Late Jurassic ichthyosaur ribs from Svalbard were denser than toothed-whale ribs
Ribs from two Late Jurassic ichthyosaurs found on Svalbard were more compact than ribs from a beluga and a harbor porpoise. Researchers at the University of Oslo Natural History Museum read the extra density as ballast that could have helped the reptiles start a dive against the buoyancy of air-filled lungs. That ballast reading is a proposal: ichthyosaurs inherited a lizard-like sideways spine and a vertical tail, which made the first downward angle harder than it is for whales.
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Svalbard ribs were more compact than whale ribs
Sci.News reports a PeerJ study from the Natural History Museum at the University of Oslo on rib microstructure in two Late Jurassic ichthyosaurs from Svalbard, about 150 million years old, compared with a beluga and a harbor porpoise. The fossils belong to the genera Keilhauia and Palvennia. Infobae covers the same comparison of thin sections from two ichthyosaur fossils and two toothed whales. In both accounts the ichthyosaur ribs came out considerably denser and more compact than the more open, spongy whale ribs. First author Jørgen Krøglid says ichthyosaur skeletons are in general quite light, but the ribs are not.[1], [2]
Extra rib weight as a proposed dive ballast
Both reports treat the heavier ribcage as a possible ballast. Ichthyosaurs and toothed whales both have lungs, so they carry buoyancy after surfacing to breathe. In whales the spine bends mainly up and down and the tail is horizontal, which helps tip the front of the body downward. Ichthyosaurs inherited a lizard-like sideways spine and a vertical tail, like sharks and fish, which made that first downward angle harder. Krøglid compares the extra weight around the air-filled lungs with the weight belts divers use. Sci.News notes that compact bones are typically found in slow movers such as manatees, so the density was unexpected in these pelagic reptiles.[1], [2]
Same fish-shaped body, different bone inside
Infobae reports that Lene Liebe Delsett leads the ECHO project on evolutionary convergence between whales and ichthyosaurs, and that the sampled bones come from collections at the Natural History Museum in Oslo. The team used computed tomography and microscopes down to cellular level. Delsett says ichthyosaurs and toothed whales are a very good example of convergent evolution in body shape, but that differences are obvious once microanatomy and cell structure are examined. Sci.News records the same contrast: the outward resemblance to dolphins and porpoises does not extend to the internal architecture of the ribs.[1], [2]
