Humeral diaphysis structure across mammals.

Long bones comprise articular ends (epiphyses) joined by transitional metaphyses and a diaphysis (shaft). The structure of the latter is often viewed as regularly tubular across tetrapods (limbed vertebrates). However, assessments of the bone structure along the whole diaphysis are rare. Here, I assess whole-diaphysis profiles of global compactness (bone fraction) of 164 species of extant and extinct therian mammals (marsupials + placentals) in a phylogenetically informed context. Generally terrestrial, mammals have acquired multiple times the highly specialized aerial, fully aquatic, and subterranean lifestyles, allowing to potentially associate specific traits with these lifestyles. I show that there is a consistent increase in global compactness along the diaphysis in most mammals. This pattern is modified in a limited number of specialized species: all aerial clades (gliders and bats) have rather uniform and low values, while cetaceans' humeral diaphysis is marked by a slightly more compact mid-diaphyseal region. Among subterranean clades, structure alterations are most obvious in fossorial talpids (true moles) and their highly modified humerus. These results call for the investigation of bone structure in whole skeletal elements of key fossils in order to reconstruct the patterns of evolutionary modifications associated with lifestyle transitions.