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Rib microstructure in thunniform ichthyosaurs and toothed whales.
Jørgen Krøglid1, Mathieu Gabriel Faure-Brac1, Lene Liebe Delsett1
1Natural History Museum, University of Oslo, Oslo, Norway.
Peerj
|July 11, 2026
Summary
Ichthyosaur rib microstructures show growth variations but lack clear growth marks, unlike gastralia. Ophthalmosaurid ribs are compact, possibly aiding buoyancy control in these marine reptiles.
Area of Science:
- Paleontology
- Comparative Anatomy
- Functional Morphology
Background:
- Rib microstructure in aquatic amniotes offers insights into physiology and locomotion.
- Ichthyosaur rib microanatomy, particularly in ophthalmosaurids, is understudied despite body shape convergence with odontocetes.
- Understanding ichthyosaur bone structure can illuminate their adaptations to marine life.
Purpose of the Study:
- To describe and compare the rib and gastralia microstructure of two Late Jurassic ichthyosaurs with extant toothed whales.
- To investigate the potential for skeletochronology in ichthyosaurian skeletal elements.
- To explore potential functional adaptations reflected in ichthyosaur rib bone compactness.
Main Methods:
- Microstructural analysis of rib and gastralia samples from *Palvennia hoybergeti* and *Keilhauia* sp.
- Comparative analysis with rib microstructures of extant odontocetes (*Delphinapterus leucas*, *Phocoena phocoena*).
- Sampling of ribs at proximal, midshaft, and distal locations for growth mark assessment.
Main Results:
- Ichthyosaur ribs exhibited alternating vascular and avascular bands due to growth variation, but lacked clear growth marks.
- Ichthyosaur gastralia showed potential for skeletochronology.
- Toothed whale ribs did not preserve growth marks.
- Ophthalmosaurid ribs displayed relatively compact bone structure compared to cetaceans.
Conclusions:
- Ichthyosaurian ribs are not ideal for skeletochronology, but gastralia may be suitable.
- The compact bone structure in ophthalmosaurid ribs might be an adaptation for buoyancy compensation.
- This study enhances understanding of ichthyosaurian adaptations and bone histology.
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