New Biogeographic insight into Bauhinia s.l. (Leguminosae): integration from fossil records and molecular analyses

Given that most species that have ever existed on earth are extinct, it stands to reason that the evolutionary history can be better understood with fossil taxa. Bauhinia is a typical genus of pantropical intercontinental disjunction among the Asian, African, and American continents. Geographic distribution patterns are better recognized when fossil records and molecular sequences are combined in the analyses. Here, we describe a new macrofossil species of Bauhinia from the Upper Miocene Xiaolongtan Formation in Wenshan County, Southeast Yunnan, China, and elucidate the biogeographic significance through the analyses of molecules and fossils.

Morphometric analysis demonstrates that the leaf shapes of B. acuminata, B. championii, B. chalcophylla, B. purpurea, and B. podopetala closely resemble the leaf shapes of the new finding fossil. Phylogenetic relationships among the Bauhinia species were reconstructed using maximum parsimony and Bayesian inference, which inferred that species in Bauhinia species are well-resolved into three main groups. Divergence times were estimated by the Bayesian Markov chain Monte Carlo (MCMC) method under a relaxed clock, and inferred that the stem diversification time of Bauhinia was ca. 62.7 Ma. The Asian lineage first diverged at ca. 59.8 Ma, followed by divergence of the Africa lineage starting during the late Eocene, whereas that of the neotropical lineage starting during the middle Miocene.

Hypotheses relying on vicariance or continental history to explain pantropical disjunct distributions are dismissed because they require mostly Palaeogene and older tectonic events. We suggest that Bauhinia originated in the middle Paleocene in Laurasia, probably in Asia, implying a possible Tethys Seaway origin or an "Out of Tropical Asia", and dispersal of legumes. Its present pantropical disjunction resulted from disruption of the boreotropical flora by climatic cooling after the Paleocene-Eocene Thermal Maximum (PETM). North Atlantic land bridges (NALB) seem the most plausible route for migration of Bauhinia from Asia to America; and additional aspects of the Bauhinia species distribution are explained by migration and long distance dispersal (LDD) from Eurasia to the African and American continents.