Measuring Lateral Iliac Flare by Different Methods Risks Obscuring Evolutionary Changes in the Pelvis

The orientation and morphology of the hominin ilium is functionally important for locomotion, yet no standard method exists for quantifying its lateral flare. This lack of standardization stems from fossil preservation issues and morphological differences between hominin species. Many techniques have been developed to measure flare based on what is preserved on a single fossil, but this makes comparisons between fossil species challenging. In some cases, methods developed based on australopith morphology are not appropriate for measuring the morphology in genus Homo. Lateral iliac flare is the angle between the iliac blade and the sagittal plane, and it affects the function and efficiency of the lesser gluteal muscles. Four different methods of quantifying lateral iliac flare are reviewed here, and assessed based on how well they reflect lateral iliac flare. The data show that these metrics measure different parts of the pelvis, none measures lateral iliac flare relative to the sagittal plane, and they yield different results for which fossil hominin is the most (or least) flared. This inconsistency puts paleoanthropologists at risk of misinterpreting the functional importance of lateral iliac flare in hominin evolution. I propose two paths forward to improve our assessment of flare: first, a geometric morphometric comparison of iliac blade variation would properly assess the full three-dimensions of the ilium; second, in hominin fossils that are not complete or numerous enough to include in a geometric morphometric analysis, two-dimensional measurements may still be used to assess functional differences in the ilium provided the researcher does not claim that they measure lateral iliac flare. Anat Rec, 300:956-963, 2017. © 2017 Wiley Periodicals, Inc.