Ecomorphological adaptations to carnivory are found in many mammalian clades, especially in the order Carnivora (Placentalia). Carnivorans are equipped with a distinctive P4/m1 carnassial pair, the carnassial shear, which displays a gradient from the more carnivorous species with bladelike carnassials (e.g., felids) to the least carnivorous species with generalized molars (e.g., bears). This evolutionary versatility contributed to their ecological success. The aim of this work is to describe the evolutionary trajectories of the carnassial lower molar within the clade Carnivora. For that purpose, we used 3D geometric morphometrics to explore the shape of the lower carnassials of 342 specimens corresponding to 71 extant species and 46 fossil species of Carnivora. With the software TNT, we optimized shape, size and diet to define ancestral nodes, and then we calculated the branch length (i.e., proportional to the cumulative change in each branch). Our results indicated that the ancestral morphotype of Feliformia had carnassials without entoconid and with trigonid higher than the talonid (characters traditionally associated with carnivory) and hypercarnivorous habits. On the contrary, the ancestral morphotype of Caniformia had generalized molars (with trigonid as high as talonid and with all the cusps) and omnivorous habits. Felidae was the node with the greatest morphological change, leading to its typical bladelike carnassials. Ailuropoda and Arctotherium nodes had the greatest morphological change in size, and acquired large carnassials convergently, maybe associated with the consumption of hard and tough food (i.e., bamboo and bones). Finally, there is no correlation between the change of shape and the change in size. However, there is an allometric pattern in Felidae, with a significant correlation of 23%. In conclusion, our analyses confirm that diet and dental anatomy are the two main factors that have driven the evolution of Carnivora, while the size had little importance except for some subclades.