Orientation of the iliac blades is a key feature that appears to distinguish extant apes from monkeys. Iliac morphology is hypothesized to reflect variation in thoracic shape that, in turn, reflects adaptations for shoulder and forearm function in anthropoids. Iliac orientation is traditionally measured relative to the acetabulum, whereas functional explanations pertain to its orientation relative to the cardinal anatomical planes. We investigated iliac orientation relative to a median plane using digital models of hipbones registered to landmark data from articulated pelves. We fit planes to the iliac surfaces, midline, and acetabulum, and investigated linear metrics that characterize geometric relationships of the iliac margins. Our results demonstrate that extant hominoid ilia are not rotated into a coronal plane from a more sagittal position in basal apes and monkeys but that the apparent rotation is the result of geometric changes within the ilia. The whole ilium and its gluteal surface are more coronally oriented in apes, but apes and monkeys do not differ in orientation of the iliac fossa. The angular differences in the whole blade and gluteal surface primarily reflect a narrower iliac tuberosity set closer to the midline in extant apes, reflecting a decrease in erector spinae muscle mass associated with stiffening of the lumbar spine. Mediolateral breadth across the ventral dorsal iliac spines is only slightly greater in extant apes than in monkeys. These results demonstrate that spinal musculature and mobility have a more significant effect on pelvic morphology than does shoulder orientation, as had been previously hypothesized. Anat Rec, 300:810-827, 2017. © 2017 Wiley Periodicals, Inc.

A cornerstone assumption for reconstructing ape evolution is that the hominoid torso differs from that of monkeys, reflecting adaptation to below‐branch arboreality. The fossil record leaves only sporadic bony remains, however, forcing paleontologists to rely on hypothesized covariation among bones to infer torso structure. Recent data suggesting more variation within the torso than previously appreciated challenges long‐held assumptions used to make these inferences. To explore these associations, linear and landmark data were collected from vertebrae, sterna, ribs, pelves, clavicles, scapulae and humeri of 256 anthropoids. Also, intact torsos of 57 cadavers were CT scanned. Data reveal significant correlations among lumbar vertebral, lower rib, and iliac morphology. Correlations between upper and lower portions of the rib cage are weak, as are relations among ribs, pectoral girdle and humerus. Patterns of allometry also vary among regions. These results reflect differing selection pressures and structural constraints on parts of the torso, and provide data with which to more accurately infer aspects of torso form, and locomotor adaptation, in fossil anthropoids.