Rib microstructure in thunniform ichthyosaurs and toothed whales

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, and quantitative microanatomical values were collected to enable a comparison to other ichthyosaurs. The ichthyosaur ribs preserve alternating vascular and avascular bands resulting from localized growth variation, but no unambiguous growth marks, whereas gastralia seem more promising for skeletochronology. The toothed whale ribs preserve no growth marks. Ichthyosaur skeletal elements often have a spongy internal structure without a thick cortex, but the ophthalmosaurid ribs in this study have relatively compact bone throughout, compared to the cetaceans, something that is possibly an adaptation to counter ballast the buoyancy of the lungs.