First record of a fossil monkey (Primates, Cercopithecidae) from the Late Pliocene of Serbia.

The cercopithecid fossil record of the Balkan Peninsula extends from the Late Miocene to the Early Pleistocene, but to date no fossils of non-human primates have been identified in Serbia. Here we report the identification of two primate teeth from Ridjake, a rich paleontological site in western Serbia. NHMBEO 042501 is an upper third molar with heavy occlusal wear and taphonomic weathering. NHMBEO 042502 is a well-preserved lower third molar with only minor damage to the cusps and root apices. We performed an analysis of non-metric traits and made bivariate comparisons of crown linear measurements in order to assess the taxonomic affinity of the molars. Both show typical papionin occlusal patterns and relatively large overall sizes. In combination with the early Villafranchian (MN16) age of the site, we attribute both Ridjake primate fossils to cf. Paradolichopithecus sp. This represents the first identification of a non-human primate in Serbia, and the first identification of any primate in the Neogene period of Serbia. Along with recent hominin discoveries, the Ridjake fossils contribute to the growing primate fossil record in Serbia, and indicate the need for increased research into fossil primates in the country.

A diminutive Pliocene guenon from Kanapoi, West Turkana, Kenya.

Although modern guenons are diverse and abundant in Africa, the fossil record of this group is surprisingly sparse. In 2012 the West Turkana Paleo Project team recovered two associated molar teeth of a small primate from the Pliocene site of Kanapoi, West Turkana, Kenya. The teeth are bilophodont and the third molar lacks a hypoconulid, which is diagnostic for Cercopithecini. The teeth are the same size as those of extant Miopithecus, which is thought to be a dwarfed guenon, as well as a partial mandible preserving two worn teeth, previously recovered from Koobi Fora, Kenya, which was also tentatively identified as a guenon possibly allied with Miopithecus. Tooth size and proportions, as well as analysis of relative cusp size and shearing crest development clearly separate the fossil from all known guenons. Based on the Kanapoi material, we erect a new genus and species, Nanopithecus browni gen. et sp. nov. The small size of the specimen suggests either that dwarfing occurred early in the lineage, or at least twice independently, depending on the relationship of the new species with extant Miopithecus. Further, the distinctive habitat and geographic separation from Miopithecus suggests that the origin of small body size is not uniquely linked to the current habitat of Miopithecus, and possibly that relatives of extant Miopithecus were much more widely distributed in the past. This in turn argues caution in using extant biogeography in models of the origins of at least some guenons.

Phylogenetic relationships of living and fossil African papionins: Combined evidence from morphology and molecules.

African papionins are a highly successful subtribe of Old World monkeys with an extensive fossil record. On the basis of both molecular and morphological data, crown African papionins are divided into two clades: Cercocebus/Mandrillus and Papio/Lophocebus/Rungwecebus/Theropithecus (P/L/R/T), though phylogenetic relationships in the latter clade, among both fossil and extant taxa, remain difficult to resolve. While previous phylogenetic studies have focused on either molecular or morphological data, here African papionin molecular and morphological data were combined using both supermatrix and molecular backbone approaches. Theropithecus is supported as the sister taxon to Papio/Lophocebus/Rungwecebus, and while supermatrix analyses using Bayesian methods are largely unresolved, analyses using parsimony are broadly similar to earlier studies. Thus, the position of Rungwecebus relative to Papio and Lophocebus remains equivocal, possibly due to complex patterns of reticulation. Parapapio is likely a paraphyletic grouping of primitive African papionins or possibly a collection of stem P/L/R/T taxa, and a similar phylogenetic position is also hypothesized for Pliopapio. ?Papio izodi is either a stem or crown P/L/R/T taxon, but does not group with other Papio taxa. Dinopithecus and Gorgopithecus are also stem or crown P/L/R/T taxa, but their phylogenetic positions remain unstable. Finally, T. baringensis is likely the most basal Theropithecus taxon, with T. gelada and T. oswaldi sister taxa to the exclusion of T. brumpti. By integrating large amounts of molecular and morphological data, combined with the application of updated parsimony and Bayesian methods, this study represents the most comprehensive analysis of African papionin phylogenetic history to date.

