One of the best known early sharks, 3-foot-long Cladoselache hunted in the warm seas of North America 375 million years ago. It had a markedly weaker jaw joint than present-day sharks, but it compensated with impressive jaw-closing muscles that are frequently fossilized. Caldoselache had a deep, forked tail and was probably a fast swimmer, which is just as well since it may have been preyed upon by the giant 15-foot armored fish known as Dunkleosteus.