What big teeth you have: disparity and convergence in the carnassial tooth shape in carnivoramorphans and ‘creodonts’

Carnivoran species exhibit remarkable diversity in their feeding habits, behavior, and morphology. A defining feature shared by carnivorans and their stem groups (Carnivoramorpha) is the presence of a highly specialized dental complex- the carnassial complex - comprising the 4th upper premolar and the 1st lower molar. Initially evolved for efficient meat slicing, carnassials have diversified in size, shape, and use, allowing carnivorans to fill varied ecological roles from herbivorous giant pandas to molluscivore sea otters and hypercarnivorous lions. However, carnivorans represent only a fraction of the diversity of carnivorous mammals that once inhabited the Earth. Other placental clades also convergently evolved a carnassial complex, albeit with differences in the number of teeth and their arrangement; they are often collectively referred to as ‘creodonts’: Hyaenodonta and Oxyaenodonta.
Whereas the evolution of carnassial teeth in mammals has been the focus of a series of morphometric studies, the conventional wisdom for carnassial function assumes a relatively restricted use for these specialized teeth for slicing vs. crushing. However, the variety of morphologies observed among carnivoramorphans and ‘creodonts’ suggests the possibility of a higher degree of functional differentiation. In this study we use high-density 3D geometric morphometrics to quantify morphological disparity in the lower carnassial by covering each tooth in 1500 semi landmarks using a semi-automated protocol. Our dataset comprises nearly 300 species, spanning 26 families, and encompassing both extant and fossil taxa. Our analyses reveal a discernible dietary signal in carnassial tooth shape of extant forms, with the most pronounced differences observed between herbivorous and carnivorous taxa, while there is less strong dietary signal in the carnassial shape of omnivorous taxa. Some dietary categories (e.g. piscivore) exhibit notably high disparity, which could be an example of many-to-one function. There are marked variations between feliformia and caniformia, even between taxa with a similar diet. Although displaying distinctive carnassial shapes, ‘creodonts’ demonstrate similarities with hypercarnivorous feliforms in the development of highly specialized cutting blades. However, unlike feliforms that exhibit diverse carnassial shapes, ‘creodonts’ likely remained restricted to a specialized hypercarnivorous niche throughout their evolution. These preliminary findings support the long-standing hypothesis that the greater dental plasticity of Carnivoramorpha conferred a competitive advantage, enabling them to occupy more diverse niches during the Eocene, while the morphologically constrained ‘creodonts’ went extinct.