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A fossil acanthodian whose preserved braincase informs early jawed-vertebrate comparisons.

The biological species concept groups naturally interbreeding biological populations with viable, fertile offspring, distinguished from other such groups by reproductive isolation. An operational test examines mating, fertilization, offspring survival and offspring reproduction, rather than only whether two adults can be paired. Sexual reproduction is its scope: fossils and organisms studied only through asexual reproduction cannot supply the same mating evidence. ^[ref]

For an original hypothetical example, imagine two separated biological populations that reproduce successfully after contact. The separation alone did not demonstrate reproductive isolation. Conversely, failure to mate in one unfamiliar chamber could reflect the assay conditions. Field observations of gene flow and tests under suitable conditions address the biological question more directly. A successful laboratory pairing does not establish frequent interbreeding in nature.

A member of the avian lineage of theropod dinosaurs. The extent of the named group can differ between crown-based and broader fossil usages.

A member of the molluscan group with a shell comprising two articulated valves. Marine bivalves provide abundant fossils for studying extinction selectivity, geographic distributions and changes in species assemblages.

A geological period beginning approximately 539 million years ago, with exceptional deposits documenting diverse early animals, including early vertebrates. Increased fossil visibility and biological diversification must be distinguished.

A Middle Jurassic mammaliaform with aquatic adaptations and preserved integument. The fossil supplies a minimum occurrence for a fur-bearing covering, not the exact origin date of endothermy. ^[ref]

The lineage of whales, dolphins and porpoises. Its Eocene fossils document a sequence from terrestrial or amphibious relatives to fully aquatic forms; living members comprise Odontoceti and Mysticeti.

Numerical solution of fluid-flow models. In fossil functional analysis, reconstructed geometry, material assumptions and boundary conditions constrain the interpretation.

The geological boundary at approximately 66 million years ago, associated with a major mass extinction. Deep lineage splits, ecological radiation and first fossil occurrence are distinct events in assessing mammalian history.

Physical and chemical alteration after burial, potentially modifying fossil tissues and isotopic compositions.

The final geological period preceding the Cambrian. Its fossil communities provide evidence of animal complexity before the conspicuous Cambrian radiation.

A highly mineralized epithelial product formed by ameloblasts, distinct from dentine. fossil tissue distributions and enamel-matrix gene expression support an ancestry in dermal coverings deeper than its familiar deployment on teeth.^[ref]

A fossil reptile proposed as an early turtle relative. Its skull anatomy, ontogeny and phylogenetic placement have been used to test the history of temporal roof closure.

Evolution is descent with inherited modification. Within a population, changes in allele frequencies provide a precise genetic description; over longer histories, branching descent connects populations through common descent. An individual growing larger is biological development, whereas an inherited variant becoming more frequent across generations is evolution. See UC Museum of Paleontology: the definition of evolution.

Mutation supplies new alleles, genetic recombination reshuffles inherited material, natural selection favors variants with greater reproductive contribution in a specified environment, and genetic drift changes frequencies through chance. Gene flow moves variants between populations. Consequently, an increasing allele frequency alone does not identify natural selection. See Clark, Douglas and Choi, Biology 2e: population genetics.

Evidence includes ordered fossils, shared inherited structures and similarities in DNA; phylogenetics uses such observations to infer relationships. A phylogenetic tree is an inferred historical account, with uncertainty about particular branches. See Clark, Douglas and Choi, Biology 2e: evidence of evolution. Evidence also comes from experiments: Blount, Borland and Lenski reported an Escherichia coli lineage able to use citrate with oxygen present by 31,500 generations, and tested historical dependence by replaying evolution from stored ancestors. See Blount, Borland and Lenski, 2008: original experimental report. Such observations constrain explanations; no single frequency equation captures every evolutionary process.

An integumentary structure whose distribution among living and fossil dinosaurs informs bird origins and functional change. Filamentous coverings and specialized flight feathers should not be treated as one invariant form.

A collection of fossil remains associated in a specified deposit or sample. Its members may have different original habitats or ages; transport of fossil remains and time averaging affect its interpretation.

Change in the crystal organization of fossil material during alteration, potentially obscuring original histological or chemical information.

Erosion of an older deposit and incorporation of its fossil material into a younger one. It can create a misleading apparent last occurrence.

Study of the solid Earth and its history. Geological evidence provides the stratigraphic and environmental context for fossils.

An inferred interval in which a lineage must have existed although it has not yet been sampled in the fossil record.