Objectives: The guenons (tribe Cercopithecini) are a diverse and primarily arboreal radiation of Old World monkeys from Africa. However, preliminary behavioral observations of the lesula (Cercopithecus lomamiensis), a little-known guenon species described in 2012, report it spending substantial amounts of time on the ground.New specimens allow us to present the first description of lesula postcranial morphology and apply a comparative functional morphology approach to supplement our knowledge of its locomotor behavior. Materials and methods:To infer the substrate use preferences of the lesula, 22 postcranial variables correlated with locomotion were assessed in a sample of 151 adult guenon specimens, including two C. lomamiensis. Using multivariate statistical analyses, we predict the amount of time the lesula spends on the ground relative to the comparative sample.Results: Results suggest that the lesula spends nearly half its time on the ground, and the two available individuals were classified as semiterrestrial and terrestrial with strong support. Comparisons with two outgroup cercopithecid taxa (Colobus guereza and Macaca mulatta) demonstrate that, as a group, guenons retain signals of a generalized, semiterrestrially adapted postcranium compared to specialized arboreal cercopithecids.Discussion: These results corroborate preliminary behavioral observations of the lesula as a semiterrestrial to terrestrial primate and imply multiple evolutionary transitions in substrate use among the guenon radiation. A broader view of cercopithecoid evolution suggests that a semiterrestrial ancestor for extant guenons is more parsimonious than an arboreal one, indicating that the arboreal members of the group are probably recently derived from a more semiterrestrial ancestor. K E Y W O R D SCercopithecini, functional morphology, locomotion, postcranium, substrate use

Objectives The little known guenon Cercopithecus dryas has a controversial taxonomic history with some recognizing two taxa (C. dryas and C. salongo) instead of one. New adult specimens from the TL2 region of the central Congo Basin allow further assessment of C. dryas morphology and, along with CT scans of the juvenile holotype, provide ontogenetically stable comparisons across all C. dryas and “C. salongo” specimens for the first time. Materials and Methods The skins and skulls of two newly acquired C. dryas specimens, male YPM MAM 16890 and female YPM MAM 17066, were compared to previously described C. dryas and “C. salongo” specimens, along with a broader guenon comparative sample (cranial sample n = 146, dental sample n = 102). Qualitative and quantitative assessments were made on the basis of commonly noted pelage features as well as craniodental characters in the form of shape ratios and multivariate discriminant analyses. Results All C. dryas specimens, including the TL2 adults, are comparatively small in overall cranial size, have relatively small I1s, and display tall molar cusps; these osteological characters, along with pelage features, are shared with known “C. salongo” specimens. Discriminant analyses of dental features separate C. dryas/salongo specimens from all other guenons. Discussion In addition to pelage‐based evidence, direct osteological evidence suggests “C. salongo” is a junior synonym of C. dryas. Combined with molecular analyses suggesting C. dryas is most closely related to Chlorocebus spp., we emend the species diagnosis and support its transfer to Chlorocebus or possibly a new genus to reflect its distinctiveness.

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