Biochronology of terrestrial mammals and Quaternary subdivisions: a case study of large mammals from the Italian peninsula.

Defining and subdividing the Quaternary on the basis of the mammalian fossil record from the continental realm is not a simple task due to the low degree of succession continuity and the scattered palaeontological evidence. Moreover, even if the approaches to the Quaternary are basically interdisciplinary and may combine many different chronological scales, establishing correlations between biochronology, biostratigraphy, chronostratigraphy, climatostratigraphy, and composite regional stratigraphy can often be very problematical. As far as biochronology is concerned, the marked geological, environmental and climatic diversity affecting different continental regions makes a correlation based on biological events difficult. Indeed, “biochronological units” represent a time span during which faunas have a degree of taxonomic homogeneity and the corresponding “faunal complexes” have to be regarded as non- overlapping and "ecologically adjusted groups of animals with specific geographic limits and chronological range" (TEDFORD, 1970). Nevertheless, the stratigraphic lowest and highest occurrences of fossil remains (stratigraphic datum) within a given geographical area do not necessarily correspond to their actual first/last appearances (palaeobiological events) in time. This is due to diachroneity in palaeobiological events (i.e. local first and last appearances are strictly linked to dispersal, and the physical and biotic factors causing local evolution and extinctions) coupled with discontinuity in the continental sedimentary record, the rarity of deposits formed in a regime of virtually continuous sedimentation, the presence of important ecological barriers (that prevent some taxa from dispersing), environmental conditions (that affect the structure of palaeocommunities), and taphonomic and sampling biases. As a result, ongoing research, continuously yielding new data, make chronological frameworks thus far outlined, even if recent, open to significant improvements, and causing biochrons to be updated. This fact prevents any detailed biochronological framework from having widespread geographical significance. Thus, only higher ranking biochronological units (Land Mammal Ages, LMAs) - whose separation is based on palaeobiological events which have a wide territorial significance - could be useful for chronological correlations. Nonetheless, the transition between successive LMAs does not always correspond to the boundaries separating marine Series or Stages. For instance, the Villafranchian LMA approximately began with the Late Pliocene (Piacentian) and the transition from the early to the middle Villafranchian LMA happened around the Pliocene/Pleistocene (Piacentian/Gelasian) boundary, whereas the transition to the late Villafranchian took place during the latest Gelasian, and those from the Villafranchian to Galerian LMA and from the Galerian to Aurelian LMA respectively predated the beginning of the Middle and Late Pleistocene.