Combing Paleontology and Developmental Biology to Understand Skull Evolution in Turtles

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The skulls of land vertebrates are characterized by a large diversity. Particularly the region behind the eye, the temporal region, shows considerable differences between major groups, such as mammals, lizards, birds, crocodiles, and frogs. The origin of this diversity - including diverse skull openings and marginal bone reductions - is rarely understood. Of particular interest are turtles that do not resample the skull shape of any living land vertebrate, although they were recently hypothesized to be closely related to extant reptilian groups.In order to understand the temporal skull region and the origin of turtles, a morphogenetic approach to study paleontological and developmental integrity is proposed.Primarily, the anatomy of the temporal region will be coded in a cladistic sense to reconstruct the evolution of this region. Therefore, fossil as well as embryological data will be observed. Geometric morphometric changes in evolution and development of the bones will be analyzed quantitatively to illustrate the pathways leading to either one morphotype and to infer phylogenetic and functional correlations.Micro-computed tomography and histological techniques will be applied to characterize the progress of bone development. Gene expression will be analyzed to illustrate embryonic skull anatomy before the bones begin to form.From a developmental perspective, shared ancestral and derived features will be detected that lead to either one morphotype. By studying fossil groups, which had an even larger diversity of the temporal region than extant groups do, detailed scenarios for the evolutionary changes will be detected. By characterizing the turtle morphotype, a precise phylogenetic conclusion will be drawn for the phylogenetic position of this group that has important impact to understand land vertebrate phylogeny as a whole. Moreover, the integration of genetic and bone development will allow a complex insight to head development in general. In this context, embryological malformations - as seen in humans - will be interpretable from a developmental perspective and their meaning for evolutionary changes will be derived.