Population level measures of skeletal pathologies in Mesozoic marine reptiles (Ichthyosauria)
Final Report Abstract
Pathologies record evidence of disease or injury incurred during the lifetime of an organism. In fossil vertebrates, detection is limited to damage affecting mineralized tissues (i.e., the skeleton). Pathologies have been described in Mesozoic reptiles, including reports of trauma and infection, however in ichthyosaurs (marine reptiles) these reports are rare. The goal of this project was to examine the range and distribution of pathologies across the ichthyosaurian skeleton, and assess the influence of individual age and body size on the prevalence of pathologies, as well as changes in prevalence over geological time. We surveyed 236 ichthyosaurs from the Early Jurassic Posidonia Shale of Germany, and identified a broad range of pathological damage to the skeleton including traumas, joint ankyloses, articular disease, and infection. Damage most frequently affected the skull and ribs. The prevalence of pathologies differed between genera, with Stenopterygius, the most abundant ichthyosaur with an adult body size 2-4 m in length, showing a significantly lower incidence of skeletal disease and trauma than all other ichthyosaurs from this time period. In the large apex predator Temnodontosaurus, almost ¼ of all skulls showed evidence of traumatic injury. Large adults of Stenopterygius were more likely to show evidence of skeletal damage than small adults or juveniles. A minor mass extinction event linked to increased ecological stress occurring during this time interval did not affect prevalence of pathologies. This portion of the project provided important baseline data, creating a standard against which the prevalence of pathologies in other geological time intervals can be compared. Previous authors have suggested that competition and predation pressure in the marine environment intensified in the Jurassic relative to the Triassic. We examined this question by comparing prevalence of pathologies between Early Jurassic and Middle Triassic ichthyosaurs. We found that the overall prevalence of pathologies was relatively constant between the two samples, despite the prevailing view in the literature that pathologies in Triassic ichthyosaurs are uncommon. However, the distribution of pathologies within the skeleton differed. In Early Jurassic ichthyosaurs, pathologies were concentrated at the front of the body (head, forelimbs, trunk), and were rare to absent in the hind limb and tail. Traumatic damage to the rib cage was frequent. In the Triassic ichthyosaurs, the prevalence of pathologies in the hind limb and tail was similar to that at the front of the body, and traumatic damage to the ribs was absent. We attribute these differences to changes in body shape and swimming style between the more elongated Triassic and fusiform Jurassic ichthyosaurs. We examined a pathological femur of a Triassic ichthyosaur using µCT, in order to obtain an accurate diagnosis. This femur shows characteristics of Paget’s disease of bone. If confirmed, this represents the oldest record of this condition. Lastly, we assessed the prevalence of pathologies in bones of ichthyosaurs from the Middle Jurassic that were completely free of sediment, to understand potential bias in the detection of pathologies associated with slab-mounted specimens. Most of the pathologies observed were visible on multiple surfaces of the affected bones, so we consider bias from this source to be minor. The prevalence, type, and distribution of pathologies was generally similar to that in the Early Jurassic Stenopterygius; however avascular necrosis (a pathology attributed to diving behaviour) was present in the Middle Jurassic but not the Early Jurassic sample. In conclusion, this survey provided detailed insights into the types and distribution of pathological bone modifications in Mesozoic marine reptiles. Methods developed during this project can be applied to a wide range of vertebrate groups.
Publications
- 2018. Ichthyosaurian palaeopathology: evidence of injury and disease in fossil 'fish lizards'. Journal of Zoology 304: 21-33
Pardo-Pérez, J. M., Kear, B.P., Gómez, M., Moroni, M., and Maxwell, E.E.
(See online at https://doi.org/10.1111/jzo.12517) - 2018. Pathological survey on Temnodontosaurus from the Early Jurassic of southern Germany. PLoS ONE 13: e0204951
Pardo-Pérez, J. M., Kear, B.P., Mallison, H., Gómez, M., Moroni, M., and Maxwell, E.E.
(See online at https://doi.org/10.1371/journal.pone.0204951) - 2019. Palaeoepidemiology in extinct vertebrate populations: factors influencing skeletal health in Jurassic marine reptiles. Royal Society Open Science 6:190264
Pardo-Pérez, J. M., Kear, B.P., and Maxwell, E.E.
(See online at https://doi.org/10.1098/rsos.190264)
