Xanthe Sifra Bijl: Step into the Synchrotron Light: Microanatomy and Evolutionary Adaptations of the Appendicular Skeleton of Palaeozoic Sarcopterygians

Abstract

The Devonian and Carboniferous water-to-land transition is one of the most drastic events in the evolutionary history of vertebrates. The shift from paired fins in sarcopterygian fish to the first emergence of digit-bearing limbs in tetrapods is central to this transition. In this thesis synchrotron tomography is used to study the bone microanatomy of the pectoral appendages of a stem lungfish and several stem tetrapods. In the first paper, the bone microanatomy of the stem lungfish Glyptolepis shows that the loss of appendicular ossification in the lungfish group was a stepwise and prolonged evolutionary transition. It also showed that its bone elongation process functioned similar to amniotes, resulting in longitudinal trabeculae in the metaphysis. The second paper used FEA to study the effects of gravity on the fin bone of a stem tetrapod, considering the microanatomy of the humerus of Eusthenopteron as a proxy for that of the first tetrapods. We found the ability to dissipate stresses through longitudinal trabeculae could have been a useful exaptation to support body weight during terrestrial locomotion. For the third paper we studied the humeral microanatomy of four Carboniferous limbed stem tetrapods, and found that despite the fact that they all had relatively thin cortical bone and an extended spongiosa, they exhibited microanatomical adaptations to different lifestyles. The fourth paper described potentially fossilised soft tissues, in the form of vascular walls and red bloods cells within them. They have the appropriate morphology and size, and are in the correct location within the vascular network of Crassigyrinus, underlining the potential of bone as a host for soft tissue preservation. The final paper describes a unique ossification pattern in the L-shaped humeri of early tetrapods. It is a unidirectional ossification process that leaves the anterior margin of these humeri unossified. We propose a hypothetical scenario to unify these morphological and microanatomical patterns during the evolutionary shift from L-shaped humeri to tubular humeri. Together these studies show the importance of bone microanatomy and the wealth of information that can be found on the inside of bones using propagation phase-contrast synchrotron microtomography.