Madagascar

Responsables : J.-J. Jaeger, Y. Chaimanee, M. Rugbumrung, M. Benammi.

 

The peopling of Madagascar by lemurian primates, a major biogeographic enigma :

Madagascar is an isolated continent-island who has been surrounded by seas since over 150 million years. But it yields a highly endemic mammalian fauna whose origins and dispersal ways are still unknown. Among them, the lemurian strepsirrhine primates are among the most famous, but there are also tenrecids, cricedid rodents and small carnivores among the living forms but also orycteropids and dwarf hippos among the extinct ones. Molecular data have been able to identify the extant sister groups of these representatives and have estimated molecular clocks ages of beginning of Madagascar peopling by each group. Three successive waves of immigration have so far been recognized. The oldest ones, between 70 and 40 Ma, concerns the lemurian primates and the tenrecids. A Miocene colonization age has been proposed for the rodents and the small carnivores and a late Miocene-Pliocene age has been proposed for the last immigrants. A general consensus and the survival of several ancient groups of mammals in Africa, support an African origin for all these groups. But it is contradicted by the data obtained from the scanty African fossil record where the first tooth comb appears about 37 Ma in Fayum (Egypt) within the sistergroup of the lemurians !. There is no potential lemurian ancestor documented so far in Africa despite some unconvincing recent claims ! The fossil record has so far established that the order Primates appeared in Asia and later on, during the Paleocene and Eocene, some representatives immigrated to Africa. Therefore, the acquisition of the lemurian comb, the most characteristic landmark of these lemurians, may have been a characteristic of some Asian immigrants to Africa, before dispersal to Madagascar, or has evolved in Africa, before the peopling of Madagascar. It could be as well originate from an Indian immigration wave to Madagascar, by island hopping using the volcanic islands generated through time by the Reunion hot spot, during the opening of the Indian Ocean. The question is still open and only new fossil discoveries can solve it.

The cricetid rodents case is also striking. Their closest molecular sister-group are a group of African endemic cricetids. Therefore, an African origin has been claimed. But it is recognized that African cricetids are lower miocene immigrants from Asia, where this group of rodent originated during middle Eocene times. Most of the asian cricetids have become extinct, only those of their African branch having survived ! therefore if some representatives of theseAsian miocene cricetid groups would still survive, some of them may have appeared even closer to the Madagascar ones than the African,. Such possibility is disqualifying the molecular approach for reconstructing biogeographic history of mammals !

The only positive method to solve these biogeographic mysteries consists of searching   for new fossils in Madagascar. But its present fossil record is very scanty, being restricted to  a few mid and late Quaternary fossils and many Holocene ones. One exception however documents the presence of a marine mammal, a sirenian, in the Middle Eocene marine deposits of the North Western coast.

            A team of IPHEP therefore decided to survey the Eocene deposits of Madagascar, during three field expeditions. As they are no terrestrial Eocene deposits other than highly weathered laterites, they concentrated on the search of limnic intercalations within the marine neritic Eocene deposits. They have discovered several bone levels which may yield some mammal remains but so far have only collected crocodiles and turtle remains.

The political context did stop our efforts but the first results are highly encouraging to bring an answer to that major biogeographic enigma !

Informations supplémentaires