A cool mini-doc on the placoderms

While we are on Placoderm Week (more fun than Shark Week!) here's a great episode from PBS' Eon series, exploring the development of life over, well, eons.  This interesting program posits that placoderm armor was not just for defense, but provided a reservoir for calcium the body needed by the cartilage skeleton couldn't store.  I'm not sure about this, given that sharks get along just fine without any bone, but it's an intriguing idea, and the whole show is very cool.