Does skeletal anatomy reflect adaptation to locomotor patterns? cortical and trabecular architecture in human and nonhuman anthropoids

Although the correspondence between habitual activity and diaphyseal cortical bone morphology has been demonstrated for the fore- and hind-limb long bones of primates, the relationship between trabecular bone architecture and locomotor behavior is less certain. If sub-articular trabecular and diaphyseal cortical bone morphology reflects locomotor patterns, this correspondence would be a valuable tool with which to interpret morphological variation in the skeletal and fossil record. To assess this relationship, high-resolution computed tomography images from both the humeral and femoral head and midshaft of 112 individuals from eight anthropoid genera (Alouatta, Homo, Macaca, Pan, Papio, Pongo, Trachypithecus, and Symphalangus) were analyzed. Within-bone (sub-articular trabeculae vs. mid-diaphysis), between-bone (forelimb vs. hind limb), and among-taxa relative distributions (femoral:humeral) were compared. Three conclusions are evident: (1) Correlations exists between humeral head sub-articular trabecular bone architecture and mid-humerus diaphyseal bone properties; this was not the case in the femur. (2) In contrast to comparisons of inter-limb diaphyseal bone robusticity, among all species femoral head trabecular bone architecture is significantly more substantial (i.e., higher values for mechanically relevant trabecular bone architectural features) than humeral head trabecular bone architecture. (3) Interspecific comparisons of femoral morphology relative to humeral morphology reveal an osteological "locomotor signal" indicative of differential use of the forelimb and hind limb within mid-diaphysis cortical bone geometry, but not within sub-articular trabecular bone architecture.     

Articular structure and function in Hylobates,Colobus, and Papio

It has been demonstrated in clinical and experimental studies that subarticular trabecular bone responds to mechanical loads transmitted across joints through changes in mass and structural organization. We investigated differences in mass, volume, and density of subarticular trabecular bone of the humeral and femoral head in Hylobates syndactylus, Colobus guereza, and Papio cynocephalus. Our hypothesis was that variations in trabecular properties between taxa may reflect differences in mechanical loading associated with different locomotor repertoires. A nondestructive method for measuring trabecular properties using optical luminance data measured from radiographs was developed. We also examined the relationship between internal trabecular properties and the external size and surface area of the humeral and femoral heads in these taxa. Our results suggest that internal and external articular structure are relatively independent of each other and may be adapted to different aspects of the mechanical environment. Differences in trabecular mass between taxa appear to correspond to differences in the magnitudes of mechanical loads borne by the joint, whereas aritcular volume and surface area are related primarily to differences in joint mobility. Because of the apparent physiological “de-coupling” of articular mass and volume, variations in articular density (mass/volume) are difficult to interpret in isolation. Comparisons of internal and external articular structure may provide new ways to reconstruct the locomotor/positional behavior of extinct taxa. © 1994 Wiley-Liss, Inc.     

Femoral head trabecular bone structure in two omomyid primates

The study of the three-dimensional structure of trabecular bone and its relationship to locomotor behavioral differences across different primate taxa provides a potentially useful analytic tool for reconstructing the behavior of extinct taxa. The purpose of the current study is to quantify the three-dimensional architecture of trabecular bone in the femoral head of Omomys carteri and Shoshonius cooperi and to compare this structure to that of several extant strepsirrhine taxa. Bone volume fraction (BV/TV) and fabric anisotropy were quantified in three dimensions using serial high-resolution X-ray computed tomography scan data collected from one femoral head from each fossil taxon. Three cubic volumes of interest (VOI) were identified within the femoral head. The BV/TV was quantified by assessing the percentage of bone voxels within each VOI and the structural anisotropy was quantified using the star volume distribution method. The Omomys femur used here has a high BV/TV with the galagine-like pattern of decreasing BV/TV from the superior to the inferior half of the femoral head. The fabric structure, however, is more lorisine-like in being relatively isotropic throughout the femoral head. The trabecular structure in Omomys is unique in its mix of features and appears to be most similar overall to the lorisines, suggesting that Omomys engaged in a quadrupedal mode of locomotion. By contrast the Shoshonius specimen possesses a relatively uniform BV/TV across the head but displays the distinctly galagine-like pattern of increasing anisotropy moving inferiorly in the femoral head. Taken as a whole, the trabecular structure in Shoshonius appears to be most like that of the galagines and is consistent with that of either an occasional leaper-quadruped or a specialized leaper. Despite the overall similarities in the external postcranial anatomy of Omomys and Shoshonius, the results of this study indicate potentially important differences in the magnitude and orientation of the external loads at the hip joint, suggesting that these animals engaged in divergent locomotor behaviors.     

