ABSTRACT

Operational definitions of biochron boundaries refer to a stratigraphic record; either the lowest known occurrence of the defining taxon in a given section (Lowest Stratigraphic Datum), or the presumed Earliest Known Record of the defining taxon in a geographic area (EKR). Operational definitions in this context are subject to a variety of geological, paleoecological, taphonomic, and sampling biases, and will often result in biochron “boundaries” that are inappropriate for consistent magnetobiochronological correlations. Theoretical definitions of biochron boundaries refer to historical events, such as the immigration or evolution of the defining taxon (First Historical Appearance; FHA), and are recommended here. In order to demonstrate that such a boundary occurs within a particular magnetozone, (1) the defining or characterizing taxa for the beginning of this biochron and (2) an assemblage confidently referable to the next older biochron must both be known from this same magnetozone.

Flynn (1986) proposed that the Bridgerian/Uintan North American Land Mammal “Age” boundary occurred during Chron C20r. However, evidence from the La Jolla Group in San Diego and the Aycross and Tepee Trail formations in Wyoming now suggests that this boundary occurred either during the latest part of Chron C21r or during C21n, and given the time scale of Berggren et al. (1995), its age is estimated at between 46.3 and 48.3 Ma.

Flynn's (1986) suggestion that the beginning of the Uintan is older than the geochron of Uinta B is probably correct, but the status of the “Shoshonian Subage” of the Uintan is problematical.