Trabecular architecture in the forelimb epiphyses of extant xenarthrans (Mammalia)

Eli Amson,John A. Nyakatura,Aurore Canoville,Patrick Arnold,Anneke H. van Heteren

doi:10.1186/s12983-017-0241-x

Abstract

BackgroundBone structure has a crucial role in the functional adaptations that allow vertebrates to conduct their diverse lifestyles. Much has been documented regarding the diaphyseal structure of long bones of tetrapods. However, the architecture of trabecular bone, which is for instance found within the epiphyses of long bones, and which has been shown experimentally to be extremely plastic, has received little attention in the context of lifestyle adaptations (virtually only in primates). We therefore investigated the forelimb epiphyses of extant xenarthrans, the placental mammals including the sloths, anteaters, and armadillos. They are characterised by several lifestyles and degrees of fossoriality involving distinct uses of their forelimb. We used micro computed tomography data to acquire 3D trabecular parameters at regions of interest (ROIs) for all extant genera of xenarthrans (with replicates). Traditional, spherical, and phylogenetically informed statistics (including the consideration of size effects) were used to characterise the functional signal of these parameters.ResultsSeveral trabecular parameters yielded functional distinctions. The main direction of the trabeculae distinguished lifestyle categories for one ROI (the radial trochlea). Among the other trabecular parameters, it is the degree of anisotropy (i.e., a preferential alignment of the trabeculae) that yielded the clearest functional signal. For all ROIs, the armadillos, which represent the fully terrestrial and fossorial category, were found as characterised by a greater degree of anisotropy (i.e., more aligned trabeculae). Furthermore, the trabeculae of the humeral head of the most fossorial armadillos were also found to be more anisotropic than in the less fossorial species.ConclusionsMost parameters were marked by an important intraspecific variability and by a size effect, which could, at least partly, be masking the functional signal. But for some parameters, the degree of anisotropy in particular, a clear functional distinction was recovered. Along with data on primates, our findings suggest that a trabecular architecture characterised by a greater degree of anisotropy is to be expected in species in which the relevant epiphyses withstand a restricted range of load directions. Trabecular architecture therefore is a promising research avenue for the reconstruction of lifestyles in extinct or cryptic species.

Highlights

Bone structure has a crucial role in the functional adaptations that allow vertebrates to conduct their diverse lifestyles
We present a dataset comprised of μCT-scan data characterizing the 3D trabecular architecture of the main forelimb epiphyses of all extant genera of xenarthrans
The important variation observed in most of their trabecular parameters offers a unique insight in the functional relevance of these parameters, as the forelimb of xenarthrans is characterized by conspicuous differences in its functional use

Summary

Introduction

Bone structure has a crucial role in the functional adaptations that allow vertebrates to conduct their diverse lifestyles. It was shown experimentally that trabecular bone adjusts accurately and sensitively throughout life to the loads applied to the bone, as part of the ‘bone functional adaptation’, commonly referred to as ‘Wolff’s law’ (e.g., [6]; for a review see [7]). Trabecular parameters such as the number of trabeculae, their mean thickness, or their main direction of orientation (i.e., their anisotropy), have the potential to be highly insightful regarding the functional adaptations of a particular skeletal element

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