A 2Ma old baboon-like monkey from Northern Greece and new evidence to support the Paradolichopithecus - Procynocephalus synonymy (Primates: Cercopithecidae).

A new fossil cranium of a large papionin monkey from the Lower Pleistocene site of Dafnero-3 in Western Macedonia, Greece, is described by means of outer and inner morphological and metric traits using high-resolution micro-computed tomography. Comparisons with modern cercopithecids and contemporaneous Eurasian fossil taxa suggest that the new cranium could equally be ascribed to either the Eurasian Paradolichopithecus or to the East Asian Procynocephalus. The combination of the available direct and indirect fossil evidence, including the new cranium from Dafnero, revives an earlier hypothesis that considers these two sparsely documented genera as synonyms. The timing and possible causes of the rise and demise of Paradolichopithecus - Procynocephalus are discussed.

The evolution of the Cercopithecini: a (post)modern synthesis.

The Cercopithecini, or African guenon monkeys, are one of the most diverse clades of living primates and comprise the most species-rich clade of Catarrhini. Species identity is announced by flamboyant coloration of the facial and genital regions and, more cryptically, by vigorous chromosomal rearrangements among taxa. Beneath the skin, however, these animals are skeletally conservative and show low levels of genetic sequence divergence consonant with recent divergence between congeneric species. The guenons clearly demonstrate that morphological, cytogenetic, and reproductive differentiation proceed at different rates during speciation. We review diverse kinds of data in an effort to understand this conundrum.

Cercopithecoid humeri from Taung support the distinction of major papionin clades in the South African fossil record.

Associated cercopithecoid postcrania are rare in the Plio-Pleistocene fossil record, particularly in the case of South African karst cave sites. However, as clear postcranial differences between major papionin clades have been documented, it should be possible to assign isolated papionin postcrania to the Cercocebus/Mandrillus and Papio/Lophocebus/Theropithecus groups wherever sufficient anatomy is preserved. Here, we demonstrate that two partial humeri preserved at Taung, UCMP 56693 and UCMP 125898, are most likely attributable to the Cercocebus/Mandrillus and Papio/Lophocebus/Theropithecus clades, respectively. Univariate analyses (ANOVAs and t-tests) and multivariate analyses (discriminant function analyses) of humeral features, combined with a phylogenetic analysis of 24 humeral characters, all support our assessment. Given that the overwhelming number of craniodental specimens at Taung are attributable to two papionin taxa, Procercocebus antiquus (a member of the Cercocebus/Mandrillus clade) and Papio izodi (a purported fossil species of the modern genus Papio), we assign UCMP 56693 to Pr. antiquus and UCMP 125868 to P. izodi with a high degree of confidence. Implications for cercopithecoid evolution and biogeography are discussed, with a particular emphasis on these two fossil taxa.

Dietary change among hominins and cercopithecids in Ethiopia during the early Pliocene.

The incorporation of C4 resources into hominin diet signifies increased dietary breadth within hominins and divergence from the dietary patterns of other great apes. Morphological evidence indicates that hominin diet became increasingly diverse by 4.2 million years ago but may not have included large proportions of C4 foods until 800 thousand years later, given the available isotopic evidence. Here we use carbon isotope data from early to mid Pliocene hominin and cercopithecid fossils from Woranso-Mille (central Afar, Ethiopia) to constrain the timing of this dietary change and its ecological context. We show that both hominins and some papionins expanded their diets to include C4 resources as early as 3.76 Ma. Among hominins, this dietary expansion postdates the major dentognathic morphological changes that distinguish Australopithecus from Ardipithecus, but it occurs amid a continuum of adaptations to diets of tougher, harder foods and to committed terrestrial bipedality. In contrast, carbon isotope data from cercopithecids indicate that C4-dominated diets of the earliest members of the Theropithecus oswaldi lineage preceded the dental specialization for grazing but occurred after they were fully terrestrial. The combined data indicate that the inclusion of C4 foods in hominin diet occurred as part of broader ecological changes in African primate communities.

