Scientists probing a vast crater off Mexico's Yucatan peninsula questioned a popular theory about dinosaurs on Monday, saying the collision that formed the crater happened too far back in time to have caused their extinction by itself. Much evidence points to the idea that an asteroid or comet gouged the Earth around 65 million years ago, triggering volcanic and climate changes that eventually wiped out the dinosaurs. When the huge, mostly underwater crater was found off Yucatan, it seemed the perfect candidate. "Since the early 1990s the Chicxulub crater on Yucatan, Mexico, has been hailed as the smoking gun that...