Mustelids are a morphofunctionally diversified group. However, there are no descriptions of the postcranial musculature of South American mustelid species except for some comments from the 19th century. Here, we present the first description of the myology of the hind limbs, and lumbar, sacral, and caudal regions of the lesser grison (Galictis cuja), a short-legged South American mustelid, including muscle maps and weight data. We interpret the function and the evolution of several muscular features within a comparative framework and through the optimization of these traits onto a phylogeny. The configuration of the axial musculature (e.g., m. quadratus lumborum with short bundles, heavy iliocostalis, and forward originated sacrocaudalis dorsalis) and the presence of strong ankle musculature are features shared with mustelines and, to a lesser degree, with other musteloids. These could be related to a high mobility of the axial skeleton and strong control of the movement of the ankle joint, in relation to the acquisition of epigean bounding gaits, a crouched locomotion, and enhanced maneuverability inside burrows. We recorded many phylogenetically significant traits, shared with other arctoids (e.g., subdivision of m. gluteus profundus and semimembranosus, presence of a single belly for m. sartorius, and absence of articularis coxae) or exclusively musteloids (e.g., frequent fusion between m. piriformis and gluteus medius). Some features (e.g., restricted origin of the caudal belly of the m. semitendinosus, absence of gluteofemoralis, and unusually complex fibularis brevis) seem to be derived conditions acquired in some mustelid clades. Our results sustain the value of myological data for functional and phylogenetic studies.

Sparassodonta is a diverse group of extinct metatherian predators that include forms with diets ranging from omnivores to hypercarnivores, including potential bone-crushers and sabre-tooth specialised species. Most of the previous dietary studies on the group were based on qualitative approaches or dental morphometric indexes and/or bite force estimations. In this study, we explore the evolution of mandible shape and diet of Sparassodonta in a comparative phylogenetic framework, using geometric morphometric tools and allometric and discriminant analyses. We analysed the mandible shape of 142 extant species of marsupials and placental carnivores, and 15 fossil sparassodont species. We found that the relationship between shape and size of the mandible is strongly structured by phylogeny, where the more derived borhyaenoids tend to possess stronger and larger mandibles. Derived borhyaenoid sparassodonts and basal borhyaenoids were classified as hypercarnivores (with short and robust mandibular body). Hathliacynid were classified as mesocarnivores or as hypercarnivores, but with lower probabilities and less specialised morphologies (with a long and slender mandible). Although dental morphology suggests that most of the species of Sparassodonta would have been hypercarnivores, the robustness of the mandible seems to be informative regarding the prey size and degree of specialisation. The relationship between mandibular size and shape, and talonid/trigonid relative size, is strongly influenced by the phylogenetic legacy, suggesting that ecological factors could have influenced the evolution of the sparassodonts.

There are a number of studies relating to skull morphology differences within the carnivoran clades of both placentals and metatherians. It is difficult to compare these studies because of differences in taxonomic sampling, for example some include fossil taxa while others include non‐carnivoran placentals. As a consequence, we studied mandible morphology in a broad range of both extant and extinct carnivorous species, including Carnivora, Marsupialia and Sparassodonta to test for differences between these clades. We used geometric morphometrics and two disparity indexes, the variance and Procrustes distances mean. When including fossil species, we found no significant differences for both disparities in some analyses, except after the exclusion of the sabretooth morphotype. This can be explained by the extreme morphology of this morphotype, which increases the variance and reduces the disparity effect of the other species in the analyses. Using Procrustes distances, we found significant differences in disparity distances between Carnivora and Metatheria for most of the analyses. We also found significant differences using the variance index in some analyses. The mandibular disparity in Carnivora is greater than in carnivorous metatherian mammals for most of the cases and this can be related with differences in evolutionary history and constraints of both groups. The pattern found in the mandible is similar to that found in the face of the skull but was not observed in the braincase, due to differences in skull function and mandible function.