Brief communication: Paleobiological inferences on the locomotor repertoire of extinct hominoids based on femoral neck cortical thickness: The fossil great ape hispanopithecus laietanus as a test‐case study

Abstract

The relationship between femoral neck superior and inferior cortical thickness in primates is related to locomotor behavior. This relationship has been employed to infer bipedalism in fossil hominins, although bipeds share the same pattern of generalized quadrupeds, where the superior cortex is thinner than the inferior one. In contrast, knuckle-walkers and specialized suspensory taxa display a more homogeneous distribution of cortical bone. These different patterns, probably related to the range of movement at the hip joint and concomitant differences in the load stresses at the femoral neck, are very promising for making locomotor inferences in extinct primates. To evaluate the utility of this feature in the fossil record, we relied on computed tomography applied to the femur of the Late Miocene hominoid Hispanopithecus laietanus as a test-case study. Both an orthograde body plan and orang-like suspensory adaptations had been previously documented for this taxon on different anatomical grounds, leading to the hypothesis that this fossil ape should display a modern ape-like distribution of femoral neck cortical thickness. This is confirmed by the results of this study, leading to the conclusion that Hispanopithecus represents the oldest evidence of a homogeneous cortical bone distribution in the hominoid fossil record. Our results therefore strengthen the utility of femoral neck cortical thickness for making paleobiological inferences on the locomotor repertoire of fossil primates. This feature would be particularly useful for assessing the degree of orthograde arboreal locomotor behaviors vs. terrestrial bipedalism in putative early hominins.

The femur of Orrorin tugenensis exhibits morphometric affinities with both Miocene apes and later hominins

“…Rose stated: 'the fact of the ubiquity of quadrupedalism as a major locomotor mode in Miocene catarrhines in general, and Miocene hominoids in particular, is an inevitable starting point for any attempt to reach a full understanding of the emergence of the suspensory, quadrumanous climbing, quadrupedal-climbing-suspensory and bipedal specializations of extant hominoids' 54 . This agrees with claims that the mechanical requirements imposed by quadrupedalism and bipedalism are more similar to each other than to those related to suspension 37,55 . Our results therefore support a modified version of the evolutionary scenario that early bipeds might have evolved from a more generalized arboreal, largely orthograde (but not specifically suspensory) Miocene ape 11,14,15,25,45 , with a more plesiomorphic postcranium than that exhibited by any of the extant African apes 8,16 .…”

“…If this is indeed the case, our results indicate an adaptive shift towards enhanced mobility at the hip joint in hominoids relative to non-hominoid anthropoids that was already present by the early Miocene (as evidenced by Proconsul). These results expand previous analyses indicating significantly larger heads relative to neck in great apes as compared with Macaca 31 , as well as inferences derived from femoral morphology in Miocene hominoids 19,[36][37][38][39] . Conversely, the primitive pelvis of Proconsul 19 suggests mosaic evolution across both sides of the hominoid hip joint.…”

The Middle Miocene Ape Pierolapithecus catalaunicus Exhibits Extant Great Ape-Like Morphometric Affinities on Its Patella: Inferences on Knee Function and Evolution

“…However, the evidence provided by Pierolapithecus [1], [2], [4], [8], [13] suggests that the acquisition of suspensory adaptations might have been decoupled from that of vertical climbing (contra [11], [14], [15])—with clear suspensory adaptations not being displayed until the late Miocene by Hispanopithecus / Rudapithecus (see discussion in [59], but also [13]–[15], [17], [52], [60]–[62]). Concerning the latter taxa, the below-branch suspensory adaptations observed on their femora [52], [60], [61], [63], [64] and other postcranial remains [13]–[15], [17], [60]–[62] lead us to predict, based on our analyses above, that the patella of Hispanopithecus (if ever found) would probably resemble those of modern great apes, like in Pierolapithecus and Oreopithecus .…”

The primate fossil record in the Iberian Peninsula

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