The colonisation of Madagascar by land‐bound vertebrates

Abstract

Despite discussions extending back almost 160 years, the means by which Madagascar's iconic land vertebrates arrived on the island remains the focus of active debate. Three options have been considered: vicariance, range expansion across land bridges, and dispersal over water. The first assumes that a group (clade/lineage) occupied the island when it was connected with the other Gondwana landmasses in the Mesozoic. Causeways to Africa do not exist today, but have been proposed by some researchers for various times in the Cenozoic. Over‐water dispersal could be from rafting on floating vegetation (flotsam) or by swimming/drifting. A recent appraisal of the geological data supported the idea of vicariance, but found nothing to justify the notion of past causeways. Here we review the biological evidence for the mechanisms that explain the origins of 28 of Madagascar's land vertebrate clades [two other lineages (the geckos Geckolepis and Paragehyra) could not be included in the analysis due to phylogenetic uncertainties]. The podocnemid turtles and typhlopoid snakes are conspicuous for they appear to have arisen through a deep‐time vicariance event. The two options for the remaining 26 (16 reptile, five land‐bound‐mammal, and five amphibian), which arrived between the latest Cretaceous and the present, are dispersal across land bridges or over water. As these would produce very different temporal influx patterns, we assembled and analysed published arrival times for each of the groups. For all, a ‘colonisation interval’ was generated that was bracketed by its ‘stem‐old’ and ‘crown‐young’ tree‐node ages; in two instances, the ranges were refined using palaeontological data. The synthesis of these intervals for all clades, which we term a colonisation profile, has a distinctive shape that can be compared, statistically, to various models, including those that assume the arrivals were focused in time. The analysis leads us to reject the various land bridge models (which would show temporal concentrations) and instead supports the idea of dispersal over water (temporally random). Therefore, the biological evidence is now in agreement with the geological evidence, as well as the filtered taxonomic composition of the fauna, in supporting over‐water dispersal as the mechanism that explains all but two of Madagascar's land‐vertebrate groups.

Ancestral African Bats Brought Their Cargo of Pathogenic Leptospira to Madagascar under Cover of Colonization Events

“…and Myotis goudoti on Madagascar [ 22 ]. It is now well-documented that the origin of the extant land mammals of Madagascar can be explained by four successful colonization events and subsequent large-scale speciation; each of the ancestral groups appears to be of African origin and reached the island via water dispersal on some form of floating vegetation [ 6 , 37 ]. However, given their flight capacity and relative ease in crossing the Mozambique Channel, bats at the family level have colonized Madagascar, probably from the eastern portions of Africa, on about 28 independent occasions [ 7 ].…”

African bushpigs exhibit porous species boundaries and appeared in Madagascar concurrently with human arrival

“…For some Malagasy taxa, such as lemurs, it has long been debated whether colonisation of Madagascar could have taken place through island hopping or temporal land bridges 19 . It is now commonly accepted that some of these taxa arrived on Madagascar by rafting on floats of vegetation and that successful colonisation events and subsequent radiation led to the diversity seen today 20 , 21 . For bushpigs, it has been proposed that the most plausible explanation is that they were introduced to Madagascar by humans, possibly through the Comoros Islands 22 , 23 ; however, this has not been conclusively verified.…”

Another step through the crux: a new microendemic rock-dwelling Paroedura (Squamata, Gekkonidae) from south-central Madagascar

“…The complete isolation of Madagascar preceded a period of drastic species decline and turnover resulting from the mass extinction of the K/T boundary (ca. 65 million years ago) ( Crottini et al 2012a ; Antonelli et al 2022 ; Ali and Hedges 2023 ). The geological history and topography of the island ( Stephenson et al 2021 ) shaped its climatic conditions, resulting today in a wide diversity of biomes, from the eastern humid rainforest to the western dry and southern sub-arid biomes, providing a complex setting for the evolution and diversification of its biota ( Crottini et al 2012a ; Brown et al 2014 ; Antonelli et al 2022 ).…”