Land bridges to Madagascar

 

Sweepstakes distribution as envisaged by G. G.Simpson

One of the first posts on this blog (here) was about sweepstakes distribution as proposed by George Gaylord Simpson. Simply put it envisaged that animals endemic to Madagascar, such as lemurs and tenrecs, as well as now extinct hippopotami, crossed the Mozambique Channel on rafts of vegetation. 

This has been widely accepted and bolstered by evidence that ocean currents in the Cenozoic might have been more favourable to rafting than they would be today (here).

Now a cross-disciplinary group has taken a fresh look at the question and reached quite a different conclusion (here). They begin by questioning whether small primates or tenrecs could survive on the crossing which they estimate could take several months. Further they dismiss the idea that a hippo could swim to Madagascar; their arguments on this point are convincing. Could a raft be strong enough to support a hippo during the crossing (as pictured in the cartoon above)?

Verreaux's Sifaka (Propithecus verreauxi)

They suggest that there has been too much focus on the width of the Mozambique Channel and not enough on its depth. They present a mass of data, bewildering to me, but supporting the possibility of land bridges from Africa to Madagascar. The first of these at the Cretaceous-Palaeocene boundary might have been used by the ancestors of lemurs and tenrecs (their African cousins are lorises and otter shrews).

A land bridge at the Eocene-Oligocene boundary could have been used by the ancestors of Malagasy rodents and Malagasy carnivores such as the fossa. Finally there was an incomplete land bridge at the Miocene-Pliocene boundary that might be a more plausible route for a hippopotamus to cross.

The obvious question is why the Malagasy fauna is so depauperate. Why are there no monkeys, large cats or ungulates? The authors suggest they may have made it to Madagascar but gone extinct. Fortunately for this argument, the fossil record on Madagascar is very poor.

And what about that other discussion of land bridges versus rafting - how did hystricognath rodents and the ancestors of Neotropical primates reach South America (see previous post)?