See a great white shark while scuba diving, and you'll want to flee, even if surfacing quickly may cause decompression sickness – the bends. In the dinosaur era, marine reptiles had a similar problem.
The dolphins of their day, ichthyosaurs were marine reptiles that breathed air and swam at high speeds. But if they moved from deep to shallow water too fast, dissolved gases in their blood formed bubbles, impeding circulation and causing damage – evident in their scarred bones.
Ichthyosaurs from the late Jurassic and Cretaceous suffered the bends, says Bruce Rothschild at the University of Kansas in Lawrence, but it was not clear if their Triassic predecessors did too.
Analysing thousands of ichthyosaur fossils from all three periods, Rothschild and his team found that virtually none from the Triassic got the bends, compared with 15 to 20 per cent for the later periods.
Why were later ichthyosaurs more prone to the bends? Rothschild says the rise of big marine predators in the Jurassic could have been to blame: the ichthyosaurs may have got the bends while fleeing for their lives.
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