搞不懂句子成分

题目

搞不懂句子成分
A common case was when shellfish fossils found across Europe were said to be imprints of the Devil's Toenail.
这个when shellfish 怎么能直接加was呢?when后面的句子怎么划分句子成分?

答案

大句子是主系表结构：
主语：a common case；was之后直到末尾是表语从句,由疑问副词when做引导词.对于疑问副词引导的表语从句,应熟悉.比如：This is where I was born.（这就是我出生的地方）.
从句中：
主语：shellfish fossils,之后的found across Europe是一个过去分词做主语的后置定语,表示被动.
were said to be直到句末的应该能看懂了吧,"据说是..."