Diversity of limb long bone morphology among proboscideans: how to be the biggest one in the family

Abstract

AbstractFrom its first small representatives to its later giant species, the Order Proboscidea evolved increasingly large forms. Limb long bones are heavily affected by shifts in body mass, and this is especially true in proboscideans, in which an increase in body mass is associated with a reorientation of the limb into the parasagittal plane, reducing the need for strong musculature. We investigated shape variation in the six long bones elements of the limb of extant and extinct proboscideans with regard to body mass evolution in this lineage. To do so, we used three‐dimensional geometric morphometrics and qualitative comparisons to describe shape variability. Our analysis indicates that the shift from a flexed posture to a columnar one results in reorientations of articular surfaces in the six bones, and a decrease in robustness, thus reducing bending stresses on the bones. We also identify two main morphotypes among graviportal proboscideans: one (MA) corresponding to bones with a thin diaphysis and narrow epiphyses, as in deinotheres and elephantids, and the other (MB) to bones with a large diaphysis and wide epiphyses, seen in gomphotheres and mammutids. Both morphotypes are observed in species of similar mass, indicating that adaptation to high body weight support has been achieved in two different ways among proboscideans, and that once acquired, it remains fixed within the taxon.