Myology of the masticatory apparatus of herbivorous mammals and a novel classification for a better understanding of herbivore diversity

Abstract

Masticatory muscle features allow for an understanding of how dietary habits and masticatory functions evolved across mammalian lineages. Herbivorous mammals were traditionally classified as pertaining either to "ungulate-grinding" or "rodent-gnawing" morphotypes, but those classes may not adequately encompass the diversity of past and present herbivores. We compiled data for 104 herbivorous (nine orders, including novel data for 31 species) and 10 non-herbivorous (five orders) extant species, and for four extinct taxa, in order to construct a more comprehensive scheme of masticatory morphotypes. A myological phylomorphospace reveals 15 morphotypes, which are statistically tested and morphofunctionally interpreted. Extant herbivorous morphotypes diversified along two main pathways, that did not reflect the traditional types. Additionally, some extinct herbivores adapted along alternative pathways to those of extant forms. Early mammaliaforms and mammals possess muscle proportions almost identical to living carnivores. Several more plesiomorphic herbivorous morphotypes were recognized, some of them retaining generalized myological proportions. The rodent disparity was remarkable, and cannot be encompassed by a single category or pathway. Features of some derived euungulates and diprotodonts resemble rodents, but only a few extinct euungulates and extant wombats closely converged with them. Criteria for selecting models for palaeobiological reconstructions are discussed.