Functional morphology of Proconsul patellas from Rusinga Island, Kenya, with implications for other Miocene-Pliocene catarrhines

Four new, nearly complete patellas of Proconsul heseloni and two of P. nyanzae have been discovered on Rusinga Island, Kenya. Until recently, little was known of the Proconsul knee joint because of the fragmentary and distorted nature of the available remains. These new patellas provide an opportunity to assess knee joint structure and function in Proconsul . Fossil patellas are also known from Kenyapithecus, Oreopithecus and Pliopithecus, providing an important comparative database of fossil hominoids with which to compare the Proconsul patellas. Two mechanical models that relate external patellar dimensions to stresses developed in the patella under mechanical loading are presented to provide a theoretical framework by which to assess functional significance of variations in patellar morphology among anthropoids. Patellas are modelled as (i) short beams subjected to three-point bending, and (ii) as pulley like structures. Predicted scaling of patellar dimensions based on these models are tested by studying a sample of extant anthropoid patellas. Four external patellar dimensions are included: proximodistal height (PD), mediolateral breadth (ML), anteroposterior thickness (AP) and proximodistal height of the articular surface (PDAS). Allometric relationships among these patellar dimensions are studied by fitting reduced major axis lines through log-transformed data, and comparing lines fitted through monkey and great ape data, respectively. Monkey patellas are taller (PD) and thicker (AP) than great ape patellas, and may have slightly taller articular surfaces (PDAS) relative to breadth (ML), reflecting an emphasis on running and leaping in their locomotion. In contrast, all fossil and extant hominoids, including Proconsul, have flat, broad, and thin patellas, reflecting a generalized pattern of knee joint function that is probably primitive for the hominoid clade.