The Role of Evolutionary Integration in the Morphological Evolution of the Skull of Caviomorph Rodents (Rodentia: Hystricomorpha)

The mammalian cranium is a complex structure composed by three partially independent modules: face, cranial base and cranial vault. At the same time, it interacts with the mandible by sharing the masticatory function. Since these units develop and work together, their function and evolution may occur through correlated changes. Here, we assessed the patterns of evolutionary shape variation and covariation (i.e. integration) of cranial modules and mandible among the highly ecomorphologically diverse caviomorph rodents, and the potential evolutionary consequences on the morphological evolution of this clade. Three-dimensional geometric morphometrics was used to describe cranio-mandibular shape. The phylogenetic signal and evolutionary allometric component of morphometric variables were analyzed; in addition, evolutionary covariation among cranial modules and mandible was assessed using phylogenetic comparative methods. Significant phylogenetic signal and evolutionary allometry were detected. Large covariance values, involving coordinated breadth increase as the main shape change, were recorded between cranial vault and base, followed by cranial vault and face, and face and mandible. Since the basicranium may be the main cranial integrator, the overall widening of the cranial base, derived from the enlargement of the auditory bullae, could be influencing the integrated evolution of skull. In caviomorphs, the cranio-mandibular morphological evolution would be the outcome of a tight covariation among the modular units, and this could be driven by several factors such as allometry and specializations to environmental niches.