Vertebrates of Gondwana: Palaeobiogeographical reconstructions by molecular phylogenies and clocks
The biogeographic origins of the extant terrestrial and freshwater vertebrate fauna of Madagascar have in the past been characterized as one of the greatest unsolved mysteries of natural history. Madagascar in the early Mesozoic was a component of the southern supercontinent Gondwana, and vicariance after Gondwana breakup has often been assumed as origin of its endemic fauna. Current geological hypotheses assume the existence of a Madagascar-India continent which separated from Africa ca. 130 million years before present (my). At ca. 90 my, India started breaking off from Madagascar and drifting northeastwards. No Tertiary fossils of terrestrial organisms exist from Madagascar, and reconstructions of the vertebrate radiations of the island therefore fully rely on molecular data of extant species.
In general, over the past 10 years, molecular reconstructions of phylogeny have challenged vicariance hypotheses and have led to a change of paradigm: because the genetic differentiation of many island organisms from their mainland relatives was lower than expected, their origin by dispersal has to be assumed. This is confirmed by molecular clock analyses that become more and more precise, although many methodological refinements still need to b applied.
In Madagascar, it seems clear that the four endemic groups of non-flying mammals arrived via overseas dispersal from Africa: lemurs, tenrecs, nesomyine rodents and a clade of small carnivores.
Read more: Poux, C., O. Madsen, E. Marquard, D. R. Vieites, W. W. de Jong & M. Vences (2005): Asynchronous colonization of Madagascar by the four endemic clades of primates, tenrecs, carnivores, and rodents as inferred from nuclear genes. Systematic Biology 54: 719-730.
Among Malagasy reptiles, several of them certainly arrived via overseas dispersal as well. Some, like pseudoxrhophiines snakes, are of a relatively old Cenozoic origin. A number of lizards are particularly successful overseas colonizers, and arrived in Madagascar in very recent times. Other groups, as recently revealed by other researchers, are of ancient origins and arrived in Madagascar from South America, at times when a land bridge Madagascar-Antarctica existed.
Nagy, Z. T., U. Joger, M. Wink, F. Glaw & M. Vences (2003): Multiple colonization of Madagascar and Socotra by colubrid snakes: evidence from nuclear and mitochondrial gene phylogenies. Proceedings of the Royal Society B 270: 2613-2621.
Vences, M., S. Wanke, D. R. Vieites, W. R. Branch, F. Glaw & A. Meyer (2004): Natural colonization or introduction? Phylogeographical relationships and morphological differentiation of house geckos (Hemidactylus) from Madagascar. Biological Journal of the Linnean Society 83: 115-130.
The situation is more complex in amphibians because these animals do not tolerate contact with water of high salinity over extended periods. Nevertheless, some Malagasy frog lineages, such as hyperoliids and probably also Ptychadena mascareniensis, certainly arrived in Cenozoic times via overseas dispersal from Africa. In the family Mantellidae, two endemic (yet undescribed) frogs inhabit the volcanic Comoro island of Mayotte, indicating that their ancestors must have reached this island over the sea from Madagascar. Such a overseas rafting may be facilitated, as also in the case of the Gulf of Guinea Islands in West Africa, by a combination of accumulated vegetational debris forming floating islands, a low surface salinity due to discharges of major rivers, and sea currents.
However, the origin of the major frog radiations of Madagascars (mantellids and cophyline+scaphiophrynine microhylids) is still under discussion. Molecular clock data indicate their origin at 55-70 million years ago, which is after the assumed separation of Madagascar and India. We favour a hypothesis in which the ancestors of these lineages arrived via dispersal to Madagascar, but at the time in question, major landmasses in the Indian Ocean may have ben relatively close to each over, so that only small sea straits needed to be crossed or even land bridges can be hypothesized. The deeply nested phylogenetic position of mantellids within a group of largely Asian ranoid taxa indicates that the mantellid ancestors may have dispersed from Asia, over India, to Madagascar.
Measey, G. J., M. Vences, R. C. Drewes, Y. Chiari, M. Melo & B. Bourles (2007): Freshwater paths across the ocean: molecular phylogeny of the frog Ptychadena newtoni gives insights into amphibian colonization of oceanic islands. Journal of Biogeography 34: 7-20.
Van der Meijden, A., M. Vences, S. Hoegg, R. Boistel, A. Channing & A. Meyer (2007): Nuclear gene phylogeny of narrow-mouthed toads (Family: Microhylidae) and a discussion of competing hypotheses concerning their biogeographical origins. Molecular Phylogenetics and Evolution 44: 1017-1030.
Vences, M., D. R. Vieites, F. Glaw, H. Brinkmann, J. Kosuch, M. Veith & A. Meyer (2003): Multiple overseas dispersal in amphibians. Proceedings of the Royal Society B 270: 2435-2442.
Vences, M., J. Kosuch, M.-O. Rödel, S. Lötters, A. Channing, F. Glaw & W. Böhme (2004): Phylogeography of Ptychadena mascareniensis suggests transoceanic dispersal in a widespread African-Malagasy frog lineage. Journal of Biogeography 31: 593-601.
Ongoing studies will assess simultaneously the age of all Malagasy vertebrate radiations, to overcome the possible biases of using different molecular markers and different (possibly incompatible) calibrations.