Abstract
Anatomical network analysis is a framework for quantitatively characterizing the topological organization of anatomical structures, thus providing a way to compare structural integration and modularity among species. Here we apply this approach to study the macroevolution of the forelimb in primates, a structure whose proportions and functions vary widely within this group. We analyzed musculoskeletal network models in 22 genera, including members of all major extant primate groups and three outgroup taxa, after an extensive literature survey and dissections. The modules of the proximal limb are largely similar among taxa, but those of the distal limb show substantial variation. Some network parameters are similar within phylogenetic groups (e.g., non-primates, strepsirrhines, New World monkeys, and hominoids). Reorganization of the modules in the hominoid hand compared to other primates may relate to functional changes such as coordination of individual digit movements, increased pronation/supination, and knuckle-walking. Surprisingly, humans are one of the few taxa we studied in which the thumb musculoskeletal structures do not form an independent anatomical module. This difference may be caused by the loss in humans of some intrinsic muscles associated with the digits or the acquisition of additional muscles that integrate the thumb more closely with surrounding structures.
Highlights
Anatomical network analysis describes the organization of topological connections between anatomical structures, yielding quantitative parameters that can be used as proxies for anatomical features such as complexity and integration
We used anatomical network analysis to obtain novel information about evolutionary changes in complexity, anisomerism, modularity, and integration in the upper limb of primates
Summary
Specimens and data collection.We included in our analysis members of all major extant primate groups, as well as of the three living groups more closely related to Primates: Glires (including rodents such as Mus), Dermoptera (or colugos, represented by Cynocephalus) and Scandentia (or tree shrews, represented by Tupaia). The musculoskeletal data for primates and these three taxa was taken from dataset provided by Diogo and Wood[20], which was based on their own dissections and an extensive review of the literature; multiple specimens were represented in this dataset, and the most common configuration reported for each taxon was coded in the network matrices. In defining nodes and links (see Fig. 1), we chose to include only bones and muscles - not ligaments - to make our results comparable to previous studies[15]. Because of this methodological choice, our networks do not include connections such as the ulnar collateral ligament between the ulna and triquetrum and pisiform in hominoids, which plays an important role in wrist
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