Abstract:

Phalangeal curvature of primates is an important indicator of arboreality and suspensory locomotion. Fourth order polynomial curve fitting on geometric morphometric landmark data (GM-PCF) provides a more precise and accurate quantitative measure of phalangeal curvature, namely normalized phalangeal curve height (NPCH), which controls for the impact of size. Coupled with phalangeal curve length (PCL), NPCH maps primate phalangeal curvature to locomotor modes more accurately. Furthermore, data on phalangeal curvature derived from a sample of 15 extant anthropoid primate taxa comprising 328 individuals and more than 5000 phalangeal specimens can be used to reconstruct the locomotor behaviors of fossil primates. In this study, the postcranial morphological adaptations and locomotor behaviors of fossil hominoids with complete II-V proximal phalanges (pedal and manual) are inferred using GM-PCF analysis of phalangeal curvature. The aim is to provide new information that can contribute to a more complete understanding of the evolution of the locomotor behavior of fossil hominoids. The results indicate that generally there are four stages in the evolution of hominoid locomotor behavior, including the generalized arboreal quadrupedalism stage of basal hominoids, the arboreal suspension stage of early hominids, the commencement of bipedalism with retention of suspension ability stage of early hominins and australopiths, and the bipedalism stage of the genus Homo. The adaptation of climbing and suspension does not follow a simple linear mode, but develops in a mosaic pattern, and occurs in different lineages of hominoids through different pathways, even multiple times until it completely disappears in the end. The manual phalangeal curvature comparable to modern humans already occurred in OH 86 from the >1.84 MaBP deposits of Olduvai, whereas the nearly contemporary Paranthropus robustus from South Africa still retained more curved manual and pedal phalanges. Homo naledi also has extraordinarily curved manual phalanges. Nevertheless, locomotor behavior needs the coordination of the whole body. The phalangeal curvature is just one line of evidence of functional morphology. When reconstructing the locomotor behavior of a certain fossil hominoid taxon, it is necessary to take not only the functional morphological feature of the whole body into consideration, but also the paleo-ecological factors. Another intriguing finding of this research is that, in hominoids, the ratio of the curve length of manual to pedal proximal phalanges is indicative of obligate or facultative bipedalism when it is larger than 1.3. In general, the more quadrupedal primates tend to have a ratio that is closer to 1, which means that their manual and pedal proximal phalanges have a similar curve length. However, Pongo is an exception, because it has a ratio of 1.03 in spite of being highly suspensory. Hylobatids and great apes other than Pongo all have a ratio larger than 1.3 and all engage in obligate or facultative bipedalism. If this is the case, the early hominin Ardipithecus ramidus, the australopiths Australopithecus afarensis and Australopithecus africanus, and Homo floresiensis should also have engaged in certain kinds of bipedalism.

Key words: hominoids, phalangeal curvature, locomotor behavior, functional morphology, GM-PCF, bipedality