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

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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.

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Acknowledgments

The authors would like to thank D. Bohaska and N.D. Pyenson from the Paleobiology Department of the Smithsonian (NMNH), J. Meng and R. O’Leary from the American Museum of Natural History (AMNH), as well as D. Brinkman, M. Fox, and Chris Norris from the Yale Peabody Museum for providing access to the specimen to be scanned. The authors also thank J. Thostenson and D.M. Boyer for facilitating the scanning of the specimens at the SMIF (Duke University). We also thank M. Hill from the AMNH Microscopy and Imaging Facility for scanning the specimens. Thank you to E. Seiffert for fruitful discussions and for providing very generous access to resources in his lab. This research was supported by an NSERC Discovery Grant to MTS, School of Graduate Studies Travel Grant from the University of Toronto, and a Research Expenses Grant from the Department of Anthropology (University of Toronto) to OCB. The authors also thank two anonymous reviewers for comments that improved the paper.

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  1. Department of Anthropology, University of Toronto Scarborough, Toronto, ON, M1C 1A4, Canada

    Ornella C. Bertrand, Farrah Amador-Mughal, Madlen M. Lang & Mary T. Silcox

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  1. Ornella C. Bertrand
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Contributions

OCB and MTS both contributed to conception and design of the study. OCB acquired the CT data, FAM and MML segmented out the fossil specimens in order to obtain the endocasts. OCB and MTS carried out the analyses and interpretations of the data. OCB drafted the article, and OCB, MML, and MTS revised it critically for important intellectual content. All authors gave final approval before submission.

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Correspondence to Ornella C. Bertrand.

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Bertrand, O.C., Amador-Mughal, F., Lang, M.M. et al. New Virtual Endocasts of Eocene Ischyromyidae and Their Relevance in Evaluating Neurological Changes Occurring Through Time in Rodentia. J Mammal Evol 26, 345–371 (2019). https://doi.org/10.1007/s10914-017-9425-6

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