South America's extinct notoungulates evolved a masticatory system with no living analogue

Abstract Reconstructing how extinct mammals fed rests on a uniformitarian assumption: that their anatomy has a functional counterpart among living species. Notoungulata, a diverse clade of extinct South American herbivorous mammals that evolved across a wide range of ecological niches during nearly 60 Myr of South American isolation, challenges this assumption. They have been likened to rodents, lagomorphs, ungulates, hyraxes, rhinoceroses, and wombats, yet each comparison captures only part of their anatomy, and none provides a complete functional match. Whether this reflects incomplete analogy or genuine functional novelty remains unresolved. We tested all proposed feeding analogues within a single integrated framework, combining geometric morphometrics, temporomandibular joint (TMJ) indices, lever mechanics and finite element analysis across 12 notoungulates (four families, Oligocene–Pleistocene) and 128 extant herbivorous and omnivorous mammals. No living group reproduced the notoungulate masticatory system. Instead, each component matched a different analogue: mandibular shape approached hyraxes and rhinoceroses, TMJ architecture suids and tapirs, molar lever mechanics ungulates, and incisor-bite stress the most resistant rodents. Integrated in a short, robust mandible with a fused symphysis, this mosaic yielded the lowest incisor-bite stresses of all sampled clades and molar-bite stresses below those of most ungulates and diprotodontians. Notoungulates formed a cohesive functional cluster distinct from extant herbivores, with disparity concentrated in typotheres and culminating in the highly mobile jaw of Pachyrukhos . Prolonged isolation thus produced a novel solution to herbivory rather than convergence on modern designs, showing that analogue-based reconstructions can mask functional innovation in extinct endemic radiations.