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Oxford Reference

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.

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