A closer look at the “Protopithecus” fossil assemblages: new genus and species from Bahia, Brazil
Introduction
Remains of Protopithecus brasiliensis were first discovered in 1836 by the Danish naturalist Peter Wilhelm Lund. A left proximal femur, UZM (Universitets Zoologisk Museum) 3530, and right distal humerus, UZM 1623, were found in the Lagoa Santa (LS) cave system in Minas Gerais, Brazil, and despite their robusticity and large size, twice that of the largest living platyrrhine, Lund understood that the bones belonged to a New World monkey (Lund, 1838). These fossils were the first to be correctly recognized as a primate at the time of their discovery (Hartwig, 1995b) and were mentioned in passing by Darwin (1859) along with several other early monkey fossil discoveries in On the Origin of Species. Lund's writings concerning Protopithecus were neither extensive nor detailed, but a later monograph by Winge (1895) describing Lund's Brazilian excavations included a brief comparative analysis and an expanded hypodigm that included several more postcranial elements but no teeth or cranial material (Fig. 1). Winge noted that the distal humerus and proximal femur were each found in the same cavern, but not at the exact same site. He concluded that they are ‘probably’ from the same individual and, using an outmoded taxonomic name (Eriodes), allocated the material to the genus we now know as Brachyteles, distinguished at the species level from Brachyteles arachnoides based on the fossil's larger size. As a consequence, the LS fossils were relegated to scientific obscurity for over 100 years (Hartwig, 1995a, Hartwig, 1995b).
Then, in 1992, the matter was reopened when two nearly complete skeletons of comparably large platyrrhines were discovered in the Toca da Boa Vista (TBV) caves in the neighboring state of Bahia, approximately 1200 km northeast of Lagoa Santa (Fig. 2). In a brief preliminary announcement (Cartelle, 1993), both were identified as conspecific with Lund's material and called Brachyteles. One, a subadult, would eventually be named a new genus and species, Caipora bambuiorum, and described as a giant spider monkey (MCL [Museu de Ciências Naturais PUC Minas Gerais] 05; Cartelle and Hartwig, 1996). And soon after Cartelle's published note, Hartwig (1995a) showed that Lund's large limbs from Lagoa Santa were misinterpreted as being muriqui-sized (see Hill, 1962), and he argued that generic status was, in fact, warranted.
Hartwig's (1995a) analysis, which was the first modern consideration of the original LS material, concluded that the Lund fossils were “grossly indistinguishable” from Brachyteles and Ateles, except for their larger size. A regression equation for estimating body mass based on femoral head volume in catarrhine primates (Ruff, 1990) returned an estimate of 23–24 kg (Hartwig, 1995a); the largest body mass reported for any extant atelid is 12–15 kg (Di Fiore and Campbell, 2007). A closer phylogenetic relationship to Brachyteles was suggested because of Lagoa Santa's location within the region of pre-Columbian Atlantic Coastal Forest where the woolly spider monkeys currently live in a very restricted distribution. Hartwig's (1995a) analysis led to the resurrection of the genus Protopithecus and the eventual transferral of the MCL 06 skeleton to the hypodigm of P. brasiliensis, based largely on similarities in long bone dimensions and estimated body mass (Hartwig and Cartelle, 1996; Table 1; Fig. 3). However, while their body mass may have been similar, the morphology of the distal humeri and proximal femora of the two individuals now referred to P. brasiliensis presents several anatomical and inferred behavioral differences that we contend are of systematic importance.
In suggesting that the LS and TBV samples constitute a mosaic wherein Protopithecus is Alouatta-like cranially and Brachyteles-like postcranially, a pattern not seen in any other extant or extinct platyrrhine, Hartwig and Cartelle's phylogenetic hypothesis (Hartwig, 1995a, Hartwig and Cartelle, 1996) is internally inconsistent, for these patterns are each derived among platyrrhines and indicative of separate atelid clades. While a subsequent, more detailed biometric analysis of the LS and TBV material found complimentary support in several traits of the forelimb and vertebral column that suggested a functional link between the fossils from both localities and the highly suspensory locomotion of extant atelines (Jones, 2008), more recent work (Halenar, 2012, Rosenberger et al., in press) raises additional questions by extending the range of cranio-skeletal similarities shared by only the TBV material and Alouatta, which includes such features as a relatively small brain and extended cranial base.