The three-dimensional structure of trabecular bone in the femoral head of strepsirrhine primates

It has been hypothesized for over a hundred years that trabecular bone plays an important structural role in the musculoskeletal system of animals and that it responds dynamically to applied loads through growth. The objectives of this study are to quantify the three-dimensional structure of femoral head trabecular bone in a sample of extant strepsirrhines and to relate patterns of interspecific variation to locomotor behavioral differences. The bone volume fraction (BV/TV) and fabric anisotropy of trabecular bone in the femoral heads of Cheirogaleus major, Avahi laniger, Galago senegalensis, Galago alleni, Loris tardigradus, Otolemur crassicaudatus, and Perodicticus potto were quantified in three dimensions using serial high-resolution X-ray computed tomography scan data. A volume based method was used to quantify the structural anisotropy in three cubic samples located inside the central portion of the femoral head. Significant structural differences were found between the predominantly leaping galagines and indriids and the nonleaping lorisines and cheirogaleids. The leapers in general have relatively anisotropic trabecular bone. The galagines display a unique pattern of decreasing bone volume and increasing anisotropy moving from the superior to the inferior half of the femoral head. By contrast, the nonleaping taxa possess relatively uniform and isotropic bone throughout the femoral head. The differences in femoral head trabecular structure among these taxa seem to be related to locomotor behavioral differences, reflecting variation in the use and loading of the hip joint during normal locomotion.     

Angular orientation of trabecular bone in the femoral head and its relationship to hip joint loads in leaping primates

The elastic properties and mechanical behavior of trabecular bone are largely determined by its three-dimensional (3D) fabric structure. Recent work demonstrating a correlation between the primary mechanical and material axes in trabecular bone specimens suggests that fabric orientation may be used to infer directional components of the material strength and, by extension, the hypothetical loading regime. Here we quantify the principal orientation of trabecular bone in the femoral head and relate these principal fabric directions to loading patterns during various locomotor behaviors. The proximal femora of a diverse sample of prosimians were scanned using a high-resolution X-ray computed tomography scanner with resolution of better than 50 mum. Spherical volumes of interest were defined within the femoral heads and the 3D fabric anisotropy was calculated using the mean intercept length and star volume distribution methods. In addition to differences in bone volume and anisotropy, significant differences were found in the spatial orientation of the principal trabecular axes depending on locomotor behavior. The principal orientations for leapers (Galago, Tarsius, Avahi) are relatively tightly clustered (alpha(95) confidence limit: 8.2; angular variance s: 18.2 degrees ) and oriented in a superoanterior direction, while those of nonleapers are more variable across a range of directions (alpha(95): 16.8; s: 42.0 degrees ). The mean principal directions are significantly different for leaping vs. nonleaping taxa. These results further suggest a relationship between bone microstructure in the hip joint and locomotor behavior and indicate a similarity of loading across leapers despite differences in kinematics and phylogeny.     

A comparison of proximal humeral cancellous bone of great apes and humans

The shoulder is the most mobile joint in the primate body, and is involved in both locomotor and manipulative activities. The presumed functional sensibility of trabecular bone can offer a way of decoding the activities to which the forelimbs of fossil primates were subjected. We examine the proximal humeral trabecular architecture in a relatively closely related group of similarly sized hominids (Pongo pygmaeus, Pan troglodytes, and Homo sapiens), in order to evaluate the effect of diverging habitual motion behaviors of the shoulder complex in a coherent phylogenetic group. In order to characterize and compare the humeral trabecular architectures of the three species, we imaged a large sample by high-resolution computed tomography (HrCT) and quantified their trabecular architectures by standard bone 3D morphometric parameters. Univariate statistical analysis was performed, showing significant differences among the species. However, univariate statistics could not highlight the structural particularity in the cancellous bone of each species. A principal component analysis also showed clear separation of the three taxa and enabled a structural characterization of the humeral trabecular bone of each species. We conclude that the differences in the architectural setup of the three hominids likely reflect multiple differences in their habitual activity patterns of their shoulder joint, although individual structural features are difficult to relate to specific loading conditions.     

