Ontogenetic, macroevolutionary and morphofunctional patterns in archosaur skulls

The Archosauria represent the most successful clade within tetrapods, having a large diversity in terms of species, diet spectra, body plans and locomotion styles. This is also true for the skull morphology, which shows a wide variety in shape and size, as well as in the common formation of beaks, crests, domes or horns. Archosaur skulls have been studied intensively in terms of their morphology, ontogeny, function, ecology and behavior in the past, but most of these studies have largely been restricted to case studies of single species or only a small number of taxa. The aim of the current thesis is to obtain better and comprehensive insight into skull shape diversity of archosaurs by using a two-dimensional geometric morphometric approach, with a special focus on ontogenetic and macroevolutionary patterns and their relation to function and ecology. Skull shape variation was quantified for Crocodylomorpha (including an ontogenetic series of the recent caimanine alligatorid Melanosuchus niger), Pterosauria, Sauropodomorpha and Theropoda. The material used for the analyses consists of skull reconstructions published in the scientific literature and photographs of skull material. The most important results of the thesis are summarized as follows: •The use of different skull reconstructions of the same specimen from the scientific literature has no significant influence on the results of morphometric analyses. However, the results could be potentially falsified by the use of reconstructions based on highly incomplete, strongly deformed or pathologic specimens. •In some cases the degree of intraspecific variation of one species can be as great as the interspecific variation of closely related species with similar ecological niches. Thus, species with great intraspecific diversity could have an impact on the results of morphometric analyses. •The skull shape of Archosauria is strongly correlated with function. A closer examination within theropod skulls reveals that the shape of the postrostrum is probably more affected by functional constraints than the snout, but the greatest correlation to the function was found in the orbital shape. The latter result supports previous studies on the biomechanics of theropod skulls. A comparison of the ontogenetic bite force performance with the cranial growth in the alligatorid Melanosuchus and biomechanical studies on crocodile skulls reveals that ontogenetic shape changes, especially in the orbital and postorbital region, are functional constrained. •Both ontogenetic and interspecific skull shape variation in archosaurs is correlated to diet preferences and feeding behaviour. A comparison between carnivorous and non-carnivorous (i.e. omnivorous and herbivorous) theropods reveals that both ecological groups occupy large areas within the morphospace without showing a significant overlap. Furthermore, small-bodied theropods tend to have a larger diet spectrum, suggesting that diet preferences within theropods are probably size related. •The distribution of taxa within the morphospace of Crocodylomorpha, Pterosauria, Sauropodomorpha and Theropoda is strongly correlated with the phylogenetic interrelationship of these clades: Closely related taxa appear closer to one another within the morphospace than more distantly related taxa. This result indicates that skull shape in archosaurs is further constrained by phylogeny. •When inferred from geometric morphometric data, disparity results proved to be similar to those based on limb measurements and discrete characters from phylogenetic analyses. This results justifies the use of geometric morphometric data as a further and equally useful proxy for addressing disparity. •Early archosaur hatchlings share features of the skull shape, including short, pointed snouts, enlarged orbits and large postorbital regions. However, ontogenetic shape changes are only congruous in terms of a relative increase of the snout length and a relative decrease of the orbit size. The degree of these changes is not uniform, so that adult specimens of different species can vary substantially in snout length or orbit shape. Furthermore, archosaurs show a huge variability of changes in the snout depth, the length of the postorbital region as well as the relative size of the antorbital fenestra and the lateral temporal fenestra during ontogeny. This variability in ontogenetic trajectories probably causes the large skull shape diversity found in archosaurs. •Due to the great variability in ontogenetic trajectories, cranial evolution of archosaurs is strongly affected by heterochronic events. Skull shape evolution of Crocodylomorpha, Sauropodomorpha, basal theropods, Tyrannosauroidea as well as derived Oviraptoridae, Dromaeosauridae and Troodontidae was probably influenced by peramorphosis. However, within Crocodylia the short skull of Osteolaemus might result from a paedomorphic event. This is also likely for the short-snouted basal theropods Daemonosaurus and Limusaurus. The great similarity in the skull shapes of the juvenile megalosaurid Sciurumimus and basal coelurosaurs reveals that the skull shapes of the latter might be also caused by paedomorphosis. Further paedomorphic trends are suspected for the skull evolution of basal Maniraptora and Avialae. The heterochronic events found seem to correlate with body size evolution.