The relative correspondence of cranial and genetic distances in papionin taxa and the impact of allometric adjustments.

The reconstruction of phylogenetic relationships in the primate fossil record is dependent upon a thorough understanding of the phylogenetic utility of craniodental characters. Here, we test three previously proposed hypotheses for the propensity of primate craniomandibular data to exhibit homoplasy, using a study design based on the relative congruence between cranial distance matrices and a consensus genetic distance matrix ("genetic congruence") for papionin taxa: 1) matrices based on cranial regions subjected to less masticatory strain are more genetically congruent than high-strain cranial matrices; 2) matrices based on cranial regions developing earlier in ontogeny are more genetically congruent than matrices based on regions that develop later; and 3) matrices based on cranial regions with greater anatomical/functional complexity are more genetically congruent than matrices based on anatomically simpler regions. Morphological distance matrices based on the shape of 15 different cranial regions, delineated on the basis of previous catarrhine studies, were statistically compared to a matrix of known genetic distances in papionins. Since sexual dimorphism and allometry are known to characterize this clade, several analytical iterations were conducted: 1) mixed-sex, male-only, and female-only analyses and 2) with and without an allometric scaling adjustment. Across all datasets, the chondrocranium matrix was the most consistently correlated with genetic distances, which is also consistent with previous studies of cercopithecoid taxa; however, there was no support for the internal predictions of the three hypotheses tested. Allometric scaling corrections had the largest impact on the genetic congruence of facial shape matrices, a result consistent with previous studies that have described facial homoplasy in papionin taxa. These findings differ from patterns described for hominoid taxa, suggesting that no single predictive criterion can explain phylogenetic utility of cranial datasets across catarrhine primate taxa. Many of the differences in morphological-genetic matrix correlations could result from different levels of phenotypic integration and evolvability in cercopithecoids and hominoids, suggesting that further study of these phenomena in extant primates is warranted.

New craniodental fossils of papionin monkeys from Cooper's D, South Africa.

Papionin monkey fossils are common in the Plio-Pleistocene aged karst cave deposits northwest of Johannesburg in South Africa. These deposits have yielded important primate and other vertebrate fauna since their discovery in the early part of the 20th century. In this article, we describe new primate cranial and dental specimens from excavations at the site of Cooper's D in the Sterkfontein Valley that date to around 1.5 million years ago. Unlike other localities in southern Africa, most of the new fossils are referred to Theropithecus oswaldi oswaldi, an extinct gramnivorous monkey related to the living gelada. Diagnostic features of T. o. oswaldi crania and teeth include large, thickly enameled molars with tall, columnar cusps, and high molar relief, an upright mandibular ramus, postorbital constriction, and anterior fusion of temporal lines. Also present in the new sample are teeth referred to Papio sp., which show low crowned bunodont molars, and a number of indeterminate papionin teeth and skull fragments. The presence of T. o. oswaldi at Cooper's D extends the list of known localities where the taxon is found, and may indicate the presence of an open, grassland environment in the area during the early Pleistocene. The abundance of theropith fossils at Cooper's suggests that Papio was not consistently the most common papionin in southern Africa over the past three million years.

Cladistic analysis of extant and fossil African papionins using craniodental data.

