Deep in the Silurian seas, some 420 million years ago, a strange structure had just emerged in the bodies of many new vertebrates. Some fish began developing a defined upper and lower jaw that allowed them to devour large and hard-shelled organisms.

Today more than 99 percent of vertebrates have these handy eating apparati. But new research shows that for all of their utility, mandibles did not take over the oceans in one swift chomp, as many scientists had previously assumed. Rather, the early, jawed fish didn't make too much of a dent in their jawless compatriots' success for some 30 million years. The new findings were published online July 6 in Nature (Scientific American is part of Nature Publishing Group).

And "when the jawless fishes do decline, we see no indication that their jawed cousins took up new functional roles," Philip Anderson, of the University of Bristol's School of Earth Sciences and co-author of the new study, said in a prepared statement. That finding has theoretical implications that reach far beyond the layers of Silurian sediment. It calls "into question old ideas of ecological replacement," that as a niche opens up, other species will fill it, he noted.

After analyzing fossilized Silurian and Devonian mandibles from 198 genera for different functional traits, the researchers also found that the emergence of this revolutionary mechanism did not set off a wild spree of evolutionary experimentation in design. Instead, after an initial bump of diversification—and a few exceptional mandibles in extinct placoderms and lungfishes—the major groups of jawed creatures whose descendents persist today, such as the ray-finned fishes and early tetrapods, seemed to stay relatively uniform. And all of this stability persisted despite "fluctuating global temperatures, shifting continental weathering patterns, sea level changes and a major extinction event," the researchers noted in the new paper.

Image of early fish jaw fossils, which show more diversity in lineages that later went extinct than in those that produced many of today's fish and terrestrial vertebrates (bottom two); courtesy of Simon Powell/Philip Anderson/Matt Friedman