Cranial dimensions as estimators of body mass and locomotor habits in extant and fossil rodents

ABSTRACT

Estimating body mass and locomotor habits of fossil rodents is challenging for taxa without available postcranial material. Although equations exist to estimate body mass from dental dimensions based on extant rodents, the applicability of such equations is doubtful given that modern rodents have evolved highly specialized teeth, whereas fossil taxa often exhibit a much less derived condition. For the present study, 11 cranial variables from a sample of 203 extant rodents of known body mass representing a range of taxonomic groups were assessed using reduced major axis (RMA) regression. The results show a strong correlation between body mass and each of the 11 cranial variables. The best estimators for body mass are skull length and cheek-tooth area, whereas the least reliable measures are palate length, and foramen magnum, as well as braincase and occipital condyle dimensions. We estimated body mass for specimens of five fossil Ischyromyidae rodents for which body mass had never been estimated (Paramys copei, P. delicatus, Reithroparamys delicatissimus, Rapamys atramontis, and Ischyromys typus). Principal components and canonical variates analyses based on 10 cranial dimensions for 103 members of Sciuromorpha demonstrate that a relationship exists between locomotor habits and cranial variables in this suborder. In these analyses, early ischyromyids are all placed in the terrestrial group with Aplodontia, Marmota, Cynomys, and Spermophilus. This contradicts previous hypotheses regarding early rodent locomotion, which suggested that they were arboreal or more generalized in their habits.

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Citation for this article: Bertrand, O. C., M. A. Schillaci, and M. T. Silcox. 2015. Cranial dimensions as estimators of body mass and locomotor habits in extant and fossil rodents. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2015.1014905.

ACKNOWLEDGMENTS

We thank the Mammalogy and Paleontology departments of the Royal Ontario Museum (ROM) and the American Museum of Natural History (AMNH) for providing access to the extant rodent collections We also thank M. Hill at the microscopy and imaging facilities at the AMNH and J. Thostenson at the high-resolution X-ray computed tomography scanner from the Shared Materials Instrumentation Facility (SMIF) at Duke University for scanning the fossil specimens. We also thank J. Meng for facilitating the scanning of the AMNH specimens, E. Westwig and J. Galkin for their help in the AMNH collections, and K. Seymour and S. Woodward for their assistance in the ROM collections. We thank two anonymous reviewers for comments that substantially improved the paper.

Handling editor: Anjali Goswami.