Endocranial shape variation in the squirrel-related clade and their fossil relatives using 3D geometric morphometrics: contributions of locomotion and phylogeny to brain shape

摘要

Landmark-based 3D geometric morphometrics has rarely been employed to understand the relationship between endocranial shape, phylogeny and ecology. The goal of this study is to examine the endocranial morphology of members of the squirrel-related clade by using these methods, and to develop a multi-faceted framework for studying brain evolution applicable to other groups. The squirrel-related clade is taxonomically and ecologically diverse, and includes tree squirrels, the mountain beaver and dormice. Virtual endocasts for Ischyromyidae, a primitive group of rodents likely to be related to the squirrel-related clade, were also included. Thirty landmarks were taken on virtual endocasts derived from 32 extant and extinct species. The results show that endocranial shape and size are significantly correlated in that smaller endocasts are relatively wider laterally than larger endocasts. The principal components analysis (PCA) reveals that endocranial shape is clearly distinct for Sciuridae, Aplodontidae, Gliridae and Ischyromyidae. Endocranial shape variation is associated with changes in the development of the neocortex, cerebellum (including the paraflocculi) and olfactory bulbs. The Kmult test shows that endocranial shape reflects phylogenetic relationships among the four families and within Sciuridae. In the PCA analysis, flying squirrels show the most distinct endocranial morphology among squirrels, overlapping the least with other tribes and subfamilies. This result suggests that gliding may have imposed specific constraints on cranial shape. The endocasts of fossil and modern fossorial Aplodontidae have a shape similar to those of Ischyromyidae. This similarity could be the result of homoplasy related to fossorial specialization in later occurring Aplodontidae. The fossil Sciurini Protosciurus is outside the range of variation for modern squirrels, suggesting that the emergence of the modern squirrel endocranial bauplan may have not been established until after the early Miocene. From the data gathered, phylogeny and locomotion both impacted endocranial shape in our rodent sample.