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,

第1句：
主语：triassic oceans
谓语动词：were （是个系动词）
表语：shark-and crocodile-free
状语：mercifully
插入语：by contrast （不作句子的成份）
第2句：
状语：in the Triassic
状语：then
主语：ichthyosaurs
谓语动词：were （是个系动词）
表语：top of the food chain
第3句：
状语：in the Jurassic and Cretaceous
主语：they
谓语动词：were （是个系动词）
表语：prey as well as predator
并列谓语动词：had to make （和were是并列关系）
宾语：a speedy exit
状语：as a result
句子可以翻译如下：
相比之下,三叠纪的海洋既无鲨鱼,也无鳄鱼,可谓善心大发.于是在这一时期,鱼龙处于食物链的顶端.在侏罗纪和白垩纪,它们既捕食,也会被捕食,因此（一旦得手）它们就得快速撤离.

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