A comparative study of the trabecular bony architecture of the talus in humans, non-human primates, and Australopithecus

This study tested the hypothesis that talar trabecular microarchitecture reflects the loading patterns in the primate ankle joint, to determine whether talar trabecular morphology might be useful for inferring locomotor behavior in fossil hominins. Trabecular microarchitecture was quantified in the anteromedial, anterolateral, posteromedial, and posterolateral quadrants of the talar body in humans and non-human primates using micro-computed tomography. Trabecular bone parameters, including bone volume fraction, trabecular number and thickness, and degree of anisotropy differed between primates, but not in a manner entirely consistent with hypotheses derived from locomotor kinematics. Humans have highly organized trabecular struts across the entirety of the talus, consistent with the compressive loads incurred during bipedal walking. Chimpanzees possess a high bone volume fraction, consisting of plate-like trabecular struts. Orangutan tali are filled with a high number of thin, connected trabeculae, particularly in the anterior portion of the talus. Gorillas and baboons have strikingly similar internal architecture of the talus. Intraspecific analyses revealed no regional differences in trabecular architecture unique to bipedal humans. Of the 22 statistically significant regional differences in the human talus, all can also be found in other primates. Trabecular thickness, number, spacing, and connectivity density had the same regional relationship in the talus of humans, chimpanzees, gorillas, and baboons, suggesting a deeply conserved architecture in the primate talus. Australopithecus tali are human-like in most respects, differing most notably in having more oriented struts in the posteromedial quadrant of the body compared with the posterolateral quadrant. Though this result could mean that australopiths loaded their ankles in a unique manner during bipedal gait, the regional variation in degree of anisotropy was similar in humans, chimpanzees, and gorillas. These results collectively suggest that the microarchitecture of the talus does not simply reflect the loading environment, limiting its utility in reconstructing locomotion in fossil primates.     

Comparative forefoot trabecular bone architecture in extant hominids

The appearance of a forefoot push-off mechanism in the hominin lineage has been difficult to identify, partially because researchers disagree over the use of the external skeletal morphology to differentiate metatarsophalangeal joint functional differences in extant great apes and humans. In this study, we approach the problem by quantifying properties of internal bone architecture that may reflect different loading patterns in metatarsophalangeal joints in humans and great apes. High-resolution x-ray computed tomography data were collected for first and second metatarsal heads of Homo sapiens (n = 26), Pan paniscus (n = 17), Pan troglodytes (n = 19), Gorilla gorilla (n = 16), and Pongo pygmaeus (n = 20). Trabecular bone fabric structure was analyzed in three regions of each metatarsal head. While bone volume fraction did not significantly differentiate human and great ape trabecular bone structure, human metatarsal heads generally show significantly more anisotropic trabecular bone architectures, especially in the dorsal regions compared to the corresponding areas of the great ape metatarsal heads. The differences in anisotropy between humans and great apes support the hypothesis that trabecular architecture in the dorsal regions of the human metatarsals are indicative of a forefoot habitually used for propulsion during gait. This study provides a potential route for predicting forefoot function and gait in fossil hominins from metatarsal head trabecular bone architecture.     

Functional-adaptive anatomy of the forelimb in the Didelphidae, and the paleobiology of the Paleocene marsupials Mayulestes ferox and Pucadelphys andinus

An attempt to determine the locomotor activities of Mayulestes ferox (Borhyaenoidea) and Pucadelphys andinus (Didelphoidea) from the early Paleocene site of Tiupampa (Bolivia) is presented. The functional anatomy of the forelimbs of these South American marsupials is compared to that of some living didelphids: Caluromys philander, Micoureus demerarae, Marmosa murina, Didelphis marsupialis, Monodelphis brevicaudata and Metachirus nudicaudatus. Deductions from bone morphology to myology and locomotor behavior in the fossils are inferred from the comparisons with living forms. Some features of the postcranial skeleton, indicative of arboreal adaptations, are found in the extinct marsupials: anteriorly projected acromion, hemispherical head of the humerus, extended humeral lateral epicondylar ridge, medially protruding humeral entepicondyle, proximal ulnar posterior convexity, and deep flexor fossa on the medial side of the ulna. But other features are related to a more terrestrial pattern: the well-developed tubercles of the humeral head, the elongated olecranon process of the ulna, and the oval shape of the radial head. Mayulestes had clear arboreal abilities, but, as a predaceous mammal, probably hunted on the ground. Pucadelphys was less specialized, close to the living Monodelphis, a terrestrial insectivorous form with some skeletal features related to arboreal locomotion that are probably plesiomorphic for marsupials.     

