It was not until the Devonian period, about 420 million years ago, that fish evolved jaws; before that, fish had to suck in their prey with their mouths. By now, only two types of fish are jawless — lampreys and hagfish. So what happened to the others? Until now, scientists have speculated that they died out rapidly because the jawed fish were much more efficient predators. But a new study in the journal Nature suggests that this is unlikely. Even after sophisticated jaw types emerged, both types of fish coexisted well for at least 10 million to 20 million years. “If they couldn’t compete, they probably should have gone out when the jawed fish reached their functional peak,” said the study’s lead author, Philip Anderson, an evolutionary paleobiologist at the University of Bristol in England. He and his co-authors studied fossils from around the world to better understand the emergence of jaws. Within 10 million years of the initial appearance of jaws, a set of diverse, distinct jaw types emerged. “There were slender jaws with sharp, pointy teeth, good for quickly snapping at evasive prey,” Dr. Anderson said, “and jaws with big, blunt plates, thick and robust and probably more powerful.” Still, jawless fish carried on, perhaps because they were not competing for the same resources. Once you have a jaw, you can do a lot more than suck prey in,” Dr. Anderson said. “You can grab prey and you can chew it; you can do a lot of things.” As for most of the jawless fish, “they died out for another reason,” he added. “We’re not sure what.”