If ichthyosaurs did(此处did表示强调） evolve an anti-decompression means,they clearly did so(so指的是an anti-decompression means) 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.
If ichthyosaurs didi evovle.means,-----条件状语从句
they clearly didi so quickly and they lost it afterwards----主句
and most strangely---插入语
But that is not what (Dr Rothschild thinks) happend----括号中的成分同上.
看懂了句子结构翻译就不成问题,看来你是专门学习生物的,我对生物知识不了解,就不班门弄斧了.翻译就交给你自己了.