Heavy ribs may have helped ichthyosaurs to dive

Two Late Jurassic ichthyosaurs from Spitsbergen were found to have unexpectedly dense ribs. In all the sections examined, these were more compact than the ribs of modern beluga whales and porpoises, with which the researchers compared the fossil reptiles.

The authors of the study suggest that the heavy ribcage may have acted as natural ballast: reducing buoyancy near the air-filled lungs and helping the ichthyosaurs to direct their bodies downwards at the start of a dive. This is a possible explanation, rather than a directly proven function of the ribs.

Details

The study was conducted by Jørgen Krøglid, Mathieu Gabriel For-Brac and Lene Libe Delsett from the Natural History Museum at the University of Oslo. The paper has been published in the peer-reviewed journal PeerJ.

The researchers studied adult specimens of two ichthyosaurs — Keilhauia sp., approximately 4.5 metres long, and Palvennia hoybergeti, approximately 5 metres long. Both specimens were found on Spitsbergen and excavated in 2011. For comparison, ribs from a modern porpoise and a beluga whale from the museum’s collection were used.

Samples were taken from three locations on each rib: closer to the spine, in the middle, and closer to the tip. Thin sections were examined under a microscope, and the internal structure was quantitatively assessed using bone compactness analysis.

The difference proved to be significant. The observed compactness index for ichthyosaurs ranged from 0.750 to 0.887, whilst for the two toothed whales it ranged from 0.526 to 0.691. In other words, even the most porous of the ichthyosaur sections examined was more compact than the densest section of a modern whale.

At the same time, ichthyosaurs themselves were generally not animals with a massive, heavy skeleton. The authors therefore consider the dense ribs to be a local specialisation of the thoracic cage rather than a general increase in body mass.

A possible explanation relates to the mechanics of swimming. In whales, the spine bends mainly up and down, whilst the tail fin is positioned horizontally. In ichthyosaurs, the tail was vertical, and the movement of the spine was predominantly lateral. Therefore, the additional weight at the front of the body may have been beneficial precisely at the moment when the animal began to descend from the surface into deeper water.

The cross-sections also preserved details of bone growth. In the ribs of Keilhauia, zones corresponding to changes in growth rate were discovered, but the researchers do not consider them to be reliable annual growth rings. Gastralia—the so-called abdominal ribs—proved to be more promising for determining age, as growth arrest lines were preserved in Palvennia.

Why this is important

Ichthyosaurs and modern whales have independently followed a similar evolutionary path: their ancestors lived on land, and their descendants subsequently adapted fully to the sea and developed a streamlined body shape. However, the new study shows that the same external outcome does not imply the same internal structure.

Thick ribs may be one such differing adaptation. They help explain how ichthyosaurs were able to control their buoyancy given their specific swimming mechanics.

However, the conclusions cannot yet be generalised to all ichthyosaurs. The study used only two fossil specimens and two modern toothed whales, with a limited set of ribs analysed from each animal. The authors explicitly note the small sample size and the need to study more species and individuals. Furthermore, the role of the ribs as ballast remains a functional hypothesis: it is impossible to directly verify the behaviour of these extinct animals.

Source

Jørgen Krøglid, Mathieu Gabriel Faure-Brac, Lene Liebe Delsett. Rib microstructure in thunniform ichthyosaurs and toothed whales. PeerJ, 2026, 14:e21486. DOI: 10.7717/peerj.21486.

A popular science summary of the findings, prepared by the University of Oslo: Fossil ribs reveal new secrets about the swimming technique of ichthyosaurs.