No abstract

Adaptations of the lower back to bipedalism are frequently discussed but infrequently demonstrated in early fossil hominins. Newly discovered lumbar vertebrae contribute to a near-complete lower back of Malapa Hominin 2 (MH2), offering additional insights into posture and locomotion in Australopithecus sediba. We show that MH2 possessed a lower back consistent with lumbar lordosis and other adaptations to bipedalism, including an increase in the width of intervertebral articular facets from the upper to lower lumbar column (‘pyramidal configuration’). These results contrast with some recent work on lordosis in fossil hominins, where MH2 was argued to demonstrate no appreciable lordosis (‘hypolordosis’) similar to Neandertals. Our three-dimensional geometric morphometric (3D GM) analyses show that MH2’s nearly complete middle lumbar vertebra is human-like in overall shape but its vertebral body is somewhat intermediate in shape between modern humans and great apes. Additionally, it bears long, cranially and ventrally oriented costal (transverse) processes, implying powerful trunk musculature. We interpret this combination of features to indicate that A. sediba used its lower back in both bipedal and arboreal positional behaviors, as previously suggested based on multiple lines of evidence from other parts of the skeleton and reconstructed paleobiology of A. sediba.

We examine how derived functional signal and phylogenetic inheritance interact in the forelimb of tree sloths, to understand the relative contribution of each in the evolution of a novel morphobehavioural suite. Molecular and craniodental data demonstrate that extant tree sloths evolved suspensory behaviours and associated morphologies from a non-suspensory ancestor independently of one another, making them a useful model system. We find that convergence in univariate traits is expressed mosaically, although the signal is largely functional. Three-dimensional analyses suggest there is greater conservatism of gross morphology in more proximal bones than in more distal elements. Convergence in some univariate scapular traits is independent of the gross morphology of the scapula itself, demonstrating that functionally relevant morphologies were mapped on to a more conserved scapular shape. Our results suggest that morphological homoplasy is expressed in a mosaic manner. The relationship between homoplasy and trait integration may be more nuanced than previously thought, even within a single adaptive system.