colugos , constituting the obscure and tiny order dermoptera , are gliding mammals confined to evergreen tropical rainforests of south - east asia . there are two extant species , now placed in separate genera : galeopterus variegatus ( malayan colugo , formerly known as cynocephalus variegatus ) and cynocephalus volans ( philippine colugo ) . their most obvious hallmark is a gliding membrane ( patagium ) surrounding almost the entire body margin . colugos are also called ' flying lemurs ' , but  as simpson aptly noted    they " are not lemurs and can not fly " . they differ from other gliding mammals ( certain rodents and marsupials ) in that the patagium also extends between the hind limbs and the short tail , even stretching between fingers and toes ( hence the name ' mitten - gliders ' ) . the lower incisors are unique : the forward - leaning ( procumbent ) crown of each tooth is subdivided into several comb - like tines . colugos are strict herbivores , predominantly eating young leaves from many tree species , and in the gastrointestinal tract the caecum is greatly enlarged . their habits are poorly documented , although a recent field study yielded valuable new information . colugos would doubtless still be languishing in obscurity but for mounting evidence indicating a connection with primate evolution . in fact , gregory   presaged this long ago by proposing the superorder archonta for elephant - shrews , tree - shrews , colugos , bats and primates . subsequently , prompted by butler , the superorder archonta was progressively resuscitated , although most authors emphatically excluded elephant - shrews ( for example ) . a quite recent major classification of mammals   united tree - shrews , colugos , bats and primates in the grand order archonta . this whole topic has been reinvigorated by molecular evidence indicating that tree - shrews , colugos and primates , at least , may be quite closely related . despite general agreement over the clustering of tree - shrews , colugos and primates together , the placement of colugos within this group in relation to primate evolution is still a matter of debate . some lines of evidence place tree - shrews and colugos together in a sister group to primates , whereas other workers in the field have advocated that colugos have a closer affinity to primates than to tree - shrews . this debate is nicely highlighted by a recent paper by nie et al .   in bmc biology , which provides cytogenetic evidence for the tree - shrews and colugos as a sister group to primates . in contrast , janecka et al .   in a recent paper last year come to the different conclusion that colugos form a sister group more closely related to primates than to tree - shrews on the basis of nuclear dna sequence data . determination of higher - level relationships among placental mammals using morphological evidence proved remarkably challenging [ 6,10 - 17 ] . despite general agreement about subdividing placental mammals into orders , recognition of deeper nodes in the tree has been tentative at best ( see for example ) , and morphological interpretations , such as the placement of colugos with bats in the grouping volitantia , frequently clash with the molecular evidence . the rapidly accumulating molecular evidence has yielded an entirely new perspective on placental mammal evolution . phylogenetic reconstructions using comprehensive dna datasets ( see for example ) have led to consistent recognition of four monophyletic superorders : afrotheria , euarchontoglires , laurasiatheria and xenarthra . those superorders were confirmed by the most extensive analysis to date , generating a supertree combining results from more than 2,500 partial trees   ( figure 1 ) . the superorder euarchontoglires ( alternatively known as supraprimates [ 21 - 23 ] ) is often divided into two subgroups : simplified tree showing relationships between 18 extant orders of placental mammals , inferred from a consensus phylogeny integrating molecular evidence . separate suborders are shown for rodentia ( n = 3 ) and primates ( n = 2 ) . note the relatively rapid diversification of placental orders between 80 and 100 million years ago ( ma ) . euarchonta ( dermoptera ( colugos ) , primates and scandentia ( tree - shrews ) ) and glires ( lagomorpha ( rabbits and hares ) and rodentia ( rodents ) ) . however , although the consensus supertree   portrays euarchontoglires as monophyletic , internal relationships between tree - shrews , glires and colugos+primates appear as an unresolved trichotomy ( figure 1 ) . it should also be noted that tree - shrews and lagomorphs ( usually linked to rodents in glires ) have emerged as sister groups in several individual studies : those of the -globin gene ; exon 28 of the von willebrand factor gene ; complete protein - coding mitochondrial dna sequences ; and complete sets of trna and rrna sequences from mitochondrial genomes . figure 2 shows a version of this part of the tree with possible sister groups indicated . inferred relationships within the superorder euarchontoglires . dashed lines with question marks indicate possible alternative links . a basal split between euarchonta and glires is often recognized , but some molecular evidence indicates a link between tree - shrews ( scandentia ) and lagomorphs . within euarchonta , colugos ( dermoptera ) have been linked either with tree - shrews   or with primates . molecular evidence has generally provided little support for a specific link between tree - shrews and primates . the now widely recognized taxon euarchonta is a radically pruned version of gregory 's archonta , excluding not only elephant - shrews ( now placed in afrotheria ) but also bats ( chiroptera , in laurasiatheria ) . strikingly , molecular evidence uniformly indicates a very deep separation between colugos and bats , soon after the common ancestor of extant placental mammals ( see figure 1 ) . similarities that led morphologists to recognize the volitantia probably reflect convergent gliding adaptations in colugos and the ( hitherto undocumented ) precursors of bats . investigation of short interspersed nuclear elements ( sines ) in euarchontoglires   identified euarchonta and glires as monophyletic sister taxa , but left the relationship between colugos , tree - shrews and primates within the euarchonta as an unresolved trichotomy ( see figure 2 ) . potentially , they are highly informative phylogenetic markers because retroposition at exactly the same site in independent lineages ( that is , convergent evolution ) is highly unlikely . sines derived from 7sl rna seem to be a shared derived feature of euarchontoglires , subsequently leading to dimeric alu sequences in primates , chimeric sequences in tree - shrews and b1 sequences in rodents . moreover , a genome search revealed five independent retroposon insertions shared by tree - shrew and human and fourteen shared by mouse , rat and rabbit , indicating a basal divergence between euarchonta and glires . but the very limited genomic information available for colugos and lagomorphs handicapped this study , such that relationships between colugos , primates and tree - shrews were left unresolved . some reports   based on mitochondrial dna sequences challenged the monophyly of primates , linking colugos to higher primates ( anthropoidea ) to the exclusion of prosimians ( lemurs , lorisiforms and tarsiers ) . schmitz et al .   