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 movi..., 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. , The file formats are STL and PLY.Â

啮齿兔形总目（Glires）涵盖啮齿类、兔形类及其化石近亲，是现生胎盘类哺乳动物中物种丰富度最高、且公认演化分异程度最强的演化支。该类群内部不同演化支演化出了基本相反的咀嚼运动模式：兔形类以横向动力咬合冲程为主，啮齿类则以前向动力咬合冲程为主，但目前啮齿兔形总目的祖先咀嚼运动状态仍不明确。
为填补这一认知空白，我们对产自中国古新世的啮齿兔形总目标本的下颌骨开展了研究，样本涵盖双门齿型（兔形类亲缘类群：拟鼠兔属*Mimotona*）与单门齿型（啮齿类亲缘类群：始鼠属*Eomylus*和重兽属*Heomys*）两个演化支。为评估下颌骨抗弯曲与抗扭转的机械性能，我们计算了截面模数（section modulus）。这些下颌骨在骨体深度、腹缘曲率、齿列弯曲程度以及最大研磨力可能施加的区域上存在差异。总体而言，古新世早期的李氏拟鼠兔*Mimotona lii*、古新世中期的强壮拟鼠兔*Mimotona robusta*以及东方重兽*Heomys orientalis*均呈现出强度随[原文截断]的变化模式。
中国科学院古脊椎动物与古人类研究所（IVPP）馆藏的古新世啮齿兔形总目标本，于中国科学院脊椎动物演化与人类起源重点实验室，使用由中国科学院高能物理研究所研制的225 kV显微CT（micro-CT）扫描仪完成扫描。本次扫描参数为：管电压120 kV、管电流120 mA，体素分辨率达12.55微米，扫描过程中样品以0.5°的步距完成360°旋转。最终通过中国科学院高能物理研究所自研的二维重建软件，将720幅投影图像重建为2048×2048像素、共1536层的断层数据。
数据集文件格式为STL与PLY。