Abstract
Analyses of phylogenetic topology and estimates of divergence timing have facilitated a reconstruction of Madagascar’s colonization events by vertebrate animals, but that information alone does not reveal the major factors shaping the island’s biogeographic history. Here, we examine profiles of Malagasy vertebrate clades through time within the context of the island’s paleogeographical evolution to determine how particular events influenced the arrival of the island’s extant groups. First we compare vertebrate profiles on Madagascar before and after selected events; then we compare tetrapod profiles on Madagascar to contemporary tetrapod compositions globally. We show that changes from the Mesozoic to the Cenozoic in the proportions of Madagascar’s tetrapod clades (particularly its increase in the representation of birds and mammals) are tied to changes in their relative proportions elsewhere on the globe. Differences in the representation of vertebrate classes from the Mesozoic to the Cenozoic reflect the effects of extinction (i.e., the non-random susceptibility of the different vertebrate clades to purported catastrophic global events 65 million years ago), and new evolutionary opportunities for a subset of vertebrates with the relatively high potential for transoceanic dispersal potential. In comparison, changes in vertebrate class representation during the Cenozoic are minor. Despite the fact that the island’s isolation has resulted in high vertebrate endemism and a unique and taxonomically imbalanced extant vertebrate assemblage (both hailed as testimony to its long isolation), that isolation was never complete. Indeed, Madagascar’s extant tetrapod fauna owes more to colonization during the Cenozoic than to earlier arrivals. Madagascar’s unusual vertebrate assemblage needs to be understood with reference to the basal character of clades originating prior to the K-T extinction, as well as to the differential transoceanic dispersal advantage of other, more recently arriving clades. Thus, the composition of Madagascar’s endemic vertebrate assemblage itself provides evidence of the island's paleogeographic history.
Highlights
Madagascar has long been recognized as one of the world’s highest biodiversity priorities [1]; its extant vertebrate terrestrial and freshwater vertebrate assemblages are highly distinctive and have high levels of species endemism across groups [2]
For each Malagasy vertebrate ‘‘clade’’, we recorded class, time of arrival, ancestor type, overwater dispersal ability, geographic source area, occurrence in the Malagasy fossil record (Mesozoic or Quaternary), and whether there are representative extant members (Table S1)
The prevailing direction of ocean current at the time of colonization was only scored for clades arriving during the Cenozoic owing to the limited availability of paleocurrent modeling for earlier times
Summary
Madagascar has long been recognized as one of the world’s highest biodiversity priorities [1]; its extant vertebrate terrestrial and freshwater vertebrate assemblages are highly distinctive and have high levels of species endemism across groups [2]. For instance, Madagascar shows conspicuous absences relative to typical African subSaharan faunas; if recent introductions by humans are excluded, only seven extant and subfossil placental orders are represented: Bibymalagasia, Afrosoricida, Primates, Chiroptera, Carnivora, Rodentia, and Cetartiodactyla. Similar patterns of disproportionate absences exist in other vertebrate higher taxa (e.g., [3,4,5]). With regard to direct evidence of the rich vertebrate fauna that once existed, the island is well known for its fossil assemblages Many forms occupy phylogenetic positions that are basal relative to continental members of their respective groups [3,6,7].
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