A recent study published in the journal PeerJ has uncovered new details about how ichthyosaurs, ancient marine reptiles, swam in the oceans. Researchers Jørgen Krøglid and Lene Liebe Delsett from the Natural History Museum at the University of Oslo compared the microscopic structure of ichthyosaur ribs to those of toothed whales, including porpoises and belugas. Using CT scans and microscopes, they examined ultra-thin slices of the bones to analyze their cellular structures and understand how these animals moved through water. Ichthyosaurs lived during the Mesozoic Era, about 250 to 90 million years ago, and were known for their fish-like bodies, which were well-suited for fast swimming. However, the study found that their ribs were more solid and less porous than those of whales. This makes ichthyosaur ribs heavier. "Whale ribs have larger cavities, making them lighter," Krøglid explained. "Ichthyosaurs also had more ribs, which added to their overall weight. Their rib cages were significantly heavier." To understand why ichthyosaurs had such heavy ribs, the researchers looked at the evolutionary backgrounds of both ichthyosaurs and whales. Both animals have lungs, which can make them float after surfacing. However, whales have a spine that moves up and down, similar to other mammals, and a horizontal tail, which helps them dive easily. "The movement of their spine and tail allows them to tip their body forward and descend effortlessly," Krøglid said. In contrast, ichthyosaurs evolved from land-dwelling reptiles and had a spine that moved sideways, like lizards. Their tails were vertical, similar to those of sharks and fish. This made it harder for them to get into a proper diving position. "The heavier ribs might have helped them counteract the buoyancy from their lungs, allowing them to start their dive. It's like divers using weight belts to get into the right position," Krøglid explained. The study highlights that while ichthyosaurs and whales evolved similar fish-like body shapes independently, their internal structures differ significantly. "They are a great example of convergent evolution, where different species develop similar traits," Delsett said. "But when you look at the microanatomy, like the structure of their bones, the differences become clear." This research adds to our understanding of how ancient marine life adapted to life in the ocean.