Remains of *Protopithecus**brasiliensis* were first discovered in 1836 by the Danish naturalist Peter Wilhelm Lund. A left proximal femur and right distal humerus were found in the Lagoa Santa cave system in Minas Gerais, Brazil, and despite their large size and robusticity, Lund recognized them as belonging to a New World monkey (Lund,1838). These fossils were the first to be correctly recognized as a primate and were included in Darwin's (1859) discussion of South American fossil monkeys in *On the Origin of Species*. Together with a nearly complete skeleton found 150 years later in the separate Toca da Boa Vista cave system in the neighboring state of Bahia, Lund's discovery makes up one-half of the known Pleistocene primate “megafauna” of Brazil. The other half is the subfossil skeleton of *Caipora bambuiorum*, also nearly complete. These two specimens include largely undescribed crania; mandibles; mostly complete upper and lower dentitions; cervical, thoracic, and caudal vertebrae; scapular and innominate fragments; complete upper and lower limb bones; carpals, tarsals, metacarpals, metatarsals, and phalanges. The reconstructed feet are the only complete cheiridia in the platyrrhine fossil record (Cartelle and Hartwig,1996; Hartwig and Cartelle,1996). The original publications describing the fossil material suggested that *Caipora* was morphologically comparable to a large *Ateles* (Cartelle and Hartwig,1996). *Protopithecus*, on the other hand, showed a more complicated mosaic pattern with frugivorous teeth in an otherwise *Alouatta*-like skull on top of a postcranial skeleton adapted for spider monkey-like suspensory locomotion (Hartwig and Cartelle,1996). Only limited additional analysis has been done on either of these fossils (Heymann,1998; Jones,2008; Halenar,2009,2010; Rosenberger et al., in review).

One of the most intriguing differences between *Protopithecus* and its living ateline relatives is the large size and robusticity of its long bones. Using a regression equation for estimating body mass based on femoral head volume in catarrhine primates (Ruff,1990), the original individual discovered by Lund in the 1800s was estimated to have a body weight of 22.7–24.1 kg (Hartwig,1995). At that time, Hartwig recognized that using catarrhines as a regression model was a fallback approach since comparable data and regression equations for living atelines did not exist. He also noted that *Protopithecus* is so much larger than modern atelines that it would fall outside of the range of any regression plot generated by a more appropriate platyrrhine reference sample (Hartwig,1995). Despite these caveats, Hartwig and Cartelle (1996) used Ruff's (1990) femoral head equation when the more complete *Protopithecus* skeleton was discovered; the new material was estimated at 25 kg, a value approximately twice that of the largest living platyrrhines. Prediction error, measures of estimate consistency and accuracy, and standard error were not provided in either publication.

A vast literature exists on the topic of body mass estimation in fossil human and non-human primates. The problems encountered by Hartwig (1995) in choosing an appropriate reference sample and dealing with fossils of large body size have been discussed at length and solved to varying degrees of satisfaction in more recent publications involving other taxa. For example, in order to estimate the body size of the truly giant subfossil lemurs, extrapolation into body size ranges outside of anything seen in living primates was required (Godfrey et al.,1995; Jungers et al.,2002,2008). The best estimates of body size were produced using humeral and femoral midshaft circumferences against a primate reference sample as well as a mammalian reference sample which represented a wide range of body sizes and positional behaviors (Godfrey et al.,1995). Similarly, Delson et al. (2000) discussed the problem of investigating very large fossils and concluded that it is the estimates for these taxa that come with the most error, regardless of the alternative materials and methods chosen. Other studies estimating the body weights of fossil primates (e.g., Dagosto and Terranova,1992; Aiello and Wood,1994) have shown that different regression models and reference samples produce not only different estimates of body mass but also different ranges and confidence intervals.

From a phylogenetic perspective, the appropriate reference sample for estimating body size of *Protopithecus* would include the four genera of ateline primates (*Alouatta, Lagothrix, Ateles*, and *Brachyteles*) suggested to be its closest living relatives (Hartwig and Cartelle,1996). Platyrrhine-only craniodental regression equations are now available from two studies (Meldrum and Kay,1997; Sears et al.,2008), although their value in this context remains somewhat limited. The lower molars of *Protopithecus* are not preserved and canine size and temporal line strength suggest the cranium belongs to a male (Hartwig and Cartelle,1996), so the regression equations of Meldrum and Kay (1997) based on female platyrrhine molar dimensions are not directly useful here. The equations of Sears et al. (2008) come with poor R^{2} values and relatively high error sum of squares (ESS) values; of the 80 craniodental variables investigated, the highest R^{2} value is 0.636 and the majority are below 0.5. It could be that there are so many variable cranial morphologies in their reference sample (i.e., callitrichine skulls are very different from *Alouatta* skulls), that those differences are making the scatter around the regression line too diffuse for accurate predictions. There are several other reasons for caution when applying the Sears et al. (2008) equations to other fossil taxa. For example, the equations produced in that study are not tested for accuracy on individuals of known body mass so there is no evidence for certain variables over- or under-estimating body mass as the authors discuss.

Despite these issues, the Sears et al. (2008) equations were applied to *Protopithecus*, and not surprisingly, the results beg further investigation. The fossil mandible is incomplete so data are not available for implementing mandibular length, their best predictor variable (R^{2} = 0.636, ESS = 0.117). Ranking second and third, bizygomatic width (R^{2} = 0.626, ESS = 0.166) and skull length (R^{2} = 0.537, ESS = 0.196) of *Protopithecus* give a mass estimate of approximately 12 kg, or about the size of a large male *Alouatta pigra*. This is much lower than either of the original estimates (Hartwig,1995; Hartwig and Cartelle,1996) and seems an unlikely value both because of the low R^{2} values of the equations and the fact that the fossil bones themselves are much larger than the bones of an extant 12 kg platyrrhine. For example, total skull length of *Brachyteles* (usually cited as the largest ateline at around 12 kg) is 115 mm while for *Protopithecus* it is 150 mm; in the postcranial skeleton, femoral length of *Brachyteles* is 202 mm and in *Protopithecus* is 237 mm (Hartwig and Cartelle,1996). Attempting to use these newly published equations again serves to highlight the fact that the choice of reference group and predictor variable will affect the final estimate. The development of more accurate platyrrhine-based regression models for application to the fossil record is a necessary step forward.

Body size of an individual or species is related to many other aspects of its biology and behavior (for recent review see Bernstein (2010)). For fossil taxa, reconstructing diet and/or locomotion are most often the focus of studies attempting to estimate body size (e.g., Fleagle,1978,1999; Jungers,1984; Kay,1985). “Kay's Threshold” of 500 g has long been used to separate small-bodied insectivores from larger-bodied folivores (Kay,1975; Gingerich,1981). Similarly, a limit of 10–12 kg has been suggested for the metabolically efficient use of ricochetal brachiation (Preuschoft and Demes,1985; see below). This article provides a new set of regression equations for predicting body weight and linear dimensions of body size based on a large series of New World monkey species and specimens. Focus is placed on the postcranial skeleton as a source of size information, especially joint surfaces that have been shown to be valuable in other studies that estimate fossil body mass in various primate groups (e.g., Jungers,1990; Godfrey et al.,1995; Delson et al.,2000; Ruff,2003). These new estimates are then used as a basis for preliminary discussion of possible locomotor behaviors used by *Protopithecus* and how the combination of body size reduction, hyoid size increase, and decrease in acrobatic locomotion influenced the evolution of extant *Alouatta*.