Tale of tails: parallelism and prehensility.
The occurrence of prehensile tails among only five platyrrhine genera--Cebus, Alouatta, Lagothrix, Ateles, and Brachyteles--might be interpreted as evidence that these are a closely related, possibly monophyletic group. In the absence of behavioral data, it is impossible to test whether all possess equivalent biological roles; such would lend credence to the idea that their tails evolved from an homologous, derived character complex. Contrariwise, the tendency for species of Cebus to have "averagely" proportioned or relatively short tails, in contrast to the relatively elongate tails of howlers and other atelines; osteological differences in caudal and sacral morphology; and a lack of ateline-like tail/neocortex correlates in Cebus, all imply that prehensility has evolved twice in parallel: once (homologously) in atelines and again in capuchins.
D000508:Alouatta / Q000033:anatomy & histology; D000818:Animals; D002427:Cebidae / Q000033:anatomy & histology*; D002428:Cebus / Q000033:anatomy & histology; D010802:Phylogeny; D013045:Species Specificity; D013623:Tail / Q000033:anatomy & histology*