The lemur fauna of Madagascar is totally endemic at the family level, and it may well also be endemic at the infraordinal level, as moderately to strongly supported by recent molecular studies. These studies lean to the conclusion that that the lemurs are indeed monophyletic, derived from a single tropical forest canopy-dwelling common ancestor (whose precise ecological preferences are hard to determine, given the diversity of adaptations displayed by both its living descendants and its presumed fossil relatives) that somehow contrived to colonize Madagascar at some point very early in the Tertiary. Molecular estimates for the basal split among the Malagasy lemurs point to a Paleocene (even early Paleocene) age for this common ancestor (Yoder and Yang, 2004): an age consistent with the remarkably ancient late Cretaceous (ca. 77 myr) date for the origin of Primates derived from various molecular data sets by Springer et al. (2003). These dates are, however, so far unsupported (though not contradicted) by the scrappy fossil record, which is also unhelpful in determining the age of the basal strepsirhine. The presence of apparent lorisids and galagids in the Fayum at ca. 40 myr ago provides a minimum fossil date for the existence of Strepsirhini, but one that is presumably highly underestimated. At the same time, the discovery of more or less contemporaneous adapids and strepsirhines in the late Eocene of the Fayum has tended to marginalize the primarily Eocene adapids as a potential source of Strepsirhini. Further, the absence of toothcombed strepsirhines in the fairly abundant European (and the more limited Asian) Eocene fossil records may suggest that this group is of African derivation, and that it spread to Asia only a substantial time after its origin. It seems likely that only improvement in the currently lamentably restricted Paleocene and Eocene fossil records of Africa (and the discovery of any Tertiary primate record at all in Madagascar) will help shed direct light on the origin of the strepsirhine primates in general and the Malagasy lemurs in particular.
Madagascar has been isolated by a very substantial water barrier since even before the very ancient Springer et al. (2003) molecular date for primate origins. The inescapable conclusion is that the ancestors of the island's four endemic terrestrial mammal orders must have arrived there via an overwater sweepstakes route in spite of the very high probabilities against such a crossing. No terrestrial mammal (other than Homo sapiens and its dependents) has, as far as we know, contrived to make this crossing at any time in the last 15 myr or so. This raises the possibility that under current geographic conditions the barrier is absolutely impermeable to such notoriously poor dispersers. If such is the case, it is necessary to inquire whether in fact the geology and topography of the ocean floor surrounding Madagascar has indeed been stable back into the farthest reaches of the Tertiary. The assumption of stability has been called into question recently by a variety of authors (McCall, 1997; Marivaux et al., 2001; deWit and Masters, 2004), who between them have evoked potential "steppingstone" routes both toward and from Africa and India. Clearly, the ocean surrounding Madagascar has acted as a powerful filter to potential mammal invaders of the island; but equally evidently the barrier has not been a totally unbreachable one, at least at certain times during the Tertiary. And it is for this reason that fuller knowledge of the geology of the seafloor surrounding Madagascar will be necessary before we can properly test hypotheses about the mechanisms of Madagascar's colonization by terrestrial mammals, including primates.
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