Bruce Rothschild of the University of Kansas knew all this when he began a study of ichthyosaur bones to find out how widespread the problem was in the past.What he particularly wanted to investigate was how ichthyosaurs adapted to the problem of decompression over the 150 million years.To this end, he and his colleagues traveled the world's natural-history museums, looking at hundreds of ichthyosaurs from the Triassic period and from the later Jurassic and Cretaceous periods.
When he started, he assumed that signs of the bends would be rarer in younger fossils, reflecting their gradual evolution of measures to deal with decompression.Instead,he was astonished to discover the opposite.More than 15% of Jurassic and Cretaceous ichthyosaurs had suffered the bends before they died,but not a single Triassic specimen(标本)showed evidence of that sort of injury.
If ichthyosaurs did evolve an anti-decompression means, they clearly did so quickly—and, most strangely, they lost it afterwards.But that is not what Dr Rothschild thinks happened.He suspects it was evolution in other animals that caused the change.
Whales that suffer the bends often do so because they have surfaced to escape a predator (捕食动物)such as a large shark.One of the features of Jurassic oceans was an abundance of large sharks and crocodiles,both of which were fond of ichthyosaur lunches.Triassic oceans,by contrast,were mercifully shark- and crocodile-free.In the Triassic, then, ichthyosaurs were top of the food chain.In the Jurassic and Cretaceous,they were prey(猎物)as well as predator—and often had to make a speedy exit as a result.
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