Bony skull development in the Argus monitor (Squamata, Varanidae, Varanus panoptes) with comments on developmental timing and adult anatomy

Varanids represent one of the most charismatic squamate clades and include the largest living lizards; however, little is known about their embryonic development and what it might reveal about the origin of their derived anatomy. In the present study, we describe external organogenesis and skull formation of Varanus panoptes in great detail. We compared timing of ossification with the patterns seen in other squamates, using three major hypotheses of squamate interrelationship as phylogenetic templates, and were able to detect heterochronic patterns in ossification that are associated with adult anatomy in each phylogeny. However, we refrain from preferring one topology given the current lack of congruence between molecular and morphological data sets. The rule of thumb that early appearance of developmental characters is correlated to larger prominence in adults is critically discussed and we conclude that such simple correlations are the exception rather than the rule. The entanglement of developmental processes detected herein highlights the non-independent formation of adult characters that are usually treated as independent in phylogenetic studies, which may bias the output of such studies. Our comprehensive descriptions of embryonic development may serve as a resource for future studies integrating the complex processes of embryogenesis into broad-scale phylogenetic analyses that are likely to show that change in embryonic timing is one of the major sources of morphological diversification.