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The genus Cebus is one of the best extant models for examining the role of failback foods in primate evolution. Cebus includes the tufted capuchins, which exhibit skeletal features for the exploitation of hard and tough foods. Paradoxically, these seemingly "specialized" taxa belong to the most ubiquitous group of closely related primates in South America, thriving in a range of different habitats. This appears to be a consequence of their ability to exploit obdurate failback foods. Here we compare the toughness of foods exploited by two tufted capuchin species at two ecologically distinct sites; C. apella in a tropical rainforest, and C. libidinosus in a cerrado forest. We include dietary data for one untufted species (C. olivaceus) to assess the degree of difference between the tufted species. These data, along with information on skeletal morphology, are used to address whether or not a faliback foraging species exhibits a given suite of morphological and behavioral attributes, regardless of habitat. Both tufted species ingest and masticate a number of exceedingly tough plant tissues that appear to be used as faliback resources, however, C. libidinosus has the toughest diet both in terms of median and maximal values. Morphologically, C. libidinosus is intermediate in absolute symphyseal and mandibular measurements, and in measures of postcranial robusticity, but exhibits a higher intermembral index than C. apella. We propose that this incongruence between dietary toughness and skeletal morphology is the consequence of C. libidinosus' use of tools while on the ground for the exploitation of fallback foods. [ABSTRACT FROM AUTHOR]

The earliest Neotropical primate fossils complete enough for taxonomic assessment, Dolichocebus, Tremacebus, and Chilecebus, date to approximately 20 Ma. These have been interpreted as either closely related to extant forms or as extinct stem lineages. The former hypothesis of morphological stasis requires most living platyrrhine genera to have diverged before 20 Ma. To test this hypothesis, we collected new complete mitochondrial genomes from Aotus lemurinus, Saimiri sciureus, Saguinus oedipus, Ateles belzebuth, and Callicebus donacophilus. We combined these with published sequences from Cebus albifrons and other primates to infer the mitochondrial phylogeny. We found support for a cebid/atelid dade to the exclusion of the pitheciids. Then, using Bayesian methods and well-supported fossil calibration constraints, we estimated that the platyrrhine most recent common ancestor (MRCA) dates to 19.5 Ma, with all major lineages diverging by 14.3 Ma. Next, we estimated catarrhine divergence dates on the basis of platyrrhine divergence scenarios and found that only a platyrrhine MRCA less than 21 Ma is concordant with the catarrhine fossil record. Finally, we calculated that 33% more change in the rate of evolution is required for platyrrhine divergences consistent with the morphologic stasis hypothesis than for a more recent radiation. We conclude that Dolichocebus, Tremacebus, and Chilecebus are likely too old to be crown platyrrhines, suggesting they were part of an extinct early radiation. We note that the crown platyrrhine radiation was concomitant with the radiation of 2 South American xenarthran lineages and follows a global temperature peak and tectonic activity in the Andes. [ABSTRACT FROM AUTHOR]