its original publication, serving as the permanent, objective standard of reference for the application of that species name. Under the International Code of Zoological Nomenclature (ICZN, 4th edition, Article 73.1), the holotype is fixed exclusively in the original publication by the original author, either through explicit designation or by monotypy when the description is based on only one specimen. Under the International Code of Nomenclature for algae, fungi, and plants (ICN, Shenzhen Code, Article 9.1), the holotype is similarly defined as the one specimen or illustration indicated by the author as the nomenclatural type, or used by the author when no type was indicated. As long as the holotype is extant, it fixes the application of the name concerned, providing an objective anchor that prevents taxonomic names from drifting in meaning regardless of how species boundaries may be redrawn by subsequent researchers. When a holotype is designated, all other specimens of the type series become paratypes, which have no name-bearing function. The ICZN mandates that for any new species-group taxon proposed after 1999, fixation of a holotype (or expressly indicated syntypes) is a requirement for nomenclatural availability. The holotype thus stands as the cornerstone of biological nomenclature, ensuring stability, universality, and reproducibility in the naming of species across all domains of life.

An index fossil is a fossil of an organism that is used to define and identify a specific, relatively narrow interval of geologic time and to correlate the strata in which it occurs with contemporaneous strata at distant localities. In biostratigraphy, index fossils serve as biological markers within sedimentary rock sequences, enabling geologists to assign relative ages to rock units and to establish temporal equivalence between geographically separated sections. For a fossil to qualify as a useful index fossil, the source organism must satisfy several key criteria simultaneously: it must have existed for only a short geologic time span (indicating rapid evolutionary turnover), it must have been geographically widespread across large regions or multiple continents, it must have been sufficiently abundant that specimens are commonly recovered, its remains must be readily preservable (typically possessing hard parts such as shells or exoskeletons), and it must be morphologically distinctive enough to be easily identified. Because marine organisms are more likely to achieve wide geographic distribution through oceanic dispersal, the most effective index fossils tend to be marine invertebrates and microfossils rather than terrestrial vertebrates. The concept of index fossils is foundational to the construction of the geologic time scale. Virtually all stratigraphic correlation above the formation level depends on biostratigraphy, and the boundaries between geologic periods, epochs, and stages are typically defined by the first appearance of a diagnostic index taxon at a designated Global Boundary Stratotype Section and Point (GSSP). Without index fossils, the relative dating framework that underpins historical geology would be impossible to establish across widely separated regions.

A skin impression is a type of fossil that preserves the surface texture and pattern of an organism's integument as a negative relief mold in sedimentary rock, without retaining the original organic tissue itself. In paleontology, this term most commonly refers to the fossilized imprints of non-avian dinosaur skin, which record the arrangement, shape, and size of epidermal scales, tubercles, and other integumentary structures. Skin impressions form when fine-grained sediment encases the outer surface of an animal's skin — whether on a carcass, a body part in contact with substrate, or the sole of a foot pressing into mud — and subsequently lithifies before the organic material decays. Because soft tissues rarely survive the fossilization process, these impressions constitute the primary direct evidence for reconstructing the external appearance and epidermal morphology of extinct vertebrates. They provide critical information on scale geometry (polygonal, tuberculate, rosette-pattern, etc.), regional variation in integument across the body, and the presence or absence of feather-like structures. Consequently, skin impressions are among the most scientifically valuable and publicly captivating fossils for understanding how dinosaurs looked in life, and they serve as key evidence for paleoartistic reconstructions, inferences about thermoregulation, locomotion, camouflage, and the evolutionary transition from scaled to feathered integument in archosaurs.