This study examines African papionin phylogenetic history through a comprehensive cladistic analysis of extant and fossil craniodental morphology using both quantitative and qualitative characters. To account for the well-documented influence of allometry on the papionin skull, the general allometric coding method was applied to characters determined to be significantly affected by allometry. Results of the analyses suggest that Parapapio, Pliopapio, and Papio izodi are stem African papionin taxa. Crown Plio-Pleistocene African papionin taxa include Gorgopithecus, Lophocebus cf. albigena, Procercocebus, Soromandrillus (new genus defined herein) quadratirostris, and, most likely, Dinopithecus. Furthermore, S. quadratirostris is a member of a clade also containing Mandrillus, Cercocebus, and Procercocebus; ?Theropithecus baringensis is strongly supported as a primitive member of the genus Theropithecus; Gorgopithecus is closely related to Papio and Lophocebus; and Theropithecus is possibly the most primitive crown African papionin taxon. Finally, character transformation analyses identify a series of morphological transformations during the course of papionin evolution. The origin of crown African papionins is diagnosed, at least in part, by the appearance of definitive and well-developed male maxillary ridges and maxillary fossae. Among crown African papionins, Papio, Lophocebus, and Gorgopithecus are further united by the most extensive development of the maxillary fossae. The Soromandrillus/Mandrillus/Cercocebus/Procercocebus clade is diagnosed by upturned nuchal crests (especially in males), widely divergent temporal lines (especially in males), medially oriented maxillary ridges in males, medially oriented inferior petrous processes, and a tendency to enlarge the premolars as an adaptation for hard-object food processing. The adaptive origins of the genus Theropithecus appear associated with a diet requiring an increase in size of the temporalis, the optimal placement of occlusal forces onto the molar battery, and an increase in the life of the posterior dentition. This shift is associated with the evolution of distinctive morphological features such as the anterior union of the temporal lines, increased enamel infoldings on the premolars and molars, a reversed curve of Spee, and delayed molar eruption.

Next-generation museomics disentangles one of the largest primate radiations.

Guenons (tribe Cercopithecini) are one of the most diverse groups of primates. They occupy all of sub-Saharan Africa and show great variation in ecology, behavior, and morphology. This variation led to the description of over 60 species and subspecies. Here, using next-generation DNA sequencing (NGS) in combination with targeted DNA capture, we sequenced 92 mitochondrial genomes from museum-preserved specimens as old as 117 years. We infer evolutionary relationships and estimate divergence times of almost all guenon taxa based on mitochondrial genome sequences. Using this phylogenetic framework, we infer divergence dates and reconstruct ancestral geographic ranges. We conclude that the extraordinary radiation of guenons has been a complex process driven by, among other factors, localized fluctuations of African forest cover. We find incongruences between phylogenetic trees reconstructed from mitochondrial and nuclear DNA sequences, which can be explained by either incomplete lineage sorting or hybridization. Furthermore, having produced the largest mitochondrial DNA data set from museum specimens, we document how NGS technologies can "unlock" museum collections, thereby helping to unravel the tree-of-life.

Evolutionary molecular cytogenetics of catarrhine primates: past, present and future.

The catarrhine primates were the first group of species studied with comparative molecular cytogenetics. Many of the fundamental techniques and principles of analysis were initially applied to comparisons in these primates, including interspecific chromosome painting, reciprocal chromosome painting and the extensive use of cloned DNA probes for evolutionary analysis. The definition and importance of chromosome syntenies and associations for a correct cladistics analysis of phylogenomic relationships were first applied to catarrhines. These early chromosome painting studies vividly illustrated a striking conservation of the genome between humans and macaques. Contemporarily, it also revealed profound differences between humans and gibbons, a group of species more closely related to humans, making it clear that chromosome evolution did not follow a molecular clock. Chromosome painting has now been applied to more that 60 primate species and the translocation history has been mapped onto the major taxonomic divisions in the tree of primate evolution. In situ hybridization of cloned DNA probes, primarily BAC-FISH, also made it possible to more precisely map breakpoints with spanning and flanking BACs. These studies established marker order and disclosed intrachromosomal rearrangements. When applied comparatively to a range of primate species, they led to the discovery of evolutionary new centromeres as an important new category of chromosome evolution. BAC-FISH studies are intimately connected to genome sequencing, and probes can usually be assigned to a precise location in the genome assembly. This connection ties molecular cytogenetics securely to genome sequencing, assuring that molecular cytogenetics will continue to have a productive future in the multidisciplinary science of phylogenomics.

