英语翻译Whales that suffer the bends often do so because they ha

英语翻译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.】请分析【】里的句子结构,我觉得很乱,然后标准翻译一下,