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 as yet unclear. To address on this knowledge gap, we studied the mandibles of Paleocene Glires from China 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 in depth, curvature of the ventral margin, the bending of the dental row and the region where the maximum grinding force was likely applied. In general, the early Paleocene Mimotona lii and the middle Paleocene Mimotona robusta and Heomys orientalis all show a pattern of increasing strength moving distally along the mandible, similar to sciurids and the mountain beaver. In contrast, the late Paleocene Eomylus sp. had a mandible that was strongest in the region of m1, a pattern seen in lagomorphs and also 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.

Methods
The Paleocene Glires specimens from the collection of Institute of Vertebrate Paleontology and Paleoanthropology CAS (IVPP), China were scanned using the 225 kV micro-CT scanner developed by the Institute of High Energy Physics, Chinese Academy of Sciences (CAS) at the Key Laboratory of Vertebrate Evolution and Human Origins, CAS. The specimens of Paleocene Glires were scanned with a beam energy of 120 kV and current of 120 mA at the resolution of 12.55µm per voxel using a 360° rotation with a step size of 0.5°. A total of 720 projections were reconstructed in a 2,048 x 2,048 matrix of 1,536 slices using a two-dimensional reconstruction software developed by the Institute of High Energy Physics, CAS.