Comparative and Experimental Analysis of Jaw Muscle Morphogenesis in Quail and Duck: A Basis for Understanding Developmental Mechanisms Underlying Evolutionary Change

more so than in other mammals of comparable size. Tardigrada (sloths) exhibit the most extreme condition, particularly among the largest forms, such as the giant ground sloths, with femoral expansion always associated with strong anteroposterior compression. Biomechanical indexes show that Tardigrada, among Xenarthra, plotted far from other mammals, except those adapted to peculiar environments and strategies (such as aquatic, semi-aquatic or bipedal habits). The lateromedial expansion of the femoral diaphysis, along with anteroposterior compression, might be due to many factors: the lack of a medullar cavity, the torsion of the hindlimb associated with a complex and relatively rigid pelvic girdle, the rearrangement of the pelvic musculature and the presence of some sacrofemoral ligaments, and an inclined attitude during a bipedal stance (as might occur during feeding or defense). Further, hindlimb torsion probably affected lateromedial expansion more than previously supposed. According to other studies, the expanded and somewhat twisted femoral diaphysis is a morphological response to torsion strains due to the nearly horizontal position of the bone during both locomotion and bipedal standing. On the other hand, the short femoral neck is invariably associated with a well developed greater trochanter, always far from the caput. A better lever arm for the gluteal muscles involved in posture is produced mainly by the displacement of their insertions.