Humeral diaphysis structure across mammals

Abstract

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.

Differing effects of size and lifestyle on bone structure in mammals

“…As with the cross-sectional shape, the aerial clades are featuring the strongest convergence, with each clade displaying a more elongate diaphysis when compared to that of their respective ancestral node. This is in accordance with the fact that these converging clades were also associated with a more regularly tubular structure of their humeral diaphysis [56]. While this trait scales with negative allometry for all other taxa (being the only humeral trait that shows a rather clear correlation with size), aerial species are clearly differing in showing positive allometry (Fig.…”

Diving dinosaurs? Caveats on the use of bone compactness and pFDA for inferring lifestyle

“…Similarly, Houssaye and Botton-Divet imaged the humerus and femur from eight species of otter, found considerable internal 3-D differences in bone microanatomy along the diaphysis, but reported only mid-diaphyseal Cg [ 134 ]. Amson scanned the humerus of specimens of 164 taxa of extant and extinct therian mammals and helpfully reported Cg values at multiple points along the proximodistal axis [ 138 ]. To facilitate comparison of bones of different lengths, the study rescaled positions along the axis to fall within the range 0 (proximal end) to 1 (distal end).…”

“…Prior research has now established that significant variation exists in Cg values unrelated to ecology or behavior. Biological factors that might affect Cg in dinosaurs as well as relevant extant taxa include: developmental variations among individuals of a species; the sex of the individual; changes in bone compactness that occur during normal ontogeny; variations in Cg among skeletal elements; and even variations among different locations along the shaft of a single bone [132,134,[137][138][139]. Diagenetic and taphonomic factors-including fracturing, deformation, infilling, and external erosion-can also introduce variations in Cg measurements.…”

Unique bone microanatomy reveals ancestry of subterranean specializations in mammals

“…The humerus of therian mammals with various lifestyles and body sizes were CT scanned at high resolution. The datasets of Amson ( 2021 ) and Amson and Bibi ( 2021 ) were extended with data acquired with micro‐CT scanners (Bruker SkyScan1272 and Nikon XTH 225 ST). Of particular importance for this work is the subterranean lifestyle, attributed based on Ellerman ( 1956 ).…”