Abstract
Organisms comprise multiple interacting parts, but few quantitative studies have analysed multi-element systems, limiting understanding of phenotypic evolution. We investigate how disparity of vertebral morphology varies along the axial column of mammalian carnivores — a chain of 27 subunits — and the extent to which morphological variation have been structured by evolutionary constraints and locomotory adaptation. We find that lumbars and posterior thoracics exhibit high individual disparity but low serial differentiation. They are pervasively recruited into locomotory functions and exhibit relaxed evolutionary constraint. More anterior vertebrae also show signals of locomotory adaptation, but nevertheless have low individual disparity and constrained patterns of evolution, characterised by low-dimensional shape changes. Our findings demonstrate the importance of the thoracolumbar region as an innovation enabling evolutionary versatility of mammalian locomotion. Moreover, they underscore the complexity of phenotypic macroevolution of multi-element systems and that the strength of ecomorphological signal does not have a predictable influence on macroevolutionary outcomes.
Highlights
Organisms comprise multiple interacting parts, but few quantitative studies have analysed multi-element systems, limiting understanding of phenotypic evolution
Morphological traits distinguish among cervical morphologies and mainly reflect variation in the length of the vertebral body and spinous process (PC1) and in the length and width of the vertebral body and of the spinous process (PC2) (Fig. 2A)
Vertebrae with more constrained patterns of evolution have low disparities, even though constraint—the extent to which evolving lineages have repeatedly explored the same set of morphologies—is quantified without reference to the amount of difference among those morphologies
Summary
Organisms comprise multiple interacting parts, but few quantitative studies have analysed multi-element systems, limiting understanding of phenotypic evolution. The vertebral column poses greater analytical difficulties than the study of any single bone[15,16,17] or of those structures composed of rigidly articulated bones such as the skull[18,19,20], pelvis[21] or the sacrum[22] because there is not a clear criterion about the homology among subunits across taxa with different counts This is due to variation in the rate of somitogenesis or to variation in the expression of Hox genes along the anteroposterior axis of the embryo[1]. Changes in the rate of somitogenesis—i.e. velocity changes in a molecular oscillator (segmentation clock) that triggers the budding of a new somite from the presomitic mesoderm—results in meristic variation, and changes in Hox gene expression results in shifts of boundaries among regions, changing the number of somites that belong to each particular region[1] These variations lead to difficulties in studies performed at interspecific level, as only homologous subunits
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