Integración morfológica entre las extremidades delantera y trasera en carnívoros con dientes de sable: ¿existe alguna evidencia de evolución desacoplada?

Abstract

The powerfully built forelimb of saber-tooth carnivorans has been traditionally interpreted as part of their prey-killing arsenal, as it is thought to be an adaption to immobilizing their prey before performing a quick and effective killing bite with their enlarged canines, with a minimum risk of fracture. Under this interpretation, the forelimbs of saber-tooths should be stouter than their hind limbs, which were not directly involved in prey capture. In spite of this, few studies have quantitatively compared the size and morphology of the fore- and hindlimb bones in saber-tooth predators. In this study, we investigate if the morphology of the fore- and the hindlimb is decoupled in saber-tooths (machairodontines, nimravids, and barbourofelids) compared to the living felines. We collected a series of 3D-landmarks in the scapula, humerus, radius, ulna, pelvis, femur, and tibia. Then, we compared Procrustes coordinates (a proxy for bone shape) and centroid size (a proxy for bone size) for each pair of fore- and hind limb major limb bones, following the criterion of serial homology. These comparisons were made using linear regressions for centroid size and two-block partial least squares (2B-PLS) for Procrustes coordinates. Our data suggest that, for both bone size and robustness (recovered by the first PLS axis of long bone comparisons), saber-tooth predators follow the same trend as modern felids. Therefore, the shape and size of their forelimb bones does not differ from the shape and size of their hind limb counterparts to a greater extent than in the living felines. This is particularly striking as some saber-tooths have a degree of limb robustness that goes far beyond the range of modern felids. Our results indicate clearly that the limbs of both the living felines and the saber-tooths share the same highly integrated pattern. The reason is probably that the powerful forelimbs of saber-tooths were adaptive for their hunting style, while their stout hind limbs were the inevitable outcome of developmental constrains, as both limbs share the same developmental processes resulting from serial homology. However, another possibility is that the powerful hind limbs of saber-tooths were also adaptive in order to withstand body weight loads while the forelimbs were used for immobilizing the struggling prey. Future research will answer if the highly integrated pattern between the fore- and the hind limb in felids and saber-tooth predators is governed internally (i.e., by developmental constrains) or externally (i.e., by natural selection).