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 d The skulls of sabertooth cats differ in bone thickness and strength d Biomechanical differences in sabertooth cats are revealed by finite element analysis d Smilodon fatalis and Homotherium serum deployed distinct killing bites d SUMMARYOver the Cenozoic, large cat-like forms have convergently evolved into specialized killers of ''megaherbivores'' that relied on their large, and laterally compressed (saber-like) canines to rapidly subdue their prey [1][2][3][4][5]. Scimitar-and dirk-toothed sabertooths are distinct ecomorphs that differ in canine tooth length, degree of serration, and postcranial features indicative of dissimilar predatory behavior [6][7][8][9][10][11][12][13]. Despite these differences, it is assumed that they used a similar ''canine-shear'' bite to kill their prey [14,15]. We investigated the killing behavior of the scimitar-toothed Homotherium serum and the dirk-toothed Smilodon fatalis using a comparative sample of living carnivores and a new quantitative approach to the analysis of skull function. For the first time, we quantified differences in the relative amount and distribution of cortical and trabecular bone in coronal sections of skulls to assess relative skull stiffness and flexibility [16][17][18][19]. We also use finite element analysis to simulate various killing scenarios that load skulls in ways that likely favor distinct proportions of cortical versus trabecular bone across the skull. Our data reveal that S. fatalis had an extremely thick skull and relatively little trabecular bone, consistent with a large investment in cranial strength for a stabbing canine-shear bite. However, H. serum had more trabecular bone and most likely deployed an unusual predatory behavior more similar to the clamp-andhold technique of the lion than S. fatalis. These data broaden the killing repertoire of sabertooths and highlight the degree of ecological specialization among members of the large carnivore guild during the Late Pleistocene of North America. RESULTS Profiles of Cortical and Trabecular BoneAs cortical bone tends to have high stiffness and trabecular bone has greater flexibility [16-19], we hypothesize that if dirk-tooths

Explaining the origin and evolution of a vertebral column with anatomically distinct regions that characterizes the tetrapod body plan provides understanding of how metameric structures become repeated and how they acquire the ability to perform different functions. However, despite many decades of inquiry, the advantages and costs of vertebral column regionalization in anatomically distinct blocks, their functional specialization, and how they channel new evolutionary outcomes are poorly understood. Here, we investigate morphological integration (and how this integration is structured [modularity]) between all the presacral vertebrae of mammalian carnivorans to provide a better understanding of how regionalization in metameric structures evolves. Our results demonstrate that the subunits of the presacral column are highly integrated. However, underlying to this general pattern, three sets of vertebrae are recognized as presacral modules—the cervical module, the anterodorsal module, and the posterodorsal module—as well as one weakly integrated vertebra (diaphragmatic) that forms a transition between both dorsal modules. We hypothesize that the strength of integration organizing the axial system into modules may be associated with motion capability. The highly integrated anterior dorsal module coincides with a region with motion constraints to avoid compromising ventilation, whereas for the posterior dorsal region motion constraints avoid exceeding extension of the posterior back. On the other hand, the weakly integrated diaphragmatic vertebra belongs to the “Diaphragmatic joint complex”—a key region of the mammalian column of exceedingly permissive motion. Our results also demonstrate that these modules do not match with the traditional morphological regions, and we propose natural selection as the main factor shaping this pattern to stabilize some regions and to allow coordinate movements in others.

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The cave bear ( Ursus spelaeus s.l .) was an iconic extinct bear that inhabited the Pleistocene of Eurasia. The cause of extinction of this species is unclear and to identify the actual factors, it is crucial to understand its feeding preferences. Here, we quantified the shape descriptor metrics in three-dimensional (3D) models of the upper teeth (P 4 –M 2 ) of the cave bear to make inferences about its controversial feeding behaviour. We used comparative samples, including representatives of all living bear species with known diets, as a template. Our topographic analyses show that the complexity of upper tooth rows in living bears is more clearly associated with the mechanical properties of the items consumed than with the type of food. Cave bears exhibit intermediate values on topographic metrics compared with the bamboo-feeder giant panda ( Ailuropoda melanoleuca ) and specialists in hard mast consumption ( Ursus arctos and Ursus thibetanus ). The crown topography of cave bear upper teeth suggests that they could chew on tough vegetal resources of low quality with high efficiency, a characteristic that no living bear currently displays. Our results align with a climate-driven hypothesis to explain the extinction of cave bear populations during the Late Pleistocene.