A review of geological evidence bearing on proposed Cenozoic land connections between Madagascar and Africa and its relevance to biogeography

For a variety of reasons, Madagascar's rich and highly distinctive faunal assemblage has long attracted attention. Recurring questions with the associated studies have concerned when and how the ancestors of the various clades reached the island. For the land-bound animals, the common view is that they arrived after the Cretaceous on ‘rafts' that washed in from Africa. However, this centuries-old discussion has been periodically spiked with proposals for land-bridges, with a recent one arguing for three separate causeways or stepping-stone chains connecting Africa and Madagascar in the early, middle, and late Cenozoic (66–60 Ma, 36–30 Ma, and 12–5 Ma). Here, the general idea of former causeways spanning the Mozambique Channel is evaluated based on an extensive survey of the related geological and geophysical literature, a large portion of which dates from the last half-decade. The analysis, which makes use of a newly-developed topological schema, indicates that only a small number of the supposed dry-land sub-paths were actually subaerial during each of the postulated colonization windows. Notably, during the Early Oligocene, a 220–250 km2 pinnacle in the Sakalaves Seamount group formed a volcanic ‘high-island’. The other offshore sectors that were exposed would have been atoll crowns: Juan de Nova Island throughout the Cenozoic and the northern Sakalaves in the Late Miocene. However, it would have been challenging for most land animals to exist on those low-elevation carbonate platforms for any length of time. Therefore, the notion of Africa and Madagascar being linked in the Cenozoic by terrestrial walkways can be regarded as falsified. Over-water dispersal best explains how 26 of Madagascar's 27 land-vertebrate clades colonized the landmass. The exception, a group of small, soil-dwelling snakes, is likely a relict lineage whose ancestors were present on the island prior to Madagascar's geographical isolation that resulted from the tectonic block's breakup with India-Seychelles c. 85 Ma.