Long before whales ruled the oceans, dolphin-shaped marine reptiles called ichthyosaurs patrolled the same waters. New research from the Natural History Museum at the University of Oslo suggests their rib cages may have done a job modern whale ribs don’t: acting as ballast to help them dive. The findings appear in the journal PeerJ.
Cryopterygius kristiansenae, a species of ophthalmosaurid ichthyosaur from the Jurassic of Svalbard. Image credit: Esther van Hulsen.
“Ichthyosaurs were secondarily aquatic reptiles that dominated the world’s oceans through much of the Mesozoic Era, with a fossil record spanning from the Early Triassic (250 million years ago) to their final demise in the early Late Cretaceous (93 million years ago),” said senior author Dr. Lene Liebe Delsett and and colleagues.
“They are among the few aquatic amniotes to acquire a fully aquatic lifestyle in the open ocean, matched only by that of modern cetaceans.”
“In both groups, the skeleton underwent extreme modifications compared to their land-dwelling ancestors, and consequently, these groups are important for understanding convergent evolution of secondarily aquatic adaptations.”
“We investigate physiological data recorded in the microstructure of ribs and gastralia to answer two questions,” they added.
“First, did ichthyosaurs have a high and consistent growth rate?”
“Second, as cetaceans are commonly seen as the modern analogues of ichthyosaurs, do their rib microanatomy and histology converge?”
In their study, the authors examined the internal microstructure of ribs from two ichthyosaur fossils unearthed on the Norwegian archipelago of Svalbard, comparing them to the ribs of two living toothed whales, the beluga and the harbor porpoise.
These ichthyosaur individuals, belonging to the genera Keilhauia and Palvennia, lived roughly 150 million years ago during the Late Jurassic period.
They had body shapes and predatory lifestyles that closely mirror those of today’s dolphins and porpoises, a phenomenon scientists call convergent evolution.
Despite the outward resemblance, the researchers found a clear difference beneath the surface.
They determined that the ichthyosaur ribs were considerably denser and more compact than those of the beluga and porpoise, which have a more open, spongy structure inside.
“Ichthyosaur skeletons are in general quite light, but the ribs are not,” said Dr. Jørgen Krøglid, first author of the study.
“This was a bit surprising to us. Compact bones are typically found in animals that move slowly, such as manatees,” Dr. Liebe Delsett said.
The scientists propose that the added weight in the ichthyosaurs’ ribs may have helped counteract the buoyancy created by their air-filled lungs, making it easier for the animals to descend and stay submerged.
“Ichthyosaurs descend from land-living reptiles, and their spine has a sideways movement, like in lizards. Their tail is vertical, like in sharks and fish,” Dr. Krøglid said.
“Therefore, ichthyosaurs need a bit of help to get the right angle when they dive. A little extra weight around the air-filled lungs may not be such a bad idea.”
“I imagine that the slightly heavier ribs can counteract this buoyancy and help ichthyosaurs initiate their dive. A bit like divers use weight belts. Once they get that downward angle, they can just keep swimming.”
“In the big picture, ichthyosaurs and toothed whales are a very good example of convergent evolution, first and foremost when it comes to body shape.”
“But when we study microanatomy and cell structure, it’s obvious that there are differences.”
The authors also looked for signs of the animals’ growth rates and ages by examining bands and lines within the bone tissue itself.
The ichthyosaur ribs showed alternating layers reflecting periods of faster and slower growth, consistent with earlier evidence that ichthyosaurs grew quickly and may have had an elevated, warm-blooded-like metabolism.
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J. Krøglid et al. 2026. Rib microstructure in thunniform ichthyosaurs and toothed whales. PeerJ 14: e21486; doi: 10.7717/peerj.21486
