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About 380 million years ago in the Devonian period, invasive species reigned rampant in the world ocean as the number of marine