Biochronology and paleobiogeography of mammals in the late Cenozoic of North America souther: Space-time approach

Abstract

By multivariate methods (similarities and biochronological parsimony) an adjustment is made in the definitions of the biochronological units NALMA's of the last 9 million years (from late Miocene to Holocene). The temporal limits of each unit time, were adjusted according to the proposed of biochronological indicator "shared-taxon". This proposal allowed each time limit to be less diachronic in its definition. On this basis, we can consider that the late Cenozoic can be divided into three hierarchical temporal categories: "Superages", "Ages" and "Subages". The first seems to be associated with the faunal succession of two Phanerozoic climatic supercycles of Fischer's: Greenhouse and Icehouse, and not with the known chrononofaunas. The second category comprises basically the mammals ages proposed by different authors. Of these, only three mammalian ages were recognized: Hemphillian, Blancan and Irvingtonian. Treating the Rancholabrean age only as an extension of the late Irvingtonian. Finally, the third category represents the "Subage" or subdivisions of each age of mammals. This is the most basic biochronological unit of the NALMA`s system, so we consider the use of inferior biochronological categories inappropriate. However, an intermediate interval between the early Blancan and late Irvingtonian is recognized, which we consider it to be part of the Irvingtonian age, it is traditionally called Blancan-Irvingtonian boundary.