Mandibular characteristics of early Glires (Mammalia) reveal mixed rodent and lagomorph morphotypes

Glires (rodents, lagomorphs and their fossil kin) is the most speciose and arguably most diversified clade of living placentals. Different lineages within the Glires evolved basically opposite chewing movements: a mostly transversal power stroke in lagomorphs, and a mostly proal power stroke in rodents, but the ancestral condition for Glires is still unclear. To address this knowledge gap, we studied the mandibles of Chinese Palaeocene Glires representing the duplicidentate (lagomorph-like; Mimotona) and simplicidentate (rodent-like; Eomylus and Heomys) lineages. To assess the mechanical resistance of mandibles to bending and torsion, we calculated the section modulus. The dentaries differ greatly in morphology and the region where the maximum grinding force was likely applied. The early Palaeocene Mimotona lii and the middle Palaeocene Mimotona robusta and Heomys orientalis all show a pattern of increasing strength moving posteriorly along the mandible, similar to sciurids and the mountain beaver. By contrast, the late Palaeocene Eomylus sp. mandible was strongest in the m1 region, a pattern seen in lagomorphs and the stem placental Zofialestes. Our results indicate the early diversification of mandible structure of Glires, demonstrate a mixture of duplicidentate and simplicidentate characters among the basal Glires and suggest an early occurrence of a lagomorph-like morphotype.This article is part of the theme issue 'The mammalian skull: development, structure and function'.

Automated summary

The Glires, including rodents and lagomorphs, are the most speciose group of placental mammals. There are different chewing movements in these lineages, with lagomorphs showing a mostly transversal power stroke and rodents showing a mostly proal power stroke. However, the ancestral condition for Glires remains unclear. This study investigates the mandibles of Chinese Palaeocene Glires and suggests an early diversification of mandible structure, with a mixture of duplicidentate and simplicidentate characters among the basal Glires and an early occurrence of a lagomorph-like morphotype.