Morphology and locomotor adaptations of the bovid femur in relation to habitat

Extant bovids inhabit a wide diversity of environments that range from forest to savanna and display locomotor patterns that are habitat specific. I report here on an investigation of the linkage between these locomotor patterns and habitat based on a study of the morphology of the bovid femur. Femoral head shape, shaft dimensions, and knee structure are examined and support a statistically significant separation of the different morphological complexes present in bovids from forest, broken cover, and savanna habitats. Morphological differences are primarily related to locomotor patterns as reflected in the degree of cursoriality displayed by bovids in different habitats. Cursorial bovids from savanna environments have laterally expanded femoral heads that act to limit the degree of abduction and axial rotation at the hip, and elliptically shaped distal femora that increase the moment arm of the extensor muscles that cross the knee. Forest bovids have spherically shaped femoral heads. This morphology permits a much higher degree of abduction and axial rotation at the hip and appears to provide greater maneuverability in a vegetationally complex habitat. Bovids living in broken cover environments that fall between the extremes of closed canopy forest and savanna display an intermediate set of femoral characters. This approach to the relationship between habitat and locomotion offers a potentially powerful means with which to examine the interplay between structural form and function in bovid evolution.     

Mechanical significance of femoral head trabecular bone structure in Loris and Galago evaluated using micromechanical finite element models

Work on the interspecific and intraspecific variation of trabecular bone in the proximal femur of primates demonstrates important architectural variation between animals with different locomotor behaviors. This variation is thought to be related to the processes of bone adaptation whereby bone structure is optimized to the mechanical environment. Micromechanical finite element models were created for the proximal femur of the leaping Galago senegalensis and the climbing and quadrupedal Loris tardigradus by converting bone voxels from high-resolution X-ray computed tomography scans of the femoral head to eight-noded brick elements. The resulting models had approximately 1.8 million elements each. Loading conditions representing takeoff phase of a leap and more generalized load orientations were applied to the models, and the models were solved using the iterative "row-by-row" matrix-vector multiplication algorithm. The principal strain and Von Mises stress results for the leaping model were similar for both species at each load orientation. Similar hip joint reaction forces in the range of 4.9 x to 12 x body weight were calculated for both species under each loading condition, but the hip reaction values estimated for Loris were higher than predicted based on locomotor behavior. These results suggest that functional adaptation to hip joint loading may not fully explain the differences in femoral head trabecular bone structure in Galago and Loris. The finite element method represents a unique and useful tool for analyzing the functional adaptation of trabecular bone in a diversity of animals and for reconstructing locomotor behavior in extinct taxa.     

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

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. Am J PhyAnthropol 2012. © Wiley Periodicals, Inc.     

Limb joint surface areas and their ratios in Malagasy lemurs and other mammals

Surface areas of humeral and femoral heads scale largely as a function of body size. However, differences in the relative sizes of these articular surfaces are correlated with differential joint mobility and force transmission through fore- and hindlimbs. They can therefore assist interpretation of the positional behavior of extinct species. In this paper, we document variation in ratios of humeral head surface area to femoral head surface area among extant primates and other mammals. We then examine a group of extinct primates: the subfossil lemurs of Madagascar. Many Malagasy le murs, including some giant extinct species with very long forelimbs and short hindlimbs, have relatively small humeral heads and large femoral heads. We explore the adaptive implications of this pattern. © 1995 Wiley-Liss, Inc.     

A comparison of the femoral head and neck trabecular architecture of Galago and Perodicticus using micro-computed tomography (microCT)

Innovations in micro-computed tomography (microCT) in the medical field have resulted in the development of techniques that allow the precise quantification of bone density and fabric related parameters of trabecular bone. For the purpose of this study, the technique was applied to a small sample of Perodicticus potto and Galago senegalensis femora to see if differences in loading environment elicit the predicted effects on trabecular structure. While the overall bone volume was approximately three times larger in the potto, there was no significant difference in the apparent volume density in the two taxa. When regional differences in the proximal femur were examined, the cancellous bone of the femoral head of Perodicticus potto and Galago senegalensis, while not differing in volume density, showed differences in trabecular orientation, with the potto having more randomly oriented trabeculae than the bushbaby. This was as hypothesized, given that the bushbaby submits its femora to more stereotypical loading environments than the potto. In the femoral neck, the cancellous bone was not only more randomly oriented, it was also denser in the potto compared with the bushbaby. This suggests that trabecular morphology may be extremely sensitive to certain differences in the loading environment and that this information, combined with information on cortical bone structure and external geometry, will result in a more complete understanding of how bone shape and composition correspond to loading and locomotor patterns. Ultimately, a synthesis of these different lines of evidence may have considerable applications in paleontological studies that attempt to reconstruct bone use from morphology.     

