Rib microstructure in thunniform ichthyosaurs and toothed whales
Abstract
The inner microstructure of aquatic amniote ribs can reveal adaptations to physiology and locomotion, but are relatively poorly known for ichthyosaurs, especially ophthalmosaurid taxa whose body shape converges to modern odontocetes. In this study, we describe the microstructure of ribs and gastralia from two Late Jurassic ichthyosaurs ( † Palvennia hoybergeti and † Keilhauia sp.) and compare them to extant odontocetes (Delphinapterus leucas and Phocoena phocoena). Ribs were sampled at proximal, midshaft, and distal locations to assess the potential for reliable growth marks. The ribs show growth bands, but no unambiguous growth marks, whereas gastralia seem more promising for skeletochronology. Ichthyosaur skeletal elements often have a spongy internal structure with a lack of a thick cortex, and quantitative histomorphometric values were collected to enable a comparison to other late ichthyosaurs. The ophthalmosaurid ichthyosaur ribs have relatively compact bone throughout, compared to the cetaceans, which is possibly an adaptation to counter ballast the buoyancy of their lungs.