What does it all mean?
Despite similarities in the overall prevalence of skeletal injury and disease between the Triassic and Jurassic ichthyosaurs, the part of the skeleton most prone to damage differed in the most abundant ichthyosaurs from each time period (Stenopterygius from the Jurassic to Mixosaurus from the Triassic).
Injuries in Mixosaurus were concentrated in the hind limbs and tail, whereas in Stenopterygius, these regions of the skeleton showed the least damage. We hypothesize that these differences relate to changes in body shape and swimming style between Triassic and Jurassic ichthyosaurs. Triassic ichthyosaurs, including Mixosaurus, had a more elongate body shape, with a small tail fin supported by tightly interlocking vertebrae and neural spines that were frequently affected by traumatic injury and joint disease.
In Jurassic ichthyosaurs, such as Stenopterygius, on the other hand, the tail fin is stabilized with soft tissues and the tail vertebrae are less tightly interlocking, so mechanical stress affecting the tail during swimming did not result in stress to the vertebral column. Changes in body shape may also explain the high frequency of broken and rehealed ribs in Stenopterygius, which were not observed in Mixosaurus: high-speed, tuna-like swimming increases the effectiveness of ramming behaviour during aggressive interactions, and results in more serious traumatic injury to the torso of the victim. We conclude that changes to the ichthyosaurian body plan appear to have had a greater effect on the type and frequency of injuries we observed than ecosystem-level changes.