In the context of a broad analysis of atelid postcranial and cranial morphology, diversity, and evolution (Halenar, 2011a, Halenar, 2011b, Halenar, 2012), we have reexamined all of the original material pertaining to these fossils and find both qualitative and quantitative indications suggesting the presence of two distinct morphologies and clades within the current ‘Protopithecus’ sample. Differences between the two specimens include the larger dimensions of the TBV material, the presence of a large brachioradialis flange on the TBV distal humerus, and the different shape and size of both the gluteal tuberosity and lesser trochanter between the two proximal femora. These femoral characters involve likely synapomorphies linking the TBV specimen with Alouatta. Their absence in the LS femur, as well as the taxonomic expression of brachioradialis flange expansion amongst extant platyrrhines, suggests that the LS fragments and TBV complete skeleton are taxonomically distinct at the genus level. As a consequence, we propose a new genus and species to accommodate the TBV material from Bahia. Data presented here support Hartwig's original observations of the ateline affinities of the LS material (Hartwig, 1995a), while the TBV material shares likely derived resemblances with Alouatta in the cranium, innominate, and proximal femur, suggesting that it is more closely related to alouattines. Without any other skeletal elements represented, we are in a similar position to Winge and other earlier authors in suggesting that the LS material is best interpreted as a large-bodied relative of Brachyteles.
Section snippets
Materials and methods
As noted, all of the original pertinent fossils have been examined by both authors. The samples, measurements and statistics employed to estimate body size in the fossils have been described elsewhere (Halenar, 2011a). Centroid size of various joint surfaces, based on three-dimensional landmark measurements, was used as the independent variable in both ordinary least squares as well as reduced major axis regression models with an atelid-only, platyrrhine-only, and primate-wide comparative
Distal humerus
The distal humeri from Lagoa Santa and Toca da Boa Vista differ from one another in their size and robusticity, especially in the width of the joint and the prominent brachioradialis flange on the lateral side of the TBV specimen, which is hardly identifiable on the LS element (Fig. 5). The difference in humeral shaft thickness between the two fossils, 4.75 mm, is larger than the differences in this measurement between the means of any of the atelid genera (Table 1). Other aspects of the joint
Systematics
The evidence presented above suggests the LS and TBV specimens that have both previously been assigned to P. brasiliensis Lund actually represent two distinct taxa. The type specimen from Lagoa Santa is morphologically similar to Brachyteles and Ateles, but comparable elements of the TBV specimen are distinctly different. When the evidence from the entire skeleton is taken into account along with its behavioral and systematic implications, the case for a conspecific allocation becomes weak. The
Conclusions
The new Bahian genus provides evidence that a third very large arboreal fossil atelid existed in the Pleistocene of Brazil. A fourth large-sized fossil from a different clade, a Lagothrix-sized cebine, is also known from the late middle Miocene of western Brazil, Acrecebus solimoensis (Kay and Cozzuol, 2006). This suggests we should rethink the common supposition, based on today's distribution, that New World monkeys are somehow size-constrained by their very nature, perhaps in connection with
Acknowledgments
This work was partially supported by National Science Foundation Doctoral Dissertation Improvement Grant #0925704. We wish to thank Castor Cartelle in Belo Horizonte, Brazil and Kim Aaris in Copenhagen, Denmark for access to the fossil material as well as Eileen Westwig at the AMNH in New York for access to the extant primate collections. Thanks go to Andi Jones for her generosity in allowing the use of her photo of the TBV skull. Walter Hartwig was also kind enough to pass along his original
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