Pliocene and Early Pleistocene carpological records of terrestrial plants from the southern border of the Po Plain (northern Italy)

The analysis of fruit and seed assemblages in Pliocene and Early Pleistocene successions of the southern border of the Po Plain provides an interesting record of the terrestrial palaeoflora, which complements the information provided by the extensive pollen records available in this area. The 14 carpoflora-bearing layers studied here, spanning from 5.1 to 0.9 Ma, have been inserted in a definite stratigraphic framework, locally with a precision of ca. 100 ka. The input of the new data into the CENOFITA database showed that the fossil record of several species was often limited to one to three chronostratigraphic stages of the Pliocene (Zanclean, Piacenzian) and Early Pleistocene (Gelasian, Calabrian). A general analysis of the northern-Italian fossil record of such species allowed us to improve the distribution chart of carpological taxa in the Pliocene and Early Pleistocene. In this new chart, the carpological species were clustered in six groups of species with similar chronologic distributions. We suggest that the analysis of such groups, in undated or poorly dated carpological assemblages, would be useful for their biochronological interpretation: the Zanclean floras can be distinguished by the Piacenzian ones in case of occurrence of Group 1 species, whereas the simultaneous occurrence of several species of Groups 2 and 3, without species of Groups 1 and 5, could be used to suggest a Piacenzian affinity for undated assemblages. The occurrence of several species of Groups 5 and 6, without species of Groups 1–3, may just point to a generic Gelasian or Calabrian affinity. The palaeoclimatic characterization of the individual species, but also of the plant assemblages, suggests that a temperature decrease is one of the causes of the palaeofloral changes, in fact the most apparent events are represented by the disappearance of thermophilous species. The appearance of new species in the studied fossil record is mostly due to the establishment of favourable local environmental conditions, rather than to evolutionary events. However, the occurrence, in chronologically well-constrained layers, of fruits and seeds of a few plant genera with a proven late Cenozoic phylogenetic differentiation (e.g. Carex, Hypericum, Thymelaea) provides important support for the dating of species divergence times.