Late persistence and deterministic extinction of “humid thermophilous plant taxa of East Asian affinity” (HUTEA) in southern Europe

Abstract

Several terrestrial plant fossils found in the late Cenozoic of Europe belong to thermophilous genera or infrageneric taxa which do not grow in this continent today, and are usually called “exotic elements”. Within this large group we singled out three more precisely defined categories based on the hypothesis that the change of geographic distribution between the late Cenozoic and the present is the result of deterministic extinctions caused by climate change. Among the taxa shared by the modern East Asian and the Plio–Pleistocene European flora, the “humid thermophilous taxa of East Asian affinity” (HUTEA) represent the central category in our study. These were traditionally considered “Pliocene” elements in Europe. In our analysis of 13 reliably dated Italian assemblages the percentage of species belonging to the HUTEA category was found to be higher in Pliocene sites, and very low to null in Pleistocene ones. Also early Pleistocene assemblages across all of Europe did not contain any HUTEA, apart from Eucommia, and Symplocos sect. Lodhra in the refugial area of the Colchis.Our analysis of fruit and seed assemblages in the San Lazzaro section (Umbria, central Italy), recently assigned to the early Pleistocene, provided contrasting evidence, which required a reconsideration of the stratigraphic and palaeontological context of another well known site in central Italy, Cava Toppetti II. Using vertebrate and continental mollusc biochronology the early Pleistocene age of this section was confirmed and its palaeontological records were compared with other assemblages in central Italy and Europe. We show that in central Italy at least three HUTEA species (Sinomenium cantalense, Symplocos casparyi, Toddalia rhenana) persisted after the Pliocene/Pleistocene boundary. We conclude that central-southern Italy offered a refugial niche that was warm and wet enough to assure the longer survival of some HUTEA, in contrast to central Europe.