Correlation between vertebral Hox code and vertebral morphology in archosaurs
The evolution of the vertebral column is marked by profound morphological changes that have a strong impact on organismal biology. The vital functions of the axial skeleton range from protecting the neural structures through sustaining the body posture to physiological aspects such as breathing. Archosaurs (crocodiles, birds and dinosaurs), as a group, display a striking variety of body plans and vertebral morphologies. This dissertation aims to contribute to the understanding of the pattern and the genetic basis for the evolution of the vertebral column in archosaurs. The transdisciplinary project comprises five chapters. Framed by a general introduction (chapter 1) and the conclusion (chapter 5), the second chapter considers, from a morphofunctional point of view, the question of (1) why differences in the vertebral column evolved. The present thesis revealed a strong link between the digitally simulated flexion pattern of the presacral vertebral column and the axial movements of modern archosaurs during related activities such as feeding and locomotion: this correlation allowed the inference of the feeding range and locomotor options in the extinct archosaur Plateosaurus. This long-necked dinosaur was primarily adapted as mid-level browser, obtaining food that was at or above the horizontal level of its head. There is currently no evidence to unambiguously interpret the locomotion style of Plateosaurus. The morphofunctional analysis supported both a quadrupedal and a bipedal posture. The third chapter addresses, from a molecular biology point of view, the question (2) of how modern taxa develop their vertebral columns. It provides insights into the genetic basis for the embryonic development of the vertebral column in modern archosaurs, which includes the highly conserved Hox genes. The Hox gene expression pattern was detected in the Nile crocodile (Crocodylus niloticus) via whole-mount in situ hybridisation experiments. Hox paralog genes 4 and 5 are expressed in the cervical region of the crocodile. The anterior expression limit of HoxC-6 marks the cervicothoracic transition. The expression of Hox paralog genes 7 and 8 is restricted to the dorsal series. The same Hox genes are expressed along the anteroposterior body axis of crocodiles, chickens and mice, but the pattern of expression is different. The comparative analysis revealed two general processes that are accompanied by evolutionary differences in the axial skeleton: 1) expansion and condensation as well as 2) a shift of genetic activity corresponding to different vertebral counts. The strong association between the anterior limits of the expression of specific Hox genes and the borders between morphological regions of the vertebral axis in a variety of vertebrate species stimulated the work presented in the fourth chapter. It considers the question (3) of whether we can infer that the development of the vertebral column took place in extinct animals. The direct correlation between vertebral Hox code and quantifiable vertebral morphology shows that the genetic code is deducible from vertebral morphology in modern crocodiles, chickens and mice. Applying these findings to the fossil relative Plateosaurus revealed that the hypothetical Hox code for the dinosaur would be generally similar to the crocodilian Hox gene expression pattern, but with the variation that the anterior region is expanded, as in birds. The integrative analysis (morphology, genes and fossils) of the vertebrae greatly enhanced our knowledge of evolutionary processes and provided valuable information about the possible reasons, genetic basis and pattern for evolutionary changes of the vertebral column in extant and extinct archosaurs.