Autecology, lifestyle and hydrodynamics of early vertebrates (autoecología, modo de vida e hidrodinámica de los primeros vertebrados)

Zusammenfassung

The fossil record of vertebrates is crucial for unravelling the early evolution of the clade, including several groups of Palaeozoic jawless and jawed stem-gnathostomes (i.e., conodonts, pteraspidomorphs, anaspids, thelodonts, galeaspids, pituriàspids, osteostracans and placoderms). Most of them have been broadly studied from taxonomical, biostratigraphic and palaeogeographic perspectives but our knowledge about their lifestyles and habitats is still very limited, which hinders to reach a good understanding about the ecological scenarios that underlid the main evolutionary transitions of the group. First inferences on the ecology of stem-gnathostome groups were collaterally derived from sedimentological studies or anatomical descriptions where morphofunctional comparisons were occasionally provided. However, most of these first attempts suffered from important potential biases and limitations and/or relied to a large extent on conjecture and speculation. In the latter two decades, the emergence of new experimental techniques and computational approaches, as well as the development of novel tools for statistical computation, has provided new opportunities to perform rigorous examination to test ecological hypotheses in palaeontology. Within this new wave of studies, this thesis aims to increase our knowledge on the ecology and lifestyles of stem-gnathostome vertebrate groups through the application of novel multidisciplinary approaches comprising a combination of both traditional and state-of-the-art techniques in palaeobiology. For this three main lines of research have been developed. Firstly, the relationship between the scale morphology and function and between squamation patterns and ecology has been assessed in extant sharks by means of multivariate morphometrics. This has allowed establishing a useful comparative framework for basing ecological inferences in extinct groups with similar squamations. Based on that, here it is provided a comprehensive overview on the lifestyles of thelodonts, revealing that this group displayed a remarkable ecological diversity. This information has been later used to explore, through the application of phylogenetic comparative methods, the ecological diversification and palaeobiogeographic history of the whole clade. This approach supports that thelodonts originated in marine shallow waters and underwent a complex ecological evolutionary history, comparable in many aspects to those of major extant groups of fishes. Further, the established framework has been proved as a useful tool for assessing ecological aspects also in other extinct and extant groups with micromeric squamations, as demonstrated by two case studies here presented. Secondly, the ecomorphology of the caudal region of living sharks is explored by means of geometric morphometrics, quantifying the shape change in relation to ecological traits and body size, and establishing thus the basis for inferring morphological aspects from palaeoecological data in extinct early vertebrates, and viceversa,. A postcranial reconstruction of the emblematic placoderm Dunkleosteus terrelli, more parsimonious with the current knowledge on its palaeoecology, is provided as a proof of concept. Finally, the morphological disparity of the osteostracans, the sister group of all jawed vertebrates, is explored from temporal and phylogenetic perspectives by different quantitative methods, and interpreted in functional and hydrodynamic terms through the novel application of computational fluid dynamics. These analyses reveals the presence of adaptations for passively controlling the flow around their body possibly conferring more manoeuvrability and versatility for adapting to different locomotory strategies than previously though. These findings contradict the generalised view that jawless stem-gnathostomes were largely sessile and benthic, and face the origin of jaws structures as the key innovations that promoted the ecological diversification of the group.