Palaeobiological inferences based on long bone epiphyseal and diaphyseal structure - the forelimb of xenarthrans (Mammalia)

Bardin

et al. 2019

Journal of Morphology

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The lifestyle of extinct tetrapods is often difficult to assess when clear morphological adaptations such as swimming paddles are absent. According to the hypothesis of bone functional adaptation, the architecture of trabecular bone adapts sensitively to physiological loadings. Previous studies have already shown a clear relation between trabecular architecture and locomotor behavior, mainly in mammals and birds. However, a link between trabecular architecture and lifestyle has rarely been examined. Here, we analyzed trabecular architecture of different clades of reptiles characterized by a wide range of lifestyles (aquatic, amphibious, generalist terrestrial, fossorial, and climbing). Humeri of squamates, turtles, and crocodylians have been scanned with microcomputed tomography. We selected spherical volumes of interest centered in the proximal metaphyses and measured trabecular spacing, thickness and number, degree of anisotropy, average branch length, bone volume fraction, bone surface density, and connectivity density. Only bone volume fraction showed a significant phylogenetic signal and its significant difference between squamates and other reptiles could be linked to their physiologies. We found negative allometric relationships for trabecular thickness and spacing, positive allometries for connectivity density and trabecular number and no dependence with size for degree of anisotropy and bone volume fraction. The different lifestyles are well separated in the morphological space using linear discriminant analyses, but a cross‐validation procedure indicated a limited predictive ability of the model. The trabecular bone anisotropy has shown a gradient in turtles and in squamates: higher values in amphibious than terrestrial taxa. These allometric scalings, previously emphasized in mammals and birds, seem to be valid for all amniotes. Discriminant analysis has offered, to some extent, a distinction of lifestyles, which however remains difficult to strictly discriminate. Trabecular architecture seems to be a promising tool to infer lifestyle of extinct tetrapods, especially those involved in the terrestrialization.

Section: Discussionsupporting

confidence: 88%

Section: Resultsmentioning

confidence: 96%

Trabecular architecture in the humeral metaphyses of non‐avian reptiles (Crocodylia, Squamata and Testudines): Lifestyle, allometry and phylogeny

Plasse

Bardin

et al. 2019

Journal of Morphology

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“…3f). Our assessment of CSS therefore illustrates quite well the spectrum of constraints acting on the humeral diaphysis, ranging from round cross-sections (with relatively thin cortex, see below) that maximise resistance to torsional stresses in volant taxa (Swartz et al, 1992) to the more elliptical cross-sections that better withstand uniaxial bending loads associated with digging (Amson and Nyakatura, 2018). The low CSS in the non-volant aerial mammals, i.e., the four gliding clades, could be reflective of the need to resist torsional loads as in bats and/or to a multidirectional bending environment, which is assumed in the case of the similarly rounder cross-sections of non-aerial, arboreal mammals (Patel et al, 2013).…”

Section: Discussionmentioning

confidence: 58%

Differing effects of size and lifestyle on bone structure in mammals

Bibi

2020

Preprint

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The skeleton is involved in most aspects of vertebrate life history. Previous macroevolutionary analyses have shown that structural, historical, and functional factors influence the gross morphology of bone. The inner structure of bone has, however, received comparatively little attention. Here we address this gap in our understanding of vertebrate evolution by quantifying bone structure in appendicular and axial elements (humerus and mid-lumbar vertebra) across therian mammals (placentals + marsupials). Our sampling captures all transitions to aerial, fully aquatic, and subterranean lifestyles in extant mammal clades. We found that mammalian inner bone structure is highly disparate. We show that vertebral structure mostly correlates with body size, but not lifestyle, while the opposite is true for humeral structure. The latter also shows a high degree of convergence among the clades that have acquired specialised lifestyles. Our results suggest that radically different extrinsic constraints can apply to bone structure in different skeletal elements.

“…3f). Our assessment of CSS therefore illustrates quite well the spectrum of constraints acting on the humeral diaphysis, ranging from round cross sections (with relatively thin cortex, see below) that maximise resistance to torsional stresses in volant taxa [49] to the more elliptical cross sections that better withstand uniaxial bending loads associated with digging [50]. Strongly elliptical sections can also be associated with pronounced bone processes, and hence affect muscle conformation.…”

Section: Lifestyles and Humeral Cross-sectional Shapementioning

confidence: 54%

Differing effects of size and lifestyle on bone structure in mammals

Bibi

2021

BMC Biol

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Background Mammals are a highly diverse group, with body mass ranging from 2 g to 170 t, and encompassing species with terrestrial, aquatic, aerial, and subterranean lifestyles. The skeleton is involved in most aspects of vertebrate life history, but while previous macroevolutionary analyses have shown that structural, phylogenetic, and functional factors influence the gross morphology of skeletal elements, their inner structure has received comparatively little attention. Here we analysed bone structure of the humerus and mid-lumbar vertebrae across mammals and their correlations with different lifestyles and body size. Results We acquired bone structure parameters in appendicular and axial elements (humerus and mid-lumbar vertebra) from 190 species across therian mammals (placentals + marsupials). Our sample captures all transitions to aerial, fully aquatic, and subterranean lifestyles in extant therian clades. We found that mammalian bone structure is highly disparate and we show that the investigated vertebral structure parameters mostly correlate with body size, but not lifestyle, while the opposite is true for humeral parameters. The latter also show a high degree of convergence among the clades that have acquired specialised (non-terrestrial) lifestyles. Conclusions In light of phylogenetic, size, and functional factors, the distribution of each investigated structural parameter reveals patterns explaining the construction of appendicular and axial skeletal elements in mammalian species spanning most of the extant diversity of the clade in terms of body size and lifestyle. These patterns should be further investigated with analyses focused on specific lifestyle transitions that would ideally include key fossils.