Jane Hu in Slate:
Once upon a time, weird fish ruled the world. The oceans teemed with primitive vertebrates that lacked eyes, ears, and even fins. Fish ate by sucking up water and debris from the ocean floor, filtering out the goodies, and then releasing the rest through their gills. Once fish evolved jaws, they began to chomp on more complex plants and animals. Scientists announced today in Nature that Metaspriggina, a primitive fish that lived roughly 505 million years ago, played a key role in the origin of jaws.
Like other fish, Metaspriggina had bones called gill arches to support its gills. But while more primitive fish had seven individual gill arches, scientists found that Metaspriggina had seven pairs of gill arches. This fish did not have full-fledged jaws, but its gill arches, with paired separate bones rather than continuous single bones, were “a staging post in the evolutionary story of vertebrates,” says paleontologist Simon Conway Morris, the study’s lead author. Morris and his co-author, Jean-Bernard Caron, propose that the pair of gill arches closest to the head evolved into the upper and lower jaw bones.Paleontologists have long predicted such a creature, but Metaspriggina is the first fossil evidence that supports the prediction. “Everyone said it should have existed, but it’s never been found,” Morris said. “It looks remarkably like the hypothetical animal that we’d talked about.”