Abstract
Investigating the morphological differences of the calcaneus in humans and great apes is crucial for reconstructing locomotor repertories of fossil hominins. However, morphological variations in the calcaneus of the great apes (chimpanzees, gorillas, and orangutans) have not been sufficiently studied. This study aims to clarify variations in calcaneal morphology among great apes based on three-dimensional geometric morphometrics. A total of 556 landmarks and semilandmarks were placed on the calcaneal surface to calculate the principal components of shape variations among specimens. Clear interspecific differences in calcaneal morphology were extracted, corresponding to the degree of arboreality of the three species. The most arboreal orangutans possessed comparatively more slender calcaneal tuberosity and deeper pivot region of the cuboid articular surface than chimpanzees and gorillas. However, the most terrestrial gorillas exhibited longer lever arm of the triceps surae muscle, larger peroneal trochlea, more concave plantar surface, more inverted calcaneal tuberosity, more everted cuboid articular surface, and more prominent plantar process than the orangutans and chimpanzees. These interspecific differences possibly reflect the functional adaptation of the calcaneus to locomotor behavior in great apes. Such information might be useful for inferring foot functions and reconstructing the locomotion of fossil hominoids and hominids.
Highlights
Investigating the morphological differences of the calcaneus in humans and great apes is crucial for reconstructing locomotor repertories of fossil hominins
We investigated whether the differences in the calcaneal morphology, the differences in the shapes of the calcaneal tuberosity and the articular surfaces of the calcaneus among great apes possibly corresponded to the differences in their locomotor behaviors and degree of arboreality
The cuboid articular surface in humans was observed to be more twisted in the inverting direction (Fig. 3c) and faced more medially than those in great apes (Fig. 3a)
Summary
Investigating the morphological differences of the calcaneus in humans and great apes is crucial for reconstructing locomotor repertories of fossil hominins. Previous studies noted that: (1) the human calcaneus is more robust, mediolaterally wider, and longitudinally straighter than that of the African apes[1,2,10]; (2) the human calcaneus possesses a large, robust tuberosity and plantarly located lateral plantar process[2,11]; (3) the human calcaneus possesses a peroneal trochlea that is smaller than that of the African apes[12,13]; (4) the calcaneocuboid articular surface is more acutely angled with respect to the longitudinal axis of the calcaneus in humans than in African apes in lateral view due to the longitudinally arched structure of the human foot[14]; (5) the calcaneocuboid articular surface is more flat and asymmetrical, constricting rotatory movement at the joint in humans than in African apes[15,16]; and (6) the posterior talar facet is flatter in the human calcaneus than in that of the African apes, reflecting more constricted subtalar joint in humans[17] These morphological differences distinguishing the human calcaneus from those of African apes are considered as morphological adaptations to obligatory bipedal locomotion, and are used to reconstruct the locomotor repertories of early hominins to understand the origin and evolution of habitual bipedal locomotion in the human lineage[1,2,11,17,18]. We investigated whether the differences in the calcaneal morphology, the differences in the shapes of the calcaneal tuberosity and the articular surfaces of the calcaneus among great apes possibly corresponded to the differences in their locomotor behaviors and degree of arboreality
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