Fossil calibration

Fossil calibration uses fossil ages to constrain when evolutionary lineages diverged. It commonly anchors molecular-clock estimates by providing minimum or, less often, maximum ages for branches in a phylogenetic tree.

Molecular clock: A method for estimating evolutionary divergence times from the accumulation of genetic changes. Fossil ages anchor the clock's estimates of when lineages separated.
Fossil: Preserved remains, traces, or chemical evidence of past life. A fossil can provide the evidence used to set a divergence-time bound.
Divergence time estimation: The estimation of when two evolutionary lineages split from a common ancestor. Fossil constraints help turn genetic differences into calendar-time estimates.
Minimum age constraint: A lower bound on an event's age, requiring that the event be no younger than a specified time. A fossil usually establishes that its lineage had already diverged by the fossil's age.
Biogeographic dating: The estimation of lineage divergence times using the geographic history of organisms and regions. It can provide temporal information where suitable fossils are scarce.
Node dating: A dating approach that assigns age constraints directly to nodes in a phylogenetic tree. It applies fossil-derived age bounds to the divergence points being estimated.
Stratigraphy: The study of layered rocks and their sequence, distribution, and age relationships. Rock-layer context helps establish a fossil's relative age.
Phylogenomics: The use of genome-scale data to infer evolutionary relationships and history. Large genetic datasets still need fossil evidence to establish absolute timescales.
Maximum age constraint: An upper bound on an event's age, requiring that the event be no older than a specified time. Older fossils outside a lineage can sometimes help bound a divergence from above.
Island biogeography: The study of how geographic isolation and island formation shape species diversity and distribution. Island emergence can offer an independent time constraint for some divergences.

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