Origins, Diversity and Relationships of Lemurs
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I review new evidence on origins and adaptive radiation of Malagasy lemurs, a remarkably diverse group containing 13% of living primate species. The number of recognized lemur species has increased significantly, partly due to research revealing specific subdivisions within known populations but mainly because of discovery of new populations through fieldwork. Some species feared to be extinct have also been rediscovered. Specific numbers have increased particularly in small-bodied, cryptic genera for which continued research will surely reveal even more species.
Adaptative radiation of lemurs has been essentially confined to Madagascar. The high density of lemur species on that island, associated with very small geographical ranges, has major implications both for their evolutionary divergence and for conservation. Reconstructions of phylogenetic relationships among primates have been considerably enhanced by DNA sequence data. Sufficient data are now available from both nuclear and mitochondrial sequences to examine relationships among and within the major groups of living primates. Most studies have confirmed that lemurs constitute a monophyletic sister-group of the lorisiform clade and all exclude a specific relationship between cheirogaleids and lorisiforms repeatedly inferred from morphological evidence. However, some analyses indicate that the aye-aye may have branched away before the divergence between other lemurs and lorisiforms. DNA sequence analyses have also yielded a broad consensus for relationships between Eulemur, Hapalemur, Lemur and Varecia: Varecia branched away first, while Lemur is more closely related to Hapalemur than to Eulemur.
As debate about phylogenetic relationships among lemurs and other primates seems to have been settled in favor of lemur monophyly (possibly excluding the aye-aye), only a single invasion of Madagascar is required; but it must still be explained how ancestral lemurs could have migrated there at an appropriate time. Separation between Madagascar and Africa was apparently complete by about 120 Ma, too far in the past for direct overland migration. A recent hypothesis suggested that uplifted land in the Mozambique Channel assisted colonization of Madagascar 26-45 Ma, seemingly agreeing with an estimated date of about 40 Ma for divergence of lemurs from other primates. However, mounting evidence suggests that divergence occurred significantly earlier. Because the earliest known fossil representatives of several modern orders of placental mammals (including primates) are dated no earlier than the early Tertiary, it is widely accepted that their divergence took place after the Cretaceous/Tertiary mass extinction. Yet the known fossil record can only yield minimum divergence times; if sampling is poor and/or biased there may be a considerable discrepancy between minimum and actual dates. There is, for example, virtually no known fossil record for lemurs in Madagascar and the earliest known representatives are subfossil lemurs, so in this case a direct reading of the fossil record would indicate that the lemurs first originated just a few thousand years ago! Examination of underestimation of times of origin because of poor sampling in the fossil record has confirmed previous suggestions that primates originated considerably earlier than generally believed. Several recent phylogenetic reconstructions based on DNA sequence data and using calibration dates derived from groups other than primates provide independent support for this inference. Overall, it now seems that primates originated at around 90 Ma rather than the 55 Ma indicated by direct reading of the known fossil record. Hence, colonization of Madagascar by lemurs would have taken place at about 80 Ma, double the date usually accepted, and should be interpreted in terms of contemporary continental relationships.
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