Sweepstakes distribution

The African-Malagasy sweepstakes envisaged by G. G. Simpson

Madagascar has been separated from mainland Africa by 300 miles of ocean since before the Age of Mammals. Any mammal found there today had to cross the ocean and the most likely scenario is that its ancestor got there on a raft of vegetation. The probability of a pair of animals, or perhaps a pregnant female, making that journey and surviving to found a dynasty is extremely low. It is like drawing the winning ticket in a lottery or sweepstakes. But George Gaylord Simpson grasped that over the span of geological time even the improbable becomes possible.

Verreaux’s sifaka (Propithecus verreauxi) Wikimedia Commons

A primate made this journey in the Palaeocene some 57 million years ago (mya). It was the ancestor of the more than 50 species of lemur living today. Tenrecs did not arrive until the Oligocene (29 mya) but they too evolved to fill all sorts of available niches. One would think this rapid radiation would be reflected in differences in placentation. In fact lemurs have a placental type not too different from other strepsirrhine primates – the lorises and bush babies of Asia and Africa. In both groups the placenta is diffuse, with interdigitating villi and an epitheliochorial barrier. Malagasy tenrecs all have the same type of placenta: discoid, labyrinthine and haemochorial. 

The capybara, a caviomorph rodent, Wikimedia Commons

Sweepstakes distribution also occurred from Africa to South America across the South Atlantic. Recently we looked at two groups of mammals that used this route. Caviomorph rodents arrived in the Middle Eocene (41 mya) and the ancestors of New World monkeys by the Late Oligocene (29 mya). Again we found that their subsequent radiation was not accompanied by major changes in placentation. Indeed the placenta of caviomorphs has the same distinctive features as in the related phiomorphs from Africa.

Perhaps the placenta is not quite as plastic as we are inclined to think.

And the hippopotamus in Simpson’s sketch – which predates the one in Dreamworks' Madagascar by 65 years? There are hippopotami in the fossil record of Madagascar, but they left no trace of their placentas.