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boundary-stratotype A specified rock section (see STRATOTYPE) within which the time-line (‘golden spike’, boundary zonal) occurs that marks the standard demarcation between chronostratigraphic units. In practice such time-lines are usually based on either the appearance or the disappearance of a key species (see INDEX FOSSIL) or other taxon. Associated faunas and sediments may transgress zonal boundaries. The term boundary-stratotype’ has also been used in the sense of the time-line itself.