article_snippet.sent: On the left in this image are fossil examples used, and on the right are living examples, with whales and sharks shown top and bottom respectively .
article_snippet.sent: Shown are the top here are echinoids (sea urchins), and at the bottom are clams. 
article_snippet.sent: Clams and mussels - so-called bivalves - had about one-tenth the extinction risk of mammals .