Geometric Morphometric Analyses and Cranial Shape Evolution in Monitor Lizards

Abstract

Understanding morphological change is a central theme in evolutionary, developmental, and comparative biology. Lizard heads are well suited to analyses of morphological evolution because of the complexity and diversity in the feeding system that integrates the cranium, jaw, hyolingual and cervical systems. Aside from feeding, the lizard skull is functionally associated with ecomorphology and sexual selection, and is additionally responsible for housing the major sensory systems of the body. Despite being a structurally complex and integrated system with numerous functions and roles, evolution in the lizard skull is characterized by the appearance of extreme morphologies.

Geometric morphometrics is a powerful tool for the quantification, visualization and analysis of morphological variation and change. This approach is being applied more frequently in a phylogenetic comparative context to assess the relative influence of size, ecology, function, and developmental constraints on morphological evolution. Geometric morphometric methods rely on homologous landmarks as the source of shape data, and the level of detail and accuracy increases with the amount of information contained in a landmark configuration. However, it may be possible to capture particular elements of shape variation by concentrating on different observation angles of a complex structure.

This research article examines how observation view (dorsal, ventral and lateral) influences 2D geometric morphometric analysis of interspecific cranial shape variation in monitor lizards (Varanus, Varanidae). I recover strong phylogenetic signal in all three views and general concordance in patterns of size-corrected shape diversification within the genus. However, I also find subtle but important differences among views in analyses of evolutionary allometry and shape variation, which may reflect both landmark configuration design and adaptive functional trends of the study system. This study shows that studies restricted to a 2D geometric morphometric analysis of a complex 3D biological structure can combine carefully designed 2D landmark configurations describing alternative planes to maximize shape coverage.

The original research article presented in my thesis provides the basis for addressing a wide range of questions associated with the analysis of skull morphology, dentition and biomechanical performance as it relates to diet in monitor lizards using 3D geometric morphometrics, bite force measurements and phylogenetic comparative methods. The future directions I propose examine alternative ways in which predators may optimize feeding success: through morphological adaptations (in the skull and teeth) that enhance prey capture, or through biomechanical adaptations that enable feeding on a wider variety of prey. Show more