Histological and remodelling patterns among neosauropods in insular environments

Sauropods are the long-necked dinosaurs best known to have evolved the largest
terrestrial animals that ever walked the Earth, typified by the iconic Diplodocus,
Brachiosaurus, and Apatosaurus. They span the entire dinosaurian era from the Late
Triassic to the K/Pg mass extinction and became a major component of the herbivory guild for most of the Jurassic. Given the gigantic dimensions they reached, sauropods became a prime target for bone histology – the study of bone tissue at the cellular level – to understand how such terrestrial animals could be exist. The extensive histological investigation of those dinosaurs documented a fast and uninterrupted growth that appears uniform throughout their evolutionary tree, which allowed the erection of histological tools of high resolution and fidelity for the clade. Under such phylogenetically constrained histology, any significant departure in the histological patterns represents virtually a palæopathological or evolutive signal. Indeed, the former is demonstrated in this thesis by the histological investigation of the basal sauropods cf. Isanosaurus and Spinophorosaurus nigerensis that revealed a palæopathological origin of an unusual bone tissue, and the latter signal is highlighted in the original histological description of the diminutive insular titanosaurs Atsinganosaurus velauciensis and Garrigatitan meridionalis from the Chalk Sea Archipelago. In concordance with other small insular titanosaurians, they appear curiously mature despite their reduced body size in comparison with more ‘standard’ representatives of the clade, and most notably share a particularly heavy bone remodelling with respect to their body size that remained unexplained since its first observation. Such striking downsizing for the clade in an insular context has been referred to insular dwarfism, however, this raises questions about the evolutionary forces driving the possible instances of reversals in the sauropod gigantism. Unfortunately, an integrated explanation from bone histology to insular palæobiogeography is still lacking to bring a consistent framework to those sauropod pygmies. To address this issue, the present thesis provides a reappraisal of the dwarfed titanosaurs inhabiting the Chalk Sea Archipelago under the prism of bone histology and the development of the IsRule Triforce, that is a new conceptual model aiming to quantify the degree of adaptation of a palæoinsular taxon to its focal island (Insularisation) and over time (Insularisation Potential). From comparisons with better constrained insular dwarves, life history theory, and body mass scaling relationships, it appears that those dwarfed titanosaurs scaled down through neotenic pædomorphism, which is a developmental heterochrony referring to a slowdown in growth compared to the ancestor that promoted the heavy remodelling described in those species. The IsRule Triforce contextualises this convergent evolution towards neotenic insular dwarfism into efficient island colonisation events and subsequent local adaptation. Over the course of immigration to the Chalk Sea Archipelago to the Maastrichtian Dinosaur Turnover, the Insularisation Potential of immigrant titanosaurian populations appear to be largely dictated by their Survivability (physiological properties including anatomical traits, reproductive strategies, gregariousness, etc., making them prone to survive locally). Both Adaptability (available resources, predative pressure, intra- and interspecific competition, etc., that allow populations to establish themselves at the longer term and engage adaptive processes) and Dispersability (island isolation and size, climate, etc., detailing the population’s migration potentials) enhance it during low sea levels but crash upon the arrival of new immigrants during the latest Cretaceous since the dwarf islanders got most probably outcompeted, pushing them to extinction. This first application of the IsRule Triforce demonstrates its comparative strengths between insular populations, even more so when considering long extinct species that thrived in a radically different world from a palæogeographical point of view. Further tests and calibration of the IsRule Triforce could open new research avenues in the palæobiogeographical realm.