Morphological systematics of the kipunji (Rungwecebus kipunji) and the ontogenetic development of phylogenetically informative characters in the Papionini.

Since its discovery and description, the systematic position of the kipunji (Rungwecebus kipunji) has been a matter of debate. Although it was first placed in the mangabey genus Lophocebus, subsequent molecular studies indicated that the kipunji is most closely related to baboons (Papio). However, the kipunji does not appear to possess cranial features typical of Papio, thus necessitating the erection of a new genus, Rungwecebus. The recovery of an M2-stage subadult male kipunji voucher specimen, in addition to the original M1-stage subadult male voucher specimen, has since allowed further study. Here, we describe the craniodental morphology of the newly acquired kipunji specimen and present a phylogenetic analysis of Rungwecebus craniodental morphology using quantitative and qualitative characters. We examined the skulls of 76 M1- and M2-stage subadult males representing all extant papionin genera, taking note of character states that are static throughout ontogeny. To control for ontogenetic changes, only those characters expressing unchanged character states between subadult and adult specimens were coded for Rungwecebus and entered into a larger, recently published 151-character matrix of adult male morphology. To account for allometry, the narrow allometric coding method and the general allometric coding method were applied. The resulting most parsimonious trees suggest that Rungwecebus is phylogenetically closest to Lophocebus, a result consistent with initial morphological descriptions. However, due to the large amount of missing data for Rungwecebus, there are low bootstrap support values associated with any relationships within the larger Theropithecus/Papio/Lophocebus/Rungwecebus grouping. Taken in combination with previous molecular, phenetic, and ecological studies, the results of this study suggest that Rungwecebus is best regarded as a distinct genus closely related to Papio, Lophocebus, and Theropithecus. Adult morphological specimens are necessary to fully understand the adult kipunji morphotype, and its phylogenetic position will only be more precisely resolved with additional morphological and molecular data.

Phylogenetic analysis of the African papionin basicranium using 3-D geometric morphometrics: the need for improved methods to account for allometric effects.

The basicranium has been argued to contain a strong phylogenetic signal in previous analyses of primate cranial morphology. Therefore, further study of basicranial morphology may offer new insights into controversial phylogenetic relationships within primate groups. In this study, I apply 3-D geometric morphometric techniques in a phylogenetic analysis of the African papionin basicranium. The effects of allometry strongly influence African papionin basicranial morphology and, unless these size effects are controlled or eliminated, phylogenetic analyses suggest traditional phylogenetic groupings of small taxa (mangabeys) and large taxa (geladas, mandrills, drills, and baboons). When the effects of allometry are eliminated by excluding size-correlated principal components (PCs) or by regression analysis with retention of residuals, phylogenetic analyses of African papionin basicranial morphology are incongruent with recent molecular and morphological studies. By contrast, a cladistic analysis of basicranial characters using the narrow allometric coding method suggests the same phylogenetic relationships as recent molecular and morphological studies. These results suggest that important phylogenetic information is contained within the size-correlated data, and this information is being discarded during the attempt to eliminate the effects of body size. Future 3-D morphometric studies of phylogeny should focus on the development of new methodologies to adjust for allometric effects, as current techniques appear to be ill-equipped to deal with the case of a size-disparate, lower-level taxonomic group.

The biogeography of introgression in the critically endangered African monkey Rungwecebus kipunji.

