Dissertations / Theses on the topic 'Marine reptile'

Jurassic marine ecosystems (ca. 201-145 million years ago) were dominated by three different lineages of reptiles - plesiosaurians, ichthyosaurs and thalattosuchian crocodylomorphs. Stratigraphic and fossil evidence indicates that these animals, like their modern counterparts, were able to coexist in the same environment for over ~50 million years from the Early Jurassic (~180 million years ago) to the Early Cretaceous (~130 million years ago). Marine reptile ecosystems were often very diverse, and included animals from different lineages, of disparate body-size and inferred ecology living alongside each other in the same environment. This unusual diversity suggests that marine reptiles formed complex ecosystems, and may have occupied analogous ecological roles today held by large fish, sharks, crocodiles, sirenians, and cetaceans. However, these comparisons are essentially qualitative, as they are based on the recurring convergent morphologies of skulls, mandibles and dentitions in aquatic tetrapods. Yet, they have never been quantitatively tested. Furthermore, although we have a comprehensive understanding of the anatomy, systematics, phylogenetic relationships, physiology and feeding ecology of these extinct animals, little is still known about the structure and evolution of their ecosystems. Thus, we do not understand what enabled marine reptiles to form complex assemblages, how their fauna changed through time, and more importantly how climatic and environmental changes shaped their long-term evolution. Answering these questions is essential because understanding past marine ecosystems may inform on whether and how modern ones can adjust to changes in the ocean temperature, chemistry and sea-level. In order to establish the reliability of these comparisons, in this project, I consider the evolution of the diverse marine reptile fossil assemblage of the Jurassic Sub-Boreal Seaway (JSBS) of the UK. The fossil record of the JSBS is an ideal case-study for many reasons. Firstly, it is a well-documented, high-diversity ecosystem, represented by hundreds of well-preserved specimens collected from the world-famous Oxford Clay Formations (OCF Callovian-early Oxfordian, late Middle to early Late Jurassic) and Kimmeridge Clay Formation (KCF - Kimmeridgian to Tithonian, Late Jurassic). These specimens have been intensively collected since the XIX century, and are available in museum collections. Secondly, the fossil record of the JSBS covers a continuous interval of ~18 million years (middle Callovian-early Tithonian ~166-148 million years ago) of marine reptile evolution, in a single seaway, during a time of well-documented environmental changes. These changes in sea-level, temperature and chemistry happened in concert with drastic changes in the composition between the OCF and KCF marine reptile faunas across the Middle-Late Jurassic boundary. Unfortunately, to date, the attempts to understand whether there is a correlation between these events have been hampered by the scarcity of fossils material from the intermediate layers of the Oxfordian 'Corallian Gap'. After a brief introduction (Chapter I), this project articulates in two parts. In the first descriptive section (Chapters II, III and IV), I set the bases for the second part by reviewing the fossil record of ichthyosaurs, plesiosaur and thalattosuchians of the JSBS. Particular emphasis was put on the systematics of thalattosuchian crocodylomorphs, and the fossil assemblage of the 'Corallian Gap'. The second part of this thesis is an analytical section (Chapters V and VI), in which, using a suite of numerical techniques, I investigate the ecology, evolution and feeding ecology of marine reptiles through time. A summary of the main conclusions and future directions are presented in Chapter VII. Chapter II is a description of a new genus and species, Ieldraan melkshamensis, a metriorhynchid thalattosuchian from the Callovian of England. The stratigraphic occurrence of this new taxon demonstrates that all the macrophagous lineages of Late Jurassic metriorhynchids originated in the Middle Jurassic, earlier than previously supposed. This also has important implications for the evolution of macropredatory features (particularly the dentition) in this group. In Chapters III and IV, I review the scarce fossil record of the Oxfordian 'Corallian Gap', the least studied stage of the considered ~18 million-year interval. The results show that despite the scarcity and poor preservation of materials compared to the underlying and overlying fossil-rich OCF and KCF, a large variety of marine reptiles lived in the JSBS during the 'Corallian Gap' (middle-late Oxfordian). The study confirms a drop in marine reptile diversity in the Oxfordian, exemplified by the demise of several OCF taxa, but partially counterbalanced by the contemporaneous radiation of some KCF lineages. This review confirms that a faunal turnover severely affected the composition of the JSBS across the Middle-Late Jurassic boundary, and I hypothesise that these faunal changes may have been driven by environmental perturbations during the Oxfordian. In Chapter