Early lagomorphs are central to our understanding of how the brain evolved in Glires (rodents, lagomorphs and their kin) from basal members of Euarchontoglires (Glires + Euarchonta, the latter grouping primates, treeshrews, and colugos). Here we report the first virtual endocast of the fossil lagomorph Megalagus turgidus, from the Orella Member of the Brule Formation, early Oligocene, Nebraska, USA. The specimen represents one of the oldest nearly complete lagomorph skulls known. Primitive aspects of the endocranial morphology in Megalagus include large olfactory bulbs, exposure of the midbrain, a small neocortex, and a relatively low encephalization quotient. Overall, this suggest a brain morphology closer to that of other basal members of Euarchontoglires (e.g., plesiadapiforms and ischyromyid rodents) than to that of living lagomorphs. However, the well-developed petrosal lobules in Megalagus, comparable to the condition in modern lagomorphs, suggest early specialization in that order for the stabilization of eye movements necessary for accurate visual tracking. Our study sheds new light on the reconstructed morphology of the ancestral brain in Euarchontoglires and fills a critical gap in the understanding of palaeoneuroanatomy of this major group of placental mammals.

The skull of Megalagus turgidus was micro-CT scanned in a high resolution Phoenix v|tome|x L 240 scanner (GE Measurement & Control Solutions) at the American Museum of Natural History (New York, NY, USA) with the following parameters: voltage 155 kV, current 145 mA, and 0.2 mm Cu filter. To accommodate the length of the specimen, the skull was scanned as a multiscan in four parts. The total of 4501 images were acquired at a resolution of 22.54 μm (isotropic voxels) with 0.33 sec of exposure. Raw data were reconstructed with Phoenix datos|x 2.0 software resulting in 16-bit TIFFs (1977x1000 pixel in size). The CT-data of the endocranial cavity of Megalagus turgidus were manually segmented in Avizo 9.0.1 (Visualization Science Group, 1995–2015) using a WACOM Cintiq 21UX tablet in each of the three parts that contain the cranial cavity, then the three resulting datasets were merged.

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