Transitional fossil

A fossil preserving a combination of ancestral and derived traits that documents evolutionary change between groups of organisms.

Evolution: The change in heritable characteristics of populations across generations. Transitional fossils document evolutionary change in the anatomical record.
Tiktaalik: A Devonian fossil vertebrate with fish features and limb-like bones in its fins. Its anatomy combines aquatic traits with structures associated with early tetrapods.
Comparative anatomy: The study of structural similarities and differences among organisms. Comparisons identify which fossil features are shared with ancestral and later groups.
Missing link: A popular but misleading expression for a supposed fossil connecting two groups. A transition is represented by branching populations, not one required link in a chain.
Punctuated equilibrium: An evolutionary model in which long periods of relative stability are interrupted by geologically brief change. It predicts that transitional forms may be scarce in some lineages and intervals.
Homology: Similarity in traits due to shared ancestry, even when structures serve different functions. Shared homologous structures help identify ancestral traits across related groups.
Archaeopteryx: A Late Jurassic fossil animal with feathers and birdlike wings alongside teeth and a long bony tail. Its mixture of traits connects early birds with non-avian theropod dinosaurs.
Stratigraphy: The study of rock layers and their sequence, composition, and age relationships. Layer position helps establish when a transitional form lived relative to other fossils.
Living fossil: A living species that has retained traits resembling those of ancient fossil relatives. It describes resemblance through time, not a fossil documenting an evolutionary transition.
Fossilization: The processes by which remains or traces of organisms become preserved as fossils. Its rarity and biases help explain why many transitions have incomplete fossil evidence.

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