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AFP A mass extinction of prehistoric fish some 360 million years ago cleared the path for the evolution of modern vertebrates, a study published Monday has found. 
"Everything was hit; the extinction was global," said lead author Lauren Sallan of the University of Chicago."It reset vertebrate diversity in every single environment, both freshwater and marine, and created a completely different world." While the mass extinction at the end of the Devonian period is among the five most significant in the earth's history, researchers are not certain as to how it was triggered. It is believed to have occurred in a series of events over the course of 20 to 25 million years and accounted for the extinction of about 20 percent of all animal families and 70 to 80 percent of all animal species. There is evidence of substantial glacier formation during the period, which would have dramatically lowered sea levels in the "Age of Fishes," and the first appearance of forests might also have produced catastrophic atmospheric changes. The armed placoderms and lobe-finned fish which dominated the Devonian Period were replaced by ray-finned fish, in a demographic shift that persists to this day. "There's some sort of pinch at the end of the Devonian," said co-author Michael Coates, a biologist and anatomist at the University of Chicago. "It's as if the roles persist, but the players change: the cast is transformed dramatically. Something happened that almost wiped the slate clean, and, of the few stragglers that made it through, a handful then re-radiate spectacularly." By analyzing the vertebrate fossil record, Sallan and Coates were able to pinpoint a critical shift in diversity to the Hangenberg extinction. While the earliest four-limbed creatures, or tetrapods, made the first tentative steps toward a land-dwelling existence prior to this extinction event, there was a 15 million year stretch of the fossil record almost barren of tetrapods. "Something that's seen after an extinction event is a gap in the records of survivors," Sallan said. "You have a very low diversity fauna, because most things have been killed off." The limited number of tetrapods which survived the extinction were likely the early ancestors of the vast majority of modern land vertebrates, the authors speculated. "Extinction events remove a huge amount of biodiversity," Coates said. "That shapes in a very significant way the patchiness of biodiversity that persists to the present day." The study was published in the Proceedings of the National Academy of Sciences.
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