In the four years since its original description, the taxonomy of the kipunji (Rungwecebus kipunji), a geographically restricted and critically endangered African monkey, has been the subject of much debate, and recent research suggesting that the first voucher specimen of Rungwecebus has baboon mitochondrial DNA has intensified the controversy. We show that Rungwecebus from a second region of Tanzania has a distinct mitochondrial haplotype that is basal to a clade containing all Papio species and the original Rungwecebus voucher, supporting the placement of Rungwecebus as the sister taxon of Papio and its status as a separate genus. We suggest that the Rungwecebus population in the Southern Highlands has experienced geographically localized mitochondrial DNA introgression from Papio, while the Ndundulu population retains the true Rungwecebus mitochondrial genome.

Bringing up baby: Developmental simulation of the adult cranial morphology of rungwecebus kipunji.

Rungwecebus kipunji is a recently discovered, critically endangered primate endemic to southern Tanzania. Although phenetically similar to mangabeys, molecular analyses suggest it is more closely related to Papio or possibly descended from an ancient population of baboon-mangabey hybrids. At present, only a single kipunji specimen, an M1-stage juvenile male, is available for study; thus, the cranial morphology of the adult kipunji is unknown. In this study, we used developmental simulation to estimate the adult kipunji's 3D cranial morphology. We examined variation in cercopithecine developmental vectors, applied selected vectors to the juvenile cranium, and compared the resulting simulated adults to actual adult male papionins. Differences between papionin developmental vectors were small and statistically insignificant. This uniformity suggests conservation of an ancestral papionin developmental program. Simulated kipunji adults were likewise extremely similar. As a group, the simulated adults were morphometrically distinct from other papionins, corroborating the kipunji's generic status. Simulated adults were phenetically most similar to Lophocebus aterrimus but were distinguished from all adult papionins by the same unique traits that characterize the kipunji juvenile: a tall neurocranium, broad face, short nasal bones, concave anteorbital profile, and dorsally rotated palate. This concordance between juvenile and estimated-adult morphologies confirms that papionin cranial shape is largely established before M1 eruption. The estimated kipunji adult's neurocranium strongly resembles that of Papio, providing the first cranial evidence supporting their phylogenetic relationship. If the kipunji does indeed have a hybrid origin, then its phenetic affinity to L. aterrimus favors Lophocebus as the proto-kipunji's paternal lineage.

Allometry, sexual dimorphism, and phylogeny: a cladistic analysis of extant African papionins using craniodental data.

This study conducts a phylogenetic analysis of extant African papionin craniodental morphology, including both quantitative and qualitative characters. We use two different methods to control for allometry: the previously described narrow allometric coding method, and the general allometric coding method, introduced herein. The results of this study strongly suggest that African papionin phylogeny based on molecular systematics, and that based on morphology, are congruent and support a Cercocebus/Mandrillus clade as well as a Papio/Lophocebus/Theropithecus clade. In contrast to previous claims regarding papionin and, more broadly, primate craniodental data, this study finds that such data are a source of valuable phylogenetic information and removes the basis for considering hard tissue anatomy "unreliable" in phylogeny reconstruction. Among highly sexually dimorphic primates such as papionins, male morphologies appear to be particularly good sources of phylogenetic information. In addition, we argue that the male and female morphotypes should be analyzed separately and then added together in a concatenated matrix in future studies of sexually dimorphic taxa. Character transformation analyses identify a series of synapomorphies uniting the various papionin clades that, given a sufficient sample size, should potentially be useful in future morphological analyses, especially those involving fossil taxa.

The primates of the Udzungwa Mountains: diversity, ecology and conservation.

