Modularity of the dorsal and lateral view of the skull in the European ground squirrel

Abstract

Modular organization is a general characteristic of biological systems from cellular to organismal level. The mammalian skull is a complex structure that can in general be divided into two functional components, the neurocranium and the viscerocranium. The two-module organisation of the skull of the European ground squirrel Spermophilus citellus (Linnaeus, 1766) has already been confirmed on the ventral cranium, while different studies of integration and modularity of squirrel skulls in general gave mixed results. Studies using 2D geometric morphometrics capture and analyse different views of the skull separately, and often the eventral cranial view is considered as the most suitable. In this study, the hypothesis of the two-module organisation of the S.citellus skull was re-evaluated and confirmed also on the dorsal and lateral cranial view. Nevertheless, the lateral cranium was more integrated than the dorsal cranium. Allometry had almost no effect on the pattern of modularity. This and the previous study of the S. citellus skull modularity show that different cranial views can give different results. Advisably, all three views should be considered also because the lateral view of the skull shows morphological variation in the sagittal plane that is not visible along the frontal plane, when only the ventral and/or dorsal views are considered.