Postcranial adaptations for climbing in Loridae (Primates)

The Loridae are an arboreal family of small primates that are specialized for slow and quiet climbing. This paper examines the relationship between lorid locomotory behaviour and postcranial skeletal morphology. Lorid humeral and femoral diaphyseal geometric cross-sectional properties, articular surface areas, and lengths are compared to those properties in other small primates with less specialized locomotory behaviour. The comparative sample includes both closely related prosimians and more distantly related platyrrhines.
Results indicate that lorids have greater humeral and femoral diaphyseal rigidity than other quadrupedal primates of similar body size, suggesting that lorid limbs are subjected to greater forces. Lorids also have relatively larger humeral and femoral articulations, corresponding to field and laboratory observations which indicate that lorid joints are highly mobilc. In addition, lorids have long humeri relative to femoral length, and compared to humeral length in less specialized prosimians of similar body mass. Long humeral length relative to femoral length is interpreted as a climbing adaptation because similar limb proportions are also seen in many non-primate climbers. Altogether, humeral and femoral diaphyseal cross-sectional properties, articular surface areas, and lengths comprise a suite of characters which have potential for identifying climbing specialists in the fossil record.     

Trabecular Bone Structure Correlates with Hand Posture and Use in Hominoids

Bone is capable of adapting during life in response to stress. Therefore, variation in locomotor and manipulative behaviours across extant hominoids may be reflected in differences in trabecular bone structure. The hand is a promising region for trabecular analysis, as it is the direct contact between the individual and the environment and joint positions at peak loading vary amongst extant hominoids. Building upon traditional volume of interest-based analyses, we apply a whole-epiphysis analytical approach using high-resolution microtomographic scans of the hominoid third metacarpal to investigate whether trabecular structure reflects differences in hand posture and loading in knuckle-walking (Gorilla, Pan), suspensory (Pongo, Hylobates and Symphalangus) and manipulative (Homo) taxa. Additionally, a comparative phylogenetic method was used to analyse rates of evolutionary changes in trabecular parameters. Results demonstrate that trabecular bone volume distribution and regions of greatest stiffness (i.e., Young''s modulus) correspond with predicted loading of the hand in each behavioural category. In suspensory and manipulative taxa, regions of high bone volume and greatest stiffness are concentrated on the palmar or distopalmar regions of the metacarpal head, whereas knuckle-walking taxa show greater bone volume and stiffness throughout the head, and particularly in the dorsal region; patterns that correspond with the highest predicted joint reaction forces. Trabecular structure in knuckle-walking taxa is characterised by high bone volume fraction and a high degree of anisotropy in contrast to the suspensory brachiators. Humans, in which the hand is used primarily for manipulation, have a low bone volume fraction and a variable degree of anisotropy. Finally, when trabecular parameters are mapped onto a molecular-based phylogeny, we show that the rates of change in trabecular structure vary across the hominoid clade. Our results support a link between inferred behaviour and trabecular structure in extant hominoids that can be informative for reconstructing behaviour in fossil primates.     

Femoral/humeral strength in early African Homo erectus

Lower-to-upper limb-bone proportions give valuable clues to locomotor behavior in fossil taxa. However, to date only external linear dimensions have been included in such analyses of early hominins. In this study, cross-sectional measures of femoral and humeral diaphyseal strength are determined for the two most complete early Homo erectus (or ergaster) associated skeletons--the juvenile KNM-WT 15000 and the adult KNM-ER 1808. Modern comparative samples include an adult human skeletal sample representative of diverse body shapes, a human longitudinal growth series, and an adult chimpanzee sample. When compared to appropriately age-matched samples, both H. erectus specimens fall very close to modern human mean proportions and far from chimpanzee proportions (which do not overlap with those of humans). This implies very similar mechanical load-sharing between the lower and upper limbs, and by implication, similar locomotor behavior in early H. erectus and modern humans. Thus, by the earliest Pleistocene (1.7 Ma), completely modern patterns of bipedal behavior were fully established in at least one early hominin taxon.     

