Fossil ribs reveal ichthyosaur swimming adaptations
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Jørgen Krøglid and Lene Liebe Delsett at the University of Oslo studied ichthyosaur fossils from Svalbard, a Norwegian archipelago, to understand how the ancient reptiles swam.
The researchers examined ultra-thin slices of ichthyosaur ribs and compared them to the bones of modern toothed whales, porpoises and belugas, using CT scans and microscopes. They discovered that ichthyosaur ribs were unexpectedly dense and heavy, and the ichthyosaurs possessed more ribs than whales.
This heavier rib cage likely helped ichthyosaurs overcome buoyancy challenges when diving. Unlike whales, which descend from four-legged mammals with up-and-down spinal movements, ichthyosaurs evolved from reptiles with side-to-side spinal movements. This difference meant ichthyosaurs needed extra weight around their lungs to achieve the correct angle for diving, similar to how modern divers use weight belts. Delsett’s project, “ECHO. Evolutionary Convergence in Historical Oceans,” investigates how different species arrive at similar body shapes.
The research confirms that while ichthyosaurs and whales share a streamlined, fish-like form, their internal structures reveal distinct evolutionary paths. The team gained access to cellular-level details within the 150-million-year-old fossils, allowing them to compare the ancient bone structure to that of living whales.
