The Phylogeny and Palaeobiology of “Protorosauria” with a Focus on the Genus Tanystropheus

Abstract

Summary “Protorosaurs” are a group of early archosauromorph reptiles (i.e. stem-archosaurs, the reptile clade containing modern birds and crocodylians). They are characterized by their long necks, which are comprised of elongate cervical vertebrae with low neural spines and long and thin cervical ribs. The group exhibits a high degree of morphological and ecological diversity, and is represented by terrestrial, fully marine, and possibly gliding taxa. As such they represent an important group for understanding the earliest evolution of the archosaur lineage as well as the composition of Triassic ecosystems in the aftermath of the Permo-Triassic mass extinction event. Well-known “protorosaurs” include the Permian Protorosaurus speneri, the Early Triassic Prolacerta broomi, and the Tanystropheidae. The most striking “protorosaurs” are the extremely long-necked Tanystropheus spp. and Dinocephalosaurus orientalis. Recent phylogenetic studies indicate that “Protorosauria” make up several lineages of non-archosauriform archosauromorphs, implying it is a polyphyletic group. This thesis is aimed at improving our understanding of this remarkable group of reptiles by investigating their morphology, taxonomy, biogeographic distribution patterns, palaeobiology, and phylogeny. A new specimen of Prolacerta broomi from the Antarctic Fremouw Formation provides new insights into its morphology and confirms that specimens of this taxon from Antarctica and South Africa belong to the same species, indicating the close faunal ties between these continents during the Early Triassic. The taxonomy of the genus Tanystropheus is reviewed and extensively revised. The number of Tanystropheus species is reduced from six to three. One species, Tanystropheus fossai, differs distinctly from other Tanystropheus species and is therefore reassigned to a separate genus, Sclerostropheus. Two other species, Tanystropheus longobardicus and the newly identified Tanystropheus hydroides are known from the famous Besano Formation of the UNESCO World Heritage Site of Monte San Giorgio. They were previously considered to represent juveniles and adults of the same species, respectively. However, distinct differences in their cranial morphology, supported by osteohistological evidence that the specimens of the smaller species T. longobardicus are skeletally mature, convincingly show that they represent separate species. The species have distinctly contrasting dentitions and thus exploited different food sources. Their co-occurrence in the Besano Formation therefore represents a convincing example of niche partitioning and reveals a surprisingly multifunctional role for the extremely long and stiff neck of Tanystropheus. The cranial morphology of both T. hydroides and Macrocnemus bassanii are studied in detail using high-resolution synchrotron radiation microtomography. This has allowed for the first comprehensive skull reconstructions of these taxa. They reveal much information on skull regions that were previously obscured such as the braincase and in the case of T. hydroides even the endocast and endosseous labyrinth. The skull of M. bassanii shows many plesiomorphic features, whereas the skull of T. hydroides is highly derived and clearly specialized for an aquatic lifestyle. The holotype of Macrocnemus fuyuanensis is redescribed and reveals additional characters that distinguish it from M. bassanii. This reveals that M. fuyuanensis occurred on both the eastern and western margins of the Tethys Ocean, since a specimen from the Besano Formation of Monte San Giorgio can be referred to this species, which is otherwise exclusively known from China. Finally, a new phylogenetic analysis comprising the most comprehensive sample of “protorosaur” taxa to date has been performed considering the new findings presented in this thesis. It confirms both the polyphyly of “Protorosauria” and the monophyly of Tanystropheidae. The position of the putative gliding reptile Ozimek volans within Tanystropheidae is corroborated. The analysis reveals that Prolacerta broomi is more distantly related to Archosauriformes (i.e. stem-archosaurs more closely related to the crown than non-archosauriform archosauromorphs) than previously considered. Additionally, a new clade of long-necked archosauromorphs, Dinocephalosauridae, is recognized, which is at least comprised of the Chinese taxa Dinocephalosaurus orientalis and Pectodens zhenyuensis.