First virtual endocasts of a fossil rodent: <i>Ischyromys typus</i> (Ischyromyidae, Oligocene) and brain evolution in rodents

<p>The evolution of the brain in rodents has rarely been studied from the perspective of the fossil record. Here we describe the first virtual endocast of a fossil rodent, pertaining to <i>Ischyromys typus</i> (ROMV 1007; Orellan North American Land Mammal Age [NALMA], Nebraska), and form comparisons with partial and complete natural endocasts pertaining to the same genus, and with the virtual endocast of a closely related extant rodent (<i>Sciurus carolinensis</i>; AMNH 258346). These data allow us to formulate the first hypotheses informed by the fossil record concerning changes in brain size and shape through time in rodents, and to make comparisons with other euarchontoglirans, including Primates. <i>Ischyromys</i> exhibits several aspects of brain morphology that can be inferred to be primitive, in part based on their presence in plesiadapiform primates (e.g., exposed midbrain), although variation exists within the genus <i>Ischyromys</i> with respect to the visibility of the inferior colliculi. There is some evidence that neocorticalization occurred in rodents through time but to a lesser degree than in Primates. Arboreality might be linked to increases in the encephalization quotient and specializations related to vision in rodents, which contrasts with the situation in Primates. Finally, Oligocene rodents had smaller olfactory bulbs compared with plesiadapiform primates from the Eocene, meaning that olfaction might have been less critical in the early evolution of rodents. These results show that the evolution of the brain in mammals does not always follow the same evolutionary trajectories and demonstrates the importance of considering ecological factors when looking at brain size.</p> <p>SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at <a href="http://www.tandfonline.com/UJVP" target="_blank">www.tandfonline.com/UJVP</a></p> <p>Citation for this article: Bertrand, O. C., and M. T. Silcox. 2016. First virtual endocasts of a fossil rodent: <i>Ischyromys typus</i> (Ischyromyidae, Oligocene) and brain evolution in rodents. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2016.1096275.</p

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

<p>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 (<i>Paramys copei, P. delicatus, Reithroparamys delicatissimus, Rapamys atramontis</i>, and <i>Ischyromys typus</i>). 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 <i>Aplodontia, Marmota, Cynomys</i>, and <i>Spermophilus</i>. This contradicts previous hypotheses regarding early rodent locomotion, which suggested that they were arboreal or more generalized in their habits.</p> <p>SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP</p> <p>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.</p

Electronic Supplementary Material: Oldest skeleton of a plesiadapiform provides additional evidence for an exclusively arboreal radiation of stem primates in the Paleocene

Materials and Methods, Geological Setting and Age, Figures S1-S4, Tables S1-S