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Biochronology is the organization of geologic time according to the irreversible process of evolution in the organic continuum. It is an ordinal framework which measures all but youngest Phanerozoic time with greater resolution (ca 1 m.y., the average age range of species in rapidly evolving lineages), if with less accuracy, than radiochronology. Both are aspects of geochronology.

In contrast, biostratigraphy (strictly speaking) is iterative, consisting of observed (not predicted) superpositional sequences of fossils without inherent chronologic significance. (This is to say that an upside-down biostratigraphy or one with huge time gaps is perfectly useful as long as it is consistent.) It is the arrangement and correlation in time of biostratigraphies that constitute the often unappreciated role of biochronology. The basis of biochronologic correlation is any notable singular occurrence, or "datum event, in the fossil record which has a geographic range overlapping the spatial limits of coeval but disjunct biostratigraphical zones. However, biostratigraphic sequences are the milieu in which datum events in the biochronology of different fossil lineages are compared and radiometrically calibrated. The concept of biochronology is illustrated by reference to the essentially isochronous First Appearance Datum (FAD) of the planktonic foraminiferal species, Globigerina nepenthes, and the three-toed horse, Hipparion, in the marine and continental strati- graphic record, respectively, during the late middle Miocene, ca 12.5 m.y.

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