Vertebrate time-tree elucidates the biogeographic pattern of a major biotic change around the K-T boundary in Madagascar

The geographic and temporal origins of Madagascar's biota have long been in the center of debate. We reconstructed a time-tree including nearly all native nonflying and nonmarine vertebrate clades present on the island, from DNA sequences of two single-copy protein-coding nuclear genes (BDNF and RAG1) and a set of congruent time constraints. Reconstructions calculated with autocorrelated or independent substitution rates over clades agreed in placing the origins of the 31 included clades in Cretaceous to Cenozoic times. The two clades with sister groups in South America were the oldest, followed by those of a putative Asian ancestry that were significantly older than the prevalent clades of African ancestry. No colonizations from Asia occurred after the Eocene, suggesting that dispersal and vicariance of Asian/Indian groups were favored over a comparatively short period during, and shortly after, the separation of India and Madagascar. Species richness of clades correlates with their age but those clades that have a large proportion of species diversity in rainforests are significantly more species-rich. This finding suggests an underlying pattern of continuous speciation through time in Madagascar's vertebrates, with accelerated episodes of adaptive diversification in those clades that succeeded radiating into the rainforests.