Land-Mammal Ages, Faunal Heterochrony, and Temporal Resolution in Cenozoic Terrestrial Sequences

Traditionally, land-mammal biochronology has subdivided the Cenozoic into discernible time intervals with an average duration of about 3.5 × 10⁶ years. Recent applications of chronological tools (particularly magnetic polarity stratigraphy) have increased the power of resolution by an order of magnitude so that time intervals of about 3 × 10⁵ years can be discerned in terrestrial sequences. This increased power of resolution has significant paleontological ramifications. Examples are cited of heterochronous (time-transgres-sive) datum planes. However, recently proposed examples of heterochrony of entire early Cenozoic land-mammal faunas result from incorrect correlations to the time scale. Given the recently increased power of temporal resolution, it is possible to assess critically the quality of chronological data that have been used to propose or support drastic paleontological hypotheses such as faunal heterochrony. Heterochrony of entire land-mammal faunas has not yet been documented.