V, I use the most common marine reptile fossils - teeth - and the revised stratigraphic occurrences of the JSBS (from the previous Chapters), to investigate the evolution of marine reptile groups, through time. Using a multivariate approach I established a quantitative system to assign species to dietary guilds based on dentition features that together with the availability of teeth, allowed examination of diversity and disparity patterns at unprecedented time, and systematic resolutions. The results show that different taxonomic/dietary groups did not overlap, suggesting partitioning of resources based on diet/feeding strategy. The analyses show a decline of shallow-water specialists, the diversification of macrophagous species, deep-diving taxa, and increasing body-size in concert with a deepening of sea-level across the Middle-Late Jurassic boundary. These trends are not accompanied by drops in disparity, but by a selective decline/increase of specific ecological guilds, that mimic the transition from shallow/nearshore to deeper/offshore habitats in modern cetacean coastal assemblages. In Chapter VI, I use a variety of multivariate techniques to present a quantitative assessment of the feeding behaviour of marine reptiles. The aim of this study is investigating the morphological and functional variation of ichthyosaur, plesiosaur and thalattosuchian lower jaws. This is done using a variety of multivariate techniques, and a biomechanical comparative approach. The analyses confirm previous qualitative observations that the ecosystems in the OCF and KCF were markedly distinct in faunal composition and structure. Phylogenetically closely related taxa preferentially cluster together, with minimal overlaps amongst groups in the morphospace. Focus examinations of key morphofunctional complexes reveals that marine reptile subclades are characterised by different combinations that are consistent with their inferred feeding ecologies (based on tooth morphology). Overall, the present quantitative results validate previous qualitative hypothetical feeding ecologies, and reveal multiple instances of morphofunctional convergent evolution. Overall my results also show that, like in modern ocean ecosystems, complex mechanisms of niche and habitat partitioning may have facilitated the coexistence of diverse marine reptile assemblages over tens of millions of years of evolutionary time.

This study has looked at an assemblage of fossilised teeth from Mesozoic marine amniotes from the Kristianstad basin in southern Sweden in order to make an estimate of the ecomorph diversity within said assemblage through looking at the morphology of the teeth. This was done as a pilot study to see if further studies would be able to produce worthwhile results. The assemblage consists largely of isolated tooth crowns, mostly from small- to medium sized mosasaurs such as Clidastes and Eonatator, but also contains larger mosasaurs, as well as a couple of plesiosaurs and one species of a marine crocodile. The analysis was performed on images of teeth using software developed for use in morphometrics. The resulting graphs imply a division into three guilds; the first represented by the short and blunt teeth of the crocodilian, the second by the elongated teeth of the plesiosaurs, and the third by the knife-like teeth of the mosasaurs. Since the mosasaurs overlap to a high degree in tooth shape, but also show quite diversity in size, it is possible the main dividing factor would have been size rather than type of prey. Further studies would be able to get a more accurate image of the ecology of this fauna by increasing the number of specimen in the analysis as well as taking into consideration more factors from other studies of similar taxa, such as jaw sizes, bite marks and gut contents.<br>Det här arbetet har studerat fossila tänder tillhörande Mesozoiska marina amnioter från Kristianstadsbassängen i södra Sverige, med avsikt att göra en uppskattning av den ekomorfa spridningen inom gruppen genom att studera tändernas form. Detta är enbart en pilotstudie för att se om fortsatta studier kan ge givande resultat. Det studerade materialet är en del av en samling till stor del bestående av enbart lösa tandkronor, mest från små till medelstora mosasaurier, såsom Clidastes och Eonatator, men även från större mosasaurier, så väl som ett par plesiosaurier och en marin krokodil-art. För analysen användes en programvara specifikt utvecklad för morfometri. De resulterande graferna antyder en indelning i tre “gillen”; det första representeras av de korta och trubbiga tänderna tillhörande krokodilen, det andra av de långsmala tänderna hos plesiosaurierna, och det tredje av de knivlika mosasaurietänderna. Eftersom mosasaurierna överlappar till stor grad gällande formen på tänderna, men skiljer sig mycket åt i storlek, så är det troligt att det snarare var storleken på bytesdjuren, och inte vilken typ, som skiljde dem åt. Framtida studier skulle kunna ge en bättre bild av den här faunans ekosystem dels genom att inkludera fler exemplar i analysen, och dels genom att inkludera andra faktorer från studier av liknande taxa, såsom käkstorlek, bitmärken och maginnehåll.