New Virtual Endocasts of Eocene Ischyromyidae and Their Relevance in Evaluating Neurological Changes Occurring Through Time in Rodentia

Abstract

Little is known about the early evolution of the brain in rodents. We report on nine new virtual endocasts for one of the most primitive family of rodents, Ischyromyidae, based on five specimens of Pseudotomus and Notoparamys (Paramyinae) and four specimens of Reithroparamys and Rapamys (Reithroparamyinae), dating from the early Eocene to the late middle Eocene of North America (Colorado, Wyoming, Utah and Montana). The virtual endocasts were obtained from high-resolution X-ray micro-computed tomography data. Comparisons with previously described ischyromyid virtual endocasts allow us to make inferences about the ancestral condition of the brain in rodents. Since Reithroparamyinae are suggested to be more closely related to the squirrel-related clade than other Ischyromyidae, comparisons were also made with the oldest virtual endocast for a squirrel, which gave us the opportunity to look at finer neurological changes occurring in the early evolution of squirrels. These new data permit a preliminary assessment of the endocranial diversity in Ischyromyidae. The results do not show evidence for a clear increase in Encephalization Quotient through time for early rodents. Instead, variation among species could be due to ecological factors (e.g., locomotion). Significant expansion in the neocortex and increase in paraflocculi ratios may have occurred in the transition from Ischyromyidae to Sciuridae, as previously hypothesized. Large olfactory bulbs and exposed midbrain are inferred to have been features present in the common ancestor of rodents, while neocortical expansion is reconstructed as having occurred twice independently within Ischyromyidae.