大句子是主系表結構：
主語：a common case；was之后直到末尾是表語從句,由疑問副詞when做引導詞.對于疑問副詞引導的表語從句,應熟悉.比如：This is where I was born.（這就是我出生的地方）.
從句中：
主語：shellfish fossils,之后的found across Europe是一個過去分詞做主語的后置定語,表示被動.
were said to be直到句末的應該能看懂了吧,"據說是..."