Allometric and group differences in the xenarthran femur

Abstract

The aim of this study is to analyze shape variation in the xenarthran femur to gain insights into their behavior and locomotion. Specimens of both Cingulata (armadillos and glyptodonts) and Pilosa (anteaters and sloths) were studied and within each group body mass varies by several orders of magnitude. The main focus of the analysis was allometric variation in femoral shape in the three groups studied, armadillos, glyptodonts, and pilosans. Three dimensional coordinates were recorded for 40 homologous landmarks on each of 51 xenarthran femurs. The data were analyzed by geometric morphometric methods, and form space analysis was used to identify the allometric variation in each of the three groups. Across all groups, larger specimens tended to have larger articular surfaces, more robust femora generally, and the shape of the femoral condyles was more suited to extended postures. In addition, in larger specimens the medial condyle was much larger than the lateral condyle and the third trochanter was located more distally. The larger armadillo femora had a greater trochanter located considerably proximal to the femoral head and this is thought to improve femoral extension, but in glyptodonts and pilosans the larger specimens had a greater trochanter that was far lateral to the femoral head and this is interpreted as enhancing femoral rotation.