replicated this aberrant finding , and conducted a test using sines . they identified a substantial set of transposable elements present in all major groups of extant primates but lacking in colugo , thus clearly supporting primate monophyly . within the archonta , colugos have sometimes been linked most closely to tree - shrews and sometimes to primates . support for the former association has now been provided by nie et al . , who generated a g - banded karyotype for the malayan colugo g. variegatus and used reciprocal chromosome painting with human and g. variegatus chromosome - specific probes to establish the first genome - wide comparative map matching galeopterus to human . this enabled them to define 44 segments in the g. variegatus genome homologous to segments in humans . comparisons across similar published maps from other species within euarchontoglires revealed that galeopterus and a tree - shrew ( tupaia belangeri ) share a unique derived association between two human syntenic segments , an association confirmed by nie et al .   by reverse painting of human chromosomes by t. belangeri and g. variegatus probes moreover , this association is borne on a large autosomal chromosome that is seemingly identical in both . thus provide more evidence for the hypothesis that scandentia and dermoptera have a closer phylogenetic relationship to each other than either of them has to primates . this is confirmation of previous studies indicating that colugos and tree - shrews constitute a monophyletic group . such a group , labeled sundatheria , was , for example , indicated by cladistic analysis of dental features , and several authors have reported molecular evidence linking colugos to tree - shrews [ 29,33 - 36 ] . but phylogenetic studies are never simple , and another recent study has interpreted dna sequence and genomic data as showing a closer association of colugos to primates . combined two independent molecular approaches to explore relationships within euarchonta : screening of almost 200,000 protein - coding exons to identify rare deletions , and generation of a phylogenetic tree using a 14-kb dna sequence dataset from nuclear genes . however , seven deletions were common to colugos and primates , whereas tree - shrews and primates shared only a single deletion . a relationship between colugos and primates was also indicated by the phylogenetic tree generated from dna sequences . overall , the results thus indicate that colugos are closer to primates than to tree - shrews . so the relationships among colugos , tree - shrews and primates still await resolution ( see figure 2 ) . it is evident , however , that any eventual solution will require convergence at the molecular level , because of the mosaic distribution of shared derived features identified in different studies . some apparent conflicts may be attributable to polymorphism in common ancestors followed by differential lineage sorting . despite remaining problems , some provisional conclusions are permissible . first , the superorder euarchontoglires is uniformly supported by molecular studies , even though no strong morphological evidence favored inclusion of colugos . the prevalent interpretation among morphologists was that bats and colugos are sister groups , a conclusion resoundingly rejected by all molecular studies . second , subdivision of euarchontoglires into two monophyletic sister groups , euarchonta and glires , has generally received most support , but there have been several divergent findings . tree - shrews have sometimes emerged as a basal offshoot in euarchontoglires or have even been linked specifically to lagomorphs , thus disrupting the monophyly of glires . third , a link between colugos and primates within euarchontoglires has frequently emerged from molecular studies , whereas inferred relationships between tree - shrews and colugos or primates have been less consistent and far more variable . a general drawback in many studies has been inclusion of only a single colugo ( usually the philippine rather than the malayan species ) . taking a single representative for an isolated mammalian group can generate misleading results because of long - branch attraction . one immediate benefit of this was demonstration of a surprisingly deep divergence between the two colugos , indicated at approximately 20 million years ago . this not only bolstered the validity of generic separation but also alleviated the effects of long - branch attraction . whatever the eventual outcome , colugos must clearly be considered in future discussions of primate evolution . morphological comparisons between colugos , tree - shrews and primates , rare in the past , are now mandatory . consideration of colugos will doubtless throw new light on key issues . to take just one example , the brain of colugos is unusually small relative to body size and morphologically very primitive . if colugos are close relatives of primates and/or tree - shrews , this means that any advanced features in tree - shrews and primates are almost certainly convergent , confirming one past interpretation . as both nie et al .   and janecka et al .   noted , proper understanding of morphological and genomic evolution of primates requires identification of the sister group , and colugos ( with or without tree - shrews ) are definite candidates . for this reason , determination of a draft genome sequence for colugo should certainly be a high priority .
substantial molecular evidence indicates that tree - shrews , colugos and primates cluster together on the mammalian phylogenetic tree . 
 previously , a sister - group relationship between colugos and primates seemed likely . 
 a new study of colugo chromosomes indicates instead an affinity between colugos and tree - shrews .