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Patterns of Integration and Modularity in the Hominoid Wrist Open Access

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Evolutionary changes in the bones of the carpus, or wrist, have long been integral to the discussion of the locomotor capabilities of fossil and extant hominoids (e.g., Lewis, 1969; Tocheri et al., 2008; Almécija et al., 2009; Williams, 2010; Nakatsukasa et al., 2016). Specifically, researchers have often turned to the wrist for evidence pointing towards the ancestral locomotor repertoire of hominoids, including the predecessors of bipedalism (e.g., Jenkins and Fleagle, 1975; Richmond and Strait, 2000; Begun, 2004; Kivell and Schmitt, 2009). Much debate has focused on whether Pan and Homo evolved from a knuckle-walking or more generalized arboreal ancestor (e.g., Tuttle, 1967; Richmond and Strait, 2000; Kivell and Schmitt, 2009; Lovejoy et al., 2009a; Williams, 2010). However, identifying the functional significance of wrist morphology, as well as other postcranial regions, in fossil skeletons is often complicated by the biological phenomenon of “mosaic evolution” observed in the hominoid fossils record (e.g., Köhler and Moyà-Solà, 1997; Moyà-Solà et al., 2004; Almécija et al., 2009; Young et al., 2010; Hamrick, 2012; Kivell et al., 2013; Tallman et al., 2013). Mosaic evolution is the concept that evolutionary change in morphology occurs at different rates among anatomical regions (Wagner and Altenberg, 1996). The hominoid fossil record shows that skeletal morphology has evolved in a mosaic fashion, where fossil skeletons often appear to exhibit a “patchwork” of conflicting adaptations. The morphological combinations observed in the appendicular skeletons of fossil apes are not found in living analogues, making it unclear whether the individual morphologies we observe in the fossil record are functionally relevant or merely retentions from an ancestral species.In addition to the issue of mosaic evolution in the wrist, the complexity within these structures is often ignored. The wrist contains several bones that work in concert during a variety of behaviors, including locomotion, foraging, and tool-making and use (Whitehead, 1993; Fleagle, 2013). Despite the strong anatomical and functional relationship among carpal bones, current studies investigating the evolution of carpal morphology often focus on single bones rather than the carpus as a cohesive unit (e.g., Tocheri et al., 2005; Kivell, 2009; Nakatsukasa et al., 2016; Ogihara et al., 2016). When bones of this region are considered in conjunction, they are often separated into two modules: a proximal and distal row (Richmond and Strait, 2000). However, modern humans have been hypothesized to have an “oval-ring” modular configuration based on the ligamentous connections between the scaphoid, lunate, capitate, and triquetrum. Furthermore, primates—particularly modern humans—differ in their patterns of wrist movement (e.g., Craigen and Stanley, 1995; Williams, 2010; Kivell et al., 2013). This variation may indicate that there may not be a single set of functional modules across primates, which may have implications for form and function of the wrist in fossil apes. It is therefore possible that these widely used functional groupings ignore the morphological and functional complexity of the wrist, as well as the phylogenetic and locomotor signals that may be driving the relationships among the bones of the wrist. To better understand the evolutionary trajectory and ancestral condition of functional adaptations of the forelimb and more accurately infer locomotion from functional morphology in fossil apes, an improved understanding of the evolutionary relationships between and within the primate carpus is needed. To interpret the functional complexity of the wrist, we can investigate trends of integration and modularity between structures in the wrists of extant and fossil primates (Young et al., 2010; Klingenberg et al., 2012; Kivell et al., 2013; Klingenberg and Marugán-Lobón, 2013; Klingenberg, 2014). Integration refers to the covariation of traits that results from developmental and evolutionary processes that produce morphology, whereas modularity pertains to units in a complex system that have high levels of trait integration within the unit, but do not covary strongly with other units (Klingenberg, 2008). Thus, trait covariation is a means by which we can evaluate integration between structures and define functional modules in the wrist. Analyses further evaluating the covariational structure of the carpal bones will supplement our hypotheses reconstructing the locomotor behavior of Miocene apes such as Ekembo and Pierolapithecus, and early hominins such as Australopithecus (Dart, 1925; Napier and David, 1959; Stern and Susman, 1983; Latimer and Lovejoy, 1989; Ward, 1993; Moyà-Solà et al., 2004; Berger et al., 2010), which will help us understand the ancestral conditions and predecessors to bipedalism. However, until now, no one has studied the complexity and modular nature of these structures using contemporary morphometric methods. Therefore, this project aims to develop our understanding of morphology in the wrists of fossil apes by establishing patterns of integration and modularity among hominoid primates with different habitual locomotor repertoires.

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