Palaeopathology part III: Triassic vs. Jurassic

Was life easier in the past?

The fossil record of vertebrates consists mostly of the skeleton, but the bones of extinct animals can tell us a lot about how they lived and died. In particular, injuries and diseases incurred during an animal’s life can leave marks on the bones, and provide us with information about behaviour and overall health. We noticed that injuries in ichthyosaurs, were very common in museum collections, and wondered if these injuries could tell us anything about the behaviour of these Mesozoic marine reptiles.

Jurassic

Dr. Judith Pardo-Pérez, a former postdoctoral researcher at the SMNS, carefully surveyed all ichthyosaur skeletons from the Early Jurassic Posidonia Shale (~182 million years old) and categorized the amount, type, and distribution of skeletal damage she could observe.

We analyzed these data, and discovered that the distribution of pathologies across the skeleton and across species was non-random- for instance, almost 25% of skulls of the top-predatorTemnodontosaurus showed lesions on the skull thought to be bite marks inflicted by other members of the same species, but in the abundant mid-sized ichthyosaur Stenopterygius, only 4% of the skulls had been injured during the life of the animal. Ribs that had been broken and had started to heal were found in all species, but injuries or bone diseases such as arthritis affecting the vertebral column were almost non-existent.

Triassic

We then wondered – do we see the same pattern in ichthyosaurs from other time periods? Skeletal injury and disease are rarely reported in Triassic marine reptiles, leading some palaeontologists to suggest that marine ecosystems were less dangerous and competitive in the Early Mesozoic. We tested this idea by comparing the prevalence of skeletal injury and disease in a sample of ichthyosaurs from the Middle Triassic (~240 million years ago) of Monte San Giorgio, Switzerland to that observed in the Early Jurassic ichthyosaurs from southwestern Germany. Judith collected the data in the exact same way as in the first part of the study, to help control for bias between different observers. We then tabulated and analyzed the data. Overall, we found that skeletal injuries were present at similar frequencies in Triassic and Jurassic species; Triassic ichthyosaurs did not have better skeletal health than their Jurassic descendants. We could therefore reject the hypothesis that competition and predation pressure in the marine environment, causing injury and skeletal disease in ichthyosaurs, increased in intensity between the Triassic and Jurassic.

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.