Allometry and adaptation in the catarrhine postcranial skeleton

Forelimb segment length proportions in extant hominoids and Australopithecus afarensis

Forelimb proportions have been used to infer locomotor adaptation in Australopithecus afarensis. However, little is known about proportions among individual forelimb segments in extant or fossil hominoids. The partial A. afarensis skeleton A.L. 438-1 and the more complete skeleton A.L. 288-1 provide the opportunity to assess relative length of the arm, forearm, wrist, and palm. We compare scaling relationships between pairs of forelimb bones of extant hominoids and A. afarensis, and length of individual forelimb elements to a body size surrogate. Hylobatids, and to a lesser extent orangutans, have the longest forelimb bones relative to size, although the carpus varies little among taxa, perhaps due to functional constraints of the wrist. Pan species are unique in having long metacarpals relative to ulnar length, demonstrating that they probably differ from the common chimp-human ancestor, and also that developmental mechanisms can be altered to results in differential growth of individual forelimb segments. A. afarensis has no forelimb bones that are significantly longer than those of humans for its size. It falls within the range of variation seen in modern humans for all comparisons relative to size, but appears to differ from the typical human brachial index due to a slightly shorter humerus and/or slightly longer ulna. It has short metacarpals like humans only among hominoids. Thus, while Pan may have elongated its metacarpus relative to ulnar length, A. afarensis may have reduced the length of its metacarpals and possibly its humerus relative to body size from the primitive condition.

Functional anatomy of the olecranon process in hominoids and plio-pleistocene hominins

This study examines the functional morphology of the olecranon process in hominoids and fossil hominins. The length of the bony lever of the triceps brachii muscle (TBM) is measured as the distance between the trochlear articular center and the most distant insertion site of the TBM, and olecranon orientation is measured as the angle that this bony lever makes with the long axis of the ulna. Results show that Homo, Pan, Gorilla, most monkeys, and the Australopithecus fossils studied have similar relative olecranon lengths. Suspensory hominoids and Ateles have shorter olecranons, suggesting, in some instances, selection for greater speed in extension. The orientation that the lever arm of the TBM makes with the long axis of the ulna varies with preferred locomotor mode. Terrestrial primates have olecranons that are more posteriorly oriented as body size increases, fitting general models of terrestrial mammalian posture. Arboreal quadrupeds have more proximally oriented lever arms than any terrestrial quadrupeds, which suggests use of the TBM with the elbow in a more flexed position. Olecranon orientation is not consistent in suspensory hominoids, although they are all characterized by orientations that are either similar or more posterior than those observed in quadrupeds. Homo and the fossils have olecranons that are clearly more proximally oriented than expected for a quadruped of their size. This suggests that Homo and Australopithecus used their TBM in a flexed position, a position most consistent with manipulatory activities.

Torso morphology and locomotion in Proconsul nyanzae

A new partial skeleton of Proconsul nyanzae from Mfangano Island, Kenya (KNM-MW 13142) includes five lumbar vertebrae, a partial sacrum, and nearly complete hipbone. Until recently, little was known of the axial and pelvic skeletons of Proconsul, or any early Miocene hominoid. Because torso morphology directly reflects locomotor behavior, these elements provide important new information about posture and locomotion of P. nyanzae. Two basic patterns of locomotor behavior and corresponding torso morphology exist among extant anthropoids. Monkeys have long, flexible spines with powerful epaxial muscles and mediolaterally narrow thoraces. This morphology represents a response to the mechanical requirements of habitual pronograde quadrupedalism. In contrast, hominoids, particularly great apes, have shorter, stiffer spines and broader torsos than do monkeys. The hominoid pattern reflects an emphasis on forelimb abduction-adduction, and on stability and control of the lower back during forelimb-dominated arboreal locomotion. KNM-MW 13142 had craniocaudally elongate vertebral bodies, and probably six lumbar vertebrae, reflecting a long, flexible vertebral column. The narrow, laterally facing iliac blades of KNM-MW 13142 imply the presence of a mediolaterally narrow, dorsoventrally deep thorax. Its wide iliac tuberosities, ventrally located lumbar transverse processes, and distinct accessory processes suggest that Proconsul nyanzae also had strap-like epaxial muscles similar to those characterizing most monkeys. Axial and pelvic morphology of KNM-MW 13142 indicate that P. nyanzae had not undergone an ape-like rearrangement of its torso to adapt to forelimb-dominated arboreality. Instead, P. nyanzae probably retained a more primitive pattern of pronograde quadrupedalism. This morphology indicates that the extant hominoid pattern of torso anatomy arose subsequent to the establishment of the hominid clade.

Allometry between length and cross-sectional dimensions of the femur and tibia in Homo sapiens sapiens

Allometric equations relating length and cross-sectional geometric properties of the femur and tibia are generated using skeletal remains from three recent human population samples. Approximate isometry, or geometric similarity, is found both within and between samples. Cross-sectional areas scale to approximately length2, while second moments of area scale to approximately length4. It is shown that this is consistent with the maintenance of equivalent mechanical stress in long bones of different length under dynamic loadings in vivo. Other evidence indicates that bending and torsional loadings are more critical than axial loadings in the determination of lower limb bone cross-sectional dimensions.

Limb bone allometry in primates: a test of the elastic similarity model

Morphometric data on postcranial elements of ten species of Old World primates are used to test the elastic similarity model of limb scaling. The data are divided into three groupings to exclude as much variation due to differences in locomotor adaptation as possible. The data on skeletal diameters fits the model reasonably well. The length results, however, are generally not in conformity with the model's predictions.