Locomotory Adaptations in 3D Humerus Geometry of Xenarthra: Testing for Convergence

Abstract

Three-dimensional (3D) models of fossil bones are increasingly available, thus opening a novel frontier in the study of organismal size and shape evolution. We provide an example of how photogrammetry can be combined with Geometric Morphometrics (GMM) techniques to study patterns of morphological convergence in the mammalian group of Xenarthra. Xenarthrans are currently represented by armadillos, sloths, and anteaters. However, this clade shows an incredibly diverse array of species and ecomorphotypes in the fossil record, including gigantic ground sloths and glyptodonts. Since the humerus is a weight-bearing bone in quadrupedal mammals and its morphology correlates with locomotor behavior, it provides an ideal bone to gain insight into adaptations of fossil species. A 3D sample of humerii belonging to extant and fossil Xenarthra allowed us to identify a significant phylogenetic signal and a strong allometric component in the humerus shape. Although no rate shift in the evolution of the humerus shape was recorded for any clade, fossorial and arboreal species humerii did evolve at significantly slower and faster paces, respectively, than the rest of the Xenarthran species. Significant evidence for morphological convergence found among the fossorial species and between the two tree sloth genera explains these patterns. These results suggest that the highly specialized morphologies of digging taxa and tree sloths represent major deviations from the plesiomorphic Xenarthran body plan, evolved several times during the history of the group.

Morphological disparity and evolutionary rates of cranial and postcranial characters in sloths (Mammalia, Pilosa, Folivora)

“…2018; Serio et al . 2020). This pattern was observed especially when anterior appendicular elements were investigated, and is in contrast to our results, which, like morphometric evaluations of scapular morphology, suggest a lesser degree of differentiation between suspensory and semi‐arboreal taxa (Toledo 2016; Grass 2019).…”

“…2014), but also with a mixture of historical and functional or ecological signals as observed in xenarthrans (Amson & Nyakatura 2018; Oliveira & Santos 2018; Serio et al . 2020; Toledo et al . 2021), which can be hard to fully disentangle, as also observed here.…”

“…2018; Grass 2019; Serio et al . 2020). Often, and especially for postcranial data, this was done in broader taxonomic contexts, along with other xenarthrans and placental mammals, providing some insights on how their morphology relates to phylogeny and/or adaptation.…”

Adaptation to graviportality in Rhinocerotoidea? An investigation through the long bone shape variation in their hindlimb

“…As we chose to work at the species level, we then computed and analysed species mean shapes (Botton-Divet et al, 2017;Serio, Raia, & Meloro, 2020). After the sliding step, we computed a first Generalized Procrustes Analysis (GPA) with all specimens to remove the effect of size, location and orientation of the different landmark conformations (Gower, 1975;Rohlf & Slice, 1990).…”

Integrative Approach Uncovers New Patterns of Ecomorphological Convergence in Slow Arboreal Xenarthrans

“…Indeed, although extinct Folivora are characterized by an astonishing ecological diversity, no fossils have provided evidence of slow arboreality (McKenna and Bell 1997;Pujos et al 2007Pujos et al , 2017Gaudin and McDonald 2008). The distinct locomotor ecology of 'tree sloths' and its constraints drove the common acquisition of numerous traits (Nyakatura and Fischer 2011;Nyakatura 2012;Amson and Nyakatura 2018b;Montañez-Rivera et al 2018;de Oliveira and Santos 2018;Serio et al 2020;Alfieri et al 2021;Toledo et al 2021).…”