The Udzungwa Mountains are one of the most important areas in Africa for primate diversity and conservation, with two endemic monkeys (Udzungwa red colobus Procolobus gordonorum and Sanje mangabey Cercocebus [galeritus] sanjei), and the near-endemic kipunji monkey Rungwecebus kipunji, a new genus and species discovered in 2004 and found in Udzungwa and Southern Highlands. With six species of galagos, or bushbabies, the area is also of exceptional importance for nocturnal primates. The form of Mountain galago Galagoides orinus occurring in the Udzungwa Mountains appears to be distinct on the base of vocalizations, and further work will be required to assess its taxonomic status. The primate community reflects the overall, exceptional biodiversity of these isolated and ancient mountains. The Udzungwa are part of the Eastern Arc mountain chain, a centre of global importance for biodiversity and endemism. Of all the Eastern Arc Mountain blocks, the Udzungwa have the largest forested area, widest altitudinal gradient and greatest habitat diversity. We review current knowledge on the diversity and distribution of the primate community of the Udzungwa Mountains, with an emphasis on ecology and conservation.

Mitochondrial evidence for the hybrid origin of the kipunji, Rungwecebus kipunji (Primates: Papionini).

Common baboons (Papio), gelada baboons (Theropithecus) and baboon-mangabeys (Lophocebus) are closely related African papionin monkeys. In 2005, the species Lophocebus kipunji was described from relict montane and submontane forests in Tanzania, based upon a single specimen and observations of living animals. Its initial assignment to Lophocebus was based on its overall morphology, but subsequent genetic studies suggesting that it was sister taxon to common baboons (Papio) led to its generic separation, as Rungwecebus. As a mangabey-like sister-taxon to Papio, Rungwecebus could be interpreted either as an arboreal derivative from a more terrestrial, baboon-like ancestor, or as a survivor of a mangabey-like common ancestor of the Lophocebus-Papio-Theropithecus clade. Here, we present a new, strongly-supported, mitochondrial DNA (mtDNA) phylogeny that includes Papio baboons from populations geographically close to the kipunji. Rather than supporting sister-taxon status, the new phylogeny not only situates the kipunji's mtDNA among Papio haplotypes, it clearly assigns it to a mitochondrial clade including geographically adjacent yellow baboons (Papio cynocephalus). This relationship suggests either that the kipunji is descended from a yellow baboon, and has converged on a mangabey-like morphology, or, much more likely, that it originated by hybridization between Papio cf.cynocephalus females and Lophocebus sp. males, about 0.65 Ma. We believe this to be the first case among mammals in which a natural occurrence of inter-generic hybridization can be shown to have resulted in a new, distinct, long-surviving taxon. More such cases can be anticipated as molecular evidence accumulates.

Knuckle walking signal in the manual digits of Pan and Gorilla.

This article examines the curvature of the manual proximal and middle phalanges of species belonging to Pan, Gorilla, Ateles, Macaca, Pongo, Hylobates, and Cebus to determine whether middle phalangeal curvature, when considered in conjunction with proximal phalangeal curvature, yields a locomotor signal. Prior studies have demonstrated the discriminatory power of proximal phalanges for separating suspensory species (including knuckle walkers) from pronograde quadrupedal species, but less emphasis has been placed on the distinguishing phalangeal characteristics of taxa within the suspensory category. This study demonstrates, first, that middle phalanges discriminate suspensory from nonsuspensory species, although not as cleanly as proximal phalanges. Finer discrimination of locomotor signals, including subtle differences among animals employing different modes of suspension, is possible through a comparison of the curvatures of the proximal phalanges and corresponding middle phalanges. Their relative curvature differs in quadrupeds, brachiators, and knuckle walkers. Knuckle walkers (Pan and Gorilla) have relatively little curvature of the middle phalanges coupled with marked curvature of the proximal phalanges, whereas brachiators (Ateles and Hylobates) display marked curvature of both proximal and middle phalanges, and pronograde quadrupeds (Cebus and Macaca) have relatively straight proximal and moderately curved middle phalanges. Quadrumanous climbers (Pongo) have a unique combination of traits, whereby curvature is high in both proximal and middle phalanges, but less so in the latter than the former. These differences, predictable on the basis of the biomechanical forces to which digits are subjected, may open a new venue for future research on the locomotor repertoire of prebipedal ancestors of hominins.