Morphometry and allometry of the primate humerus

The primate distal humerus has been used both in phylogenetic reconstruction and in assessing locomotor and postural adaptations. This study uses an allometric approach to predict locomotor patterns of extant primates regardless of phylogenetic position. By showing the relationship between form and function in living primate taxa it will be possible to use this data set to predict locomotor behavior of extinct primates. Several linear measurements were taken from the distal humerus of 71 extant primate species (anthropoids and prosimians). Allometric regressions of each measurement were performed with mandibular M₂ area as a surrogate for body size. These measurements were used to determine if significant differences in distal humerus morphology exist among locomotor groups. The results were then used to test several hypotheses about the relationship between humeral form and function. For example, the hypothesis that suspensory primates have a large medial epicondyle is confirmed; the hypothesis that terrestrial quadrupeds have a deep olecranon fossa could not be confirmed with quantitative data. In addition to this hypothesis testing, the residuals from the allometric regressions of the humeral measurements were used in a discriminant functions analysis to estimate locomotor behavior from distal humerus morphology. The discriminant functions analysis correctly reclassified 64/71 (90%) species.     

Three-dimensional analysis of nonhuman primate trabecular architecture using micro-computed tomography

Until recently, detailed analyses of the architecture of nonhuman primate cancellous bone have not been possible due to a combination of methodological constraints, including poor resolution imaging or destructive protocols. The development of micro-computed tomography (microCT) and morphometric methods associated with this imaging modality offers anthropologists a new means to study the comparative architecture of cancellous bone. Specifically, microCT will allow anthropologists to investigate the relationship between locomotor behavior and trabecular structure. We conducted a preliminary study on the trabecular patterns in the proximal humerus and femur of Hylobates lar, Ateles paniscus, Macaca mulatta, and Papio anubis to investigate the quantitative differences in their trabecular architecture and evaluate the potential of microCT in anthropological inquiry. MicroCT allows the researcher to evaluate variables beyond simple two-dimensional orientations and radiographic densities. For example, this methodology facilitates the study of trabecular thickness and bone volume fraction using three-dimensional data. Results suggest that density-related parameters do not reliably differentiate suspensory-climbing species from quadrupedal species. However, preliminary results indicate that measurements of the degree of anisotropy, a measure of trabecular orientation uniformity, do distinguish suspensory-climbing taxa from more quadrupedal species. The microCT method is an advance over conventional radiography and medical CT because it can accurately resolve micron-sized struts that make up cancellous bone, and from these images a wide array of parameters that have been demonstrated to be related to cancellous bone mechanical properties can be measured. Methodological problems pertinent to any comparative microCT study of primate trabecular architecture are discussed.     

Postcranial morphology and the locomotor habits of living and extinct carnivorans

Members of the order Carnivora display a broad range of locomotor habits, including cursorial, scansorial, arboreal, semiaquatic, aquatic, and semifossorial species from multiple families. Ecomorphological analyses from osteological measurements have been used successfully in prior studies of carnivorans and rodents to accurately infer the locomotor habits of extinct species. This study uses 20 postcranial measurements that have been shown to be effective indicators of locomotor habits in rodents and incorporates an extensive sample of over 300 individuals from more than 100 living carnivoran species. We performed statistical analyses, including analysis of variance (ANOVA) and stepwise discriminant function analysis, using a set of 16 functional indices (ratios). Our ANOVA results reveal consistent differences in postcranial skeletal morphology among locomotor groups. Cursorial species display distal elongation of the limbs, gracile limb elements, and relatively narrow humeral and femoral epicondyles. Aquatic and semiaquatic species display relatively robust, shortened femora and elongate metatarsals. Semifossorial species display relatively short, robust limbs with enlarged muscular attachment sites and elongate claws. Both semiaquatic and semifossorial species have relatively elongate olecranon process of the ulna and enlarged humeral and femoral epicondyles. Terrestrial, scansorial, and arboreal species are characterized by having primarily intermediate features, but arboreal species do show relatively elongate manual digits. Morphological indices effectively discriminate locomotor groups, with cursorial and arboreal species more accurately classified than terrestrial, scansorial, or semiaquatic species. Both within and between families, species with similar locomotor habits converge toward similar postcranial morphology despite their independent evolutionary histories. The discriminant analysis worked particularly well to correctly classify members of the Canidae, but not as well for members of the Mustelidae or Ursidae. Results are used to infer the locomotor habits of extinct carnivorans, including members of several extinct families, and also 12 species from the Pleistocene of Rancho La Brea. J. Morphol., 2013. © 2012 Wiley Periodicals, Inc.