An ancient origin for the enigmatic flat-headed frogs (Bombinatoridae: Barbourula) from the islands of Southeast Asia.

David C Blackburn,Rafe M Brown,David P Bickford,Djoko T Iskandar,Arvin C Diesmos,M Thomas P Gilbert

doi:10.1371/journal.pone.0012090

Abstract

BackgroundThe complex history of Southeast Asian islands has long been of interest to biogeographers. Dispersal and vicariance events in the Pleistocene have received the most attention, though recent studies suggest a potentially more ancient history to components of the terrestrial fauna. Among this fauna is the enigmatic archaeobatrachian frog genus Barbourula, which only occurs on the islands of Borneo and Palawan. We utilize this lineage to gain unique insight into the temporal history of lineage diversification in Southeast Asian islands.Methodology/Principal FindingsUsing mitochondrial and nuclear genetic data, multiple fossil calibration points, and likelihood and Bayesian methods, we estimate phylogenetic relationships and divergence times for Barbourula. We determine the sensitivity of focal divergence times to specific calibration points by jackknife approach in which each calibration point is excluded from analysis. We find that relevant divergence time estimates are robust to the exclusion of specific calibration points. Barbourula is recovered as a monophyletic lineage nested within a monophyletic Costata. Barbourula diverged from its sister taxon Bombina in the Paleogene and the two species of Barbourula diverged in the Late Miocene.Conclusions/SignificanceThe divergences within Barbourula and between it and Bombina are surprisingly old and represent the oldest estimates for a cladogenetic event resulting in living taxa endemic to Southeast Asian islands. Moreover, these divergence time estimates are consistent with a new biogeographic scenario: the Palawan Ark Hypothesis. We suggest that components of Palawan's terrestrial fauna might have “rafted” on emergent portions of the North Palawan Block during its migration from the Asian mainland to its present-day position near Borneo. Further, dispersal from Palawan to Borneo (rather than Borneo to Palawan) may explain the current day disjunct distribution of this ancient lineage.

Highlights

In the species-rich islands of Southeast Asia, the temporal patterns of biological diversification remain largely unexplored
We offer a new biogeographic scenario, the ‘Palawan Ark Hypothesis’ (Fig. 1), that combines aspects of the above: (1) divergence between Barbourula and Bombina is correlated with and/or precedes isolation of Barbourula on Palawan with the opening of the South China Sea (.30 mya; [6,16]); and (2) divergence between B. busuangensis and B. kalimantanensis occurred when Palawan was near to Borneo (,15 mya; [16])
We evaluated whether the estimation of divergence times at three specific nodes were sensitive to the calibration points used by utilizing a jackknife approach

Summary

Introduction

In the species-rich islands of Southeast Asia, the temporal patterns of biological diversification remain largely unexplored. Dispersal and vicariance events in the Pleistocene have received the most attention, though recent studies suggest a potentially more ancient history to components of the terrestrial fauna. Among this fauna is the enigmatic archaeobatrachian frog genus Barbourula, which only occurs on the islands of Borneo and Palawan. We utilize this lineage to gain unique insight into the temporal history of lineage diversification in Southeast Asian islands

Methods

Results

Conclusion