Mechanisms for the acquisition of habitual bipedality: are there biomechanical reasons for the acquisition of upright bipedal posture?

Morphology and biomechanics are linked by causal morphogenesis (‘Wolff's law’) and the interplay of mutations and selection (Darwin's ‘survival of the fittest’). Thus shape-based selective pressures can be determined. In both cases we need to know which biomechanical factors lead to skeletal adaptation, and which ones exert selective pressures on body shape. Each bone must be able to sustain the greatest regularly occurring loads. Smaller loads are unlikely to lead to adaptation of morphology. The highest loads occur primarily in posture and locomotion, simply because of the effect of body weight (or its multiple). In the skull, however, it is biting and chewing that result in the greatest loads. Body shape adapted for an arboreal lifestyle also smooths the way towards bipedality. Hindlimb dominance, length of the limbs in relation to the axial skeleton, grasping hands and feet, mass distribution (especially of the limb segments), thoracic shape, rib curvatures, and the position of the centre of gravity are the adaptations to arboreality that also pre-adapt for bipedality. Five divergent locomotor/morphological types have evolved from this base: arm-swinging in gibbons, forelimb-dominated slow climbing in orang-utans, quadrupedalism/climbing in the African apes, an unknown mix of climbing and bipedal walking in australopithecines, and the remarkably endurant bipedal walking of humans. All other apes are also facultative bipeds, but it is the biomechanical characteristics of bipedalism in orang-utans, the most arboreal great ape, which is closest to that in humans. If not evolutionary accident, what selective factor can explain why two forms adopted bipedality? Most authors tend to connect bipedal locomotion with some aspect of progressively increasing distance between trees because of climatic changes. More precise factors, in accordance with biomechanical requirements, include stone-throwing, thermoregulation or wading in shallow water. Once bipedality has been acquired, development of typical human morphology can readily be explained as adaptations for energy saving over long distances. A paper in this volume shows that load-carrying ability was enhanced from australopithecines to Homo ergaster (early African H. erectus), supporting an earlier proposition that load-carrying was an essential factor in human evolution

Torsion and bending in the neck and tail of sauropod dinosaurs and the function of cervical ribs: insights from functional morphology and biomechanics.

The long necks of sauropods have been subject to many studies regarding their posture and flexibility. Length of the neck varies among groups. Here, we investigate neck posture and morphology in several clades from a mechanical viewpoint. Emphasis is put on comparing sauropod necks and tails with structures in living archosaurs and mammals. Differences in the use made of necks and tails lead to clear-cut differences in the mechanical loads occurring in the same models. Ways of sustaining loads are identified by theoretical considerations. If the observed skeletal structures are suited to resist the estimated loading in a particular posture, this concordance is taken as an argument that this posture or movement was of importance during the life of the individual. Apart from the often-discussed bending in side view, we analyze the often overlooked torsion. Because torsional stresses in a homogenous element concentrate near the periphery, a cylindrical cross section gives greatest strength, and the direction of forces is oblique. In a vertebrate neck, during e.g. shaking the head and twisting the neck, oblique muscles, like the mm. scaleni, if activated unilaterally initiate movement, counterbalance the torsional moments and keep the joints between neck vertebrae in equilibrium. If activated bilaterally, these muscles keep the neck balanced in an energy-saving upright posture. The tendons of the mm. scaleni may have ossified as cervical ribs The long cervical ribs in brachiosaurids and mamenchisaurids seem to have limited flexibility, whereas the shorter cervical ribs in Diplodocidae allowed free movement. The tails of sauropods do not show pronounced adaptation to torsion, and seem to have been carried more or less in a horizontal, extended posture. In this respect, sauropod tails resemble the necks of herbivorous cursorial mammals. These analyses provide an improved understanding of neck use that will be extended to other sauropods in subsequent studies

Deducing the body posture of extinct large vertebrates from the shape of the vertebral column

Volume: 39Start Page: 801End Page: 81

From sprawling to parasagittal locomotion in Therapsida: A preliminary study of historically collected museum specimens

Therapsids covered the entire spectrum of terrestrial locomotion from sprawling to parasagittal. Switching between sprawling and more erect locomotion may have been possible in earlier taxa. First, the axial skeleton shows little regionalization and allows lateral undulation, evolving then increasingly towards regionalization enabling dorsoventral swinging. During terrestrial locomotion, every step invokes a ground reaction force and functional loadings which the musculoskeletal system needs to accomodate. First insights into the functional loading regime of the fore- and hindlimb skeleton and the body stem of therapsids presented herein are based on the assessment and preliminary measurements of the historical collection of therapsids exhibited in the Paleontological Collection of Eberhard Karls Universität Tübingen, Germany. The specimens included are the archosaur Hyperodapedon sanjuanensis, the early synapsid Dimetrodon limbatus for comparison, and the therapsids Keratocephalus moloch, Sauroctonus parringtoni, Tetragonias njalilus, and Belesodon magnificus. The vertebral columns and ribs of the mounts were carefully assessed for original fossil material and, when preserved, ribs, sacral, and anterior caudal vertebrae were measured. The body of a tetrapod is exposed to forces as well as bending and torsional moments. To resist these functional stresses, certain musculoskeletal specializations evolved. These include: 1) compression resistant plate-like pectoral and pelvic girdle bones, 2) a vertebral column combined with tendinous and muscular structures to withstand compressive and tensile forces and moments, and 3) ribs and intercostal muscles to resist the transverse forces and torsional moments. The legs are compressive stress-resistant, carry the body weight, and support the body against gravity. Tail reduction leads to restructuring of the musculoskeletal system of the pelvic girdle

Proximal part of a schematic neck seen from on top illustrates flexibility.

<p>A) Elongation of the segments (cervical vertebrae) makes the radius of curvature longer. Note that the cervical ribs do not contact vertebrae because they deviate ventrally from the axes of the centra. B) The segments (cervical vertebrae) 12 – 18 are deflected by 20° each. This corresponds to the lateral deflection observed by Dzemski (2006) in the ostrich. In addition, (long) cervical ribs are shown on both sides of the vertebrae. C) Shortening of the segments leads to a sharper curvature of the neck.</p

Influence of neck flexibility on the feeding envelope.

<p>A) A flexible neck with limited excursion angles allows harvesting of a sector (yellow) of the theoretically possible entire feeding envelope. B) Free excursions at the basis of an otherwise stiff and inflexible neck give access to only a peripheral part of the entire potential feeding envelope. C) Long-necked Canadian geese can and do flex their necks freely. In relaxed resting as well as in watching positions, the necks are kept upright. Both neck positions keep energy requirements low. While feeding, birds usually reduce the bending moments acting along their necks by assuming a sigmoid neck posture: near the trunk the usual downward convexity, near the head a convexity directed upward. These curvatures of the neck reduce the lever lengths, specifically the distances between the neck base and the segment weights contained in the neck. Abbreviations: b  =  forelimb length, n  =  neck length, d  =  the distance c overed during a given time. All these values are of the same size in A and B.</p