Population level measures of skeletal pathologies in Mesozoic marine reptiles (Ichthyosauria)

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Paleopathology is the study of illness and injury in fossil organisms. Its most spectacular application has arguably been to infer paleoecology, behaviour and trophic relationships in Mesozoic vertebrates. The frequency of traumatic modification in fossilized hard tissues at the population level can reveal important information about ancient ecosystem dynamics, although the ultimate cause of the trauma is often difficult to identify. Critically, a lack of adequate quantitative testing has rendered the overall prevalence of skeletal pathologies in the fossil record unclear, and has led to a dearth of explicit hypotheses. Our current project has undertaken the first quantitative assessment of paleopathologies in ichthyosaurs, a group of dolphin-like secondarily marine reptiles famously documented from the Early Jurassic Posidonienschiefer Formation of southwestern Germany. We compiled a baseline data set of skeletal pathologies from five ichthyosaurian genera over this stratigraphically restricted interval to test for variability in the rate of pathological occurrence and distribution. We observed a broad range of pathologies in the Posidonia Shale ichthyosaur fauna, with injuries to the skull, ribs, gastralia, and forelimb being particularly prevalent. Pathological skeletal modifications were observed in all species. We tested the relationship of skeletal damage during life with body size and ontogenetic status (juvenile vs. adult). Results show that pathologies are more frequent in adults than in juveniles, and are relatively more frequent in larger than in smaller species. However, these preliminary observations are not entirely consistent with the published record of ichthyosaur skeletal pathologies. For instance, the frequency of articular disease is much lower in the Posidonia Shale ichthyosaur fauna than in other geological units, and the incidence of traumatic injuries to the ribs appears to be much higher. In order to evaluate this data more rigorously, and correlate through both space and time, we propose to survey the frequency of osteopathologies in comparable ichthyosaur faunas, ranging from the Middle Triassic through to Early Cretaceous. We hypothesize that the relative frequency of pathologies will remain constant across successive time bins, and is likely more intrinsically influenced by body size and ontogeny. This project extension will establish a novel paradigm for inferring paleobiology from preserved traces of injury and disease in fossil vertebrates.