Ontogeny in Dysalotosaurus lettowvorbecki Dissertation der Fakultät für Geowissenschaften der Ludwig-Maximilians-Universität München von Diplom-Geologe Tom Hübner Antrag auf Zulassung zur Promotion: 11.08.2010 1. Gutachter: Dr. O.W.M. Rauhut Bayerische Staatssammlung für Paläontologie und Geobiologie München 2. Gutachter: Prof. Dr. P.M. Sander Steinmann-Institut für Geologie, Mineralogie und Paläontologie der Universität Bonn Tag der Disputation: 12.01.2011 „Jemand hat mir mal gesagt, die Zeit würde uns wie ein Raubtier ein Leben lang verfolgen. Ich möchte viel lieber glauben, dass die Zeit unser Gefährte ist, der uns auf unserer Reise begleitet und uns daran erinnert, jeden Moment zu genießen, denn er wird nicht wiederkommen. Was wir hinterlassen ist nicht so wichtig wie die Art, wie wir gelebt haben. Denn letztlich [...] sind wir alle nur sterblich.“ Jean Luc Picard The new mount of a skeleton of Dysalotosaurus lettowvorbecki (individual “dy I”), on display in the Museum für Naturkunde, Berlin. „Mein alter Glitschball-Trainer pflegte immer zu sagen: Finde raus, was Du nicht gut kannst, und dann lass es bleiben!“ Gordon Shumway Summary This study was inspired by many recent scientific projects, which gave new insight into the ontogeny of an increasing number of extinct species in more detail. The knowledge of the ontogeny and its development is very important for understanding the taphonomy and paleobiology, the taxonomic value of phylogenetic characters, and the evolutionary relationship of an extinct species. Several qualitative and quantitative methods were used in these studies including the observation of suture closure, bone surface texture, bivariate and multivariate statistics, morphometrics, and bone histology. However, most of the published studies dealt only with one or two of these topics and further concentrated only on one or two methods. The importance of the combination of methods to get a more complete picture on the life history and phylogenetic relationships of extinct animals is recognized, however, and more recent studies combine the discovery of a new species with statements on its ontogenetic stage or the description of monodominant bonebeds with the reconstruction of growth patterns and life history for instance. This scientific frame is the starting point for the present study, in which almost all of the mentioned methods were combined for the first time resulting in the most comprehensive ontogenetic study on a single dinosaur species up to date. The small Upper Jurassic ornithopod dinosaur Dysalotosaurus lettowvorbecki was the subject in this study. It was found during the famous German Tendaguru Expedition (1909-1913), which is named after the type locality of the respective Formation, the Tendaguru Hill. This hill is located approximately 60 kilometers west of the seaport of Lindi in the Southeast of Tanzania, East-Africa. In contrast to the gigantic sauropod dinosaurs and the stegosaur Kentrosaurus, Dysalotosaurus is known from only a single locality 2.5 kilometers northwest of Tendaguru Hill, but the numbers of preserved bones within the two bonebeds were extraordinarily high (more than 14000 catalogue numbers). Additionally, several ontogenetic stages were preserved for the majority of skeletal elements. The bones were mostly isolated, but often well preserved. Plenty of material is housed in collections in Berlin, Göttingen, London, Munich, Stuttgart, and Tübingen, making this dinosaur one of the best known ornithopods of the world and the ideal object for an ontogenetic study. The methods used here contain qualitative observations of the timing and degree of suture closure, of the bone surface texture, and of morphological changes in single elements during growth. Furthermore, quantitative calculations were carried out by using the values of the measured bones mainly for bivariate allometric statistics. Ratios between long bones, multivariate statistics, and morphometrics were abandoned due to the lack of articulated material and due to the rather high amount of missing values. The study of the bone histology was also extensive, because each of the chosen skeletal elements was numerously represented in several ontogenetic stages. Thus, variation of bone microstructure of Dysalotosaurus could be evaluated and its life history could be reconstructed by the calculation of growth curves. Dysalotosaurus belongs to the basal Iguanodontia. This is a derived and highly diverse group of ornithopod dinosaurs, which comprises the most primitive member Tenontosaurus from the Early Cretaceous of North America, the European Late Cretaceous rhabdodontids, the dryosaurids including Dysalotosaurus, the transatlantic Upper Jurassic genus Camptosaurus, and all more derived large ornithopods including the famous Iguanodon and the hadrosaurs. Dysalotosaurus can be treated as the perfect intermediate taxon within ornithopods because it represents the connection between primitive ornithopods and more derived basal iguanodontians on one hand and between small and large ornithopods on the other hand. It was therefore very interesting to explore ontogenetic changes within Dysalotosaurus in an evolutionary and size-related context. In 1977, Dysalotosaurus was synonymized with its close relative Dryosaurus from the Upper Jurassic of North America due to many morphological similarities. Numerous differences in skull and postcranial bones challenge this view, however. Even the morphology of the pelvic and hindlimb skeleton, which is actually relatively conservative among small ornithopods, shows more differences than between different genera of hadrosaurs or ceratopsids. Thus, the name Dysalotosaurus is resurrected for now and therefore also used throughout this study. A very important first step before starting observations and analyses of ontogenetic characters was the revised interpretation of the taphonomy of the two monodominant bonebeds of Dysalotosaurus. Mainly due to the lack of information on the sedimentology and stratigraphy of the locality and on the spatial arrangement of the bones, it was not clear, if this mass accumulation represents the result of one or two catastrophic mass death events or just the result of attritional mortality. Several lines of evidence let now conclude that the bonebeds indeed represent a single mass death event of a Dysalotosaurus herd, because there are no taphonomic differences between them and there are also no traces of preburial abrasion caused by long transport or of preburial weathering due to long exposure on the surface. The two-peaked size frequency distribution, usually interpreted as the result of attritional mortality, is also only unambiguously usable for populations with one offspring per female per year. This is highly unlikely for any dinosaur. The lack of hatchlings in the size frequency distribution could also be explained by the time of the death event, which probably took place well outside the breeding season. The underrepresentation of young age classes is probably the result of slight sorting of the bones in favor of large and robust elements. Finally, the underrepresentation of mid-sized age classes is explainable by the banishment or higher mortality rate around the time of sexual maturity, as in many modern gregarious mammals. In the end, the evidence for a single mass death event dominates and possible hints for attritional mortality can be explained alternatively, so that the hypothesis of a single Dysalotosaurus herd could be used as the null hypothesis in all following ontogenetic studies on this dinosaur. The ontogeny of the skull of Dysalotosaurus was carried out separately, because the best preserved skull, housed in the collections of the Bayerische Staatssammlung für Paläontologie and Geobiologie in Munich, was hitherto unstudied. This juvenile specimen is fully described and reconstructed. Apart from the palatine and the quadratojugal, all elements of the skull are now known and the comparison with all other preserved cranial material revealed several ontogenetic trends for the skull. The suture closure pattern, although variable in the timing, could be reconstructed. The elements of the basicranium and the parietals fuse first and the elements of the pre-orbital region fuse obviously very late or never during life. The overall skull proportions are also changing to relatively smaller orbits, an elevated posterior skull roof, and a longer snout. This indicates that peramorphic heterochrony is a dominant evolutionary tendency in the skull shape of ornithopods, supporting the development towards larger size and full herbivory. Further juvenile ontogenetic characters could be identified during this study: (1) the basioccipital has a characteristic rhomboidal shape, with the condyle neck thicker than the condyle itself in very young individuals, (2) the frontals are very slender and long, with just a small and flat central dome, (3) the deepest point of the postorbital suture has an anterior position on the postorbital process of the jugal, and (4) the tooth number is smaller (ten compared with up to 13 in older ones) in lower and upper jaws, among others. These results were then used in two other ornithopod species. The juvenile stage of the holotype skull of Gasparinisaura could be independently confirmed and the example of Thescelosaurus has demonstrated that more than one or two characters are necessary to determine the ontogenetic stage of an extinct animal unambiguously. The intermediate stage of Dysalotosaurus between less derived small