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—

Triassic oceans/（ by contrast插入语）were mercifully / shark- and crocodile-free.
相反的，三叠纪海洋里 （ 仁慈地--可以不译） 没有鲨鱼或鳄鱼。
In the Triassic（ then 插入语） ichthyosaurs were top of the food chain.
在三叠纪，鱼龙处于食物链的最顶端。
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.
在侏罗纪和白垩纪，它们既是猎物，也是捕食者，而且最后一般都快速捕食离开。

你确定要分析【】里的句子结构？看上去很正常啊……

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1. Triassic oceans, by contrast, were mercifully shark- and crocodile-free.

   = By contrast , triassic oceans were (mercifully) shark-free and crocodile-free.

    相反，三叠纪时期的海洋，很仁慈的木有神马鲨鱼或者鳄鱼。

   Note: noun + "-" + "free" is a common form of adjective: e.g. alcohol-free

    

2. In the Triassic, then, ichthyosaurs were top of the food chain.

   ...then 就是个表示“然后呢”的词，理不理它都没事...

   然后呢，在三叠纪时期，鱼龙是食物链顶端的物种。（吐槽:地位堪比贝爷...）

    

3. In the Jurassic and Cretaceous, they were prey as well as predator - and often had to make a speedy exit as a result.

   ...I'm supposing "they" here is to represent the "ichthyosaurs" in the previous sentence...

   在侏罗纪和白垩纪时期，它们（指代鱼龙）既是猎物又是捕猎者——结果经常是不得不匆匆退出（退出什么？我猜是“历史舞台”之类的东西）。

   Note: " - " here is used as a conjuction, linking two sentence.

    

 以上。

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...

相比之下，在三叠纪的海洋，幸运的是没有鲨鱼和鳄鱼。于是，在三叠纪鱼龙处于食物链的顶端。在侏罗纪和白垩纪，它们既是捕猎者，同时也是猎物-结果它们经常要迅速地逃离。