Colugos (also misleadingly labeled flying lemurs, although they are not lemurs and do not fly) exhibit a mosaic of features, some of which show individual similarities to bats, insectivores, or prosimian primates, while others are unique. Their most striking characteristics are adaptations for gliding. Because of their many unique features, the two colugo species have long been classified in an independent order of placental mammals (Dermoptera, literally meaning "skin-wings").
Although colugos are now generally regarded as an unusual and isolated mammalian lineage, they have been variously linked to insectivores (Insectivora), bats (Chiroptera), and/or primates (Primates). Some affinity to bats has often been suspected because of the potential link between gliding and actual flight. Perhaps the most influential suggestion has been that colugos, bats, tree shrews, and primates (with or without elephant shrews) should be allocated to a superorder labeled Archonta. However, the apparent morphological affinities between mammals allocated to the Archonta may be largely or exclusively attributable to retention of primitive adaptations for arboreal life that were present in ances-
tral placental mammals. The unusual molar morphology of colugos, their small, very primitive brains, and their highly unusual ear region all indicate that there is no real link to primates. Molecular evidence concerning the affinities of colugos has been equivocal, reflecting the fact that it is difficult to resolve the position of an isolated, species-poor lineage that diverged at a very early stage. Indeed, evidence from complete mitochondrial DNA sequences seems to indicate that colugos are actually closer to higher primates than the latter are to prosimians. However, other molecular evidence clearly conflicts with this highly unlikely interpretation. While an early link between colugos and primates in the mammalian tree cannot be ruled out, there is still no strong evidence to support it.
As might be expected from the existence of only two modern species, the fossil record of colugos is very poorly documented. It was long accepted that the Plagiomenidae, a
poorly documented group of North American fossil mammals from mid-Paleocene to early Eocene deposits (approximately 50-60 million years old), are related to the colugos. However recent re-examinations of the Plagiomenidae indicate that supposed dental similarities to colugos are superficial and that there is no real affinity. A more recent alternative suggestion is that North American Paleocene forms in the family Mixodectidae are related to colugos, but this possibility requires further examination. In fact, a more likely prospect is provided by dental fossils discovered in Eocene deposits of Thailand, attributed to the genus Dermotherium major and identified more convincingly as related to colugos. There have been suggestions that some members of the Ple-siadapiformes (a largely Paleocene group of mammals from Europe and North America usually allocated to the order Primates) show affinities to colugos. Indirect evidence was cited to support the interpretation that some plesiadapiforms (notably members of the family Paromomyidae) were adapted for gliding. This led to a variety of suggested relationships between colugos, Plesidapiformes, and primates of modern aspect. However, the supposed evidence for gliding adaptations in plesiadapiforms has now been largely discredited and there is little else to suggest a link to colugos.
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