Fossil datum and paleobiological event terms, paleontostratigraphy, chronostratigraphy, and the definition of Land Mammal “Age” boundaries

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Abstract
The confusion between operational and theoretical terms has been common in stratigraphic paleontology. This science includes the descriptive subdiscipline of biostratigraphy and the more interpretational subdisciplines of biochronology and ecostratigraphy. Paleontostratigraphic terms ending in “-zone” refer to intervals of rock, while those ending in “-chron” refer to spans of time. Fossil datum terms (Lowest/Highest Stratigraphie Datum = LSD/HSD) refer to the lowest/highest stratigraphic occurrence of fossils of a taxon in a given section, and define the boundaries of biostratigraphic units (fossizones = intervals of strata). Fossizones may be divided into epizones (the known extent of the fossizone) and entozones (the actual extent of the fossizone). Fossil datum terms with intrinsic temporal connotations are Oldest/Youngest Known Record (OKR/YKR). The time between the presumed OKR and YKR of a taxon in a given area is the apparent temporal range (parachron), which must be separated from the even more theoretical concept of the true temporal range (biochron). Paleobiological event terms (First/Last Historical Appearance = FHA/LHA) refer to the evolution, immigration, dispersal, emigration, and extinction of living individuals of a taxon, and define the boundaries of biochronologic units (biochrons = spans of time). Biochrons may be divided into holochrons (temporal range of the defining taxon/taxa in either a given continent or the whole world) and teilchrons (temporal range in either an individual basin or paleobio-geographic province). Geochronologic units (geochrons) are spans of time defined by two “golden spikes” in designated boundary stratotype sections, and geochronostratigraphic units (geochronozones) are the sets of rock formed during such geochrons. In contrast, biochronostratigraphic units (biochronozones) are the sets of rock formed during biochrons, without reference to any particular stratigraphie section. Land Mammal “Ages” (LMAs) should not be redefined as formal geochronologic/geochronostratigraphic units. Single-taxon definitions of LMA boundaries may be appropriate in cases where few immigrational events seem to have occurred over an appreciable interval of time on a given continent. However, in view of the relatively low potential for continent-wide biochronologic resolution generally available in terrestrial sections, most LMAs should be regarded as Oppel disjunctive biochrons, whose beginnings are theoretically defined as the point in time corresponding to whichever of the immigrational or evolutionary appearances of a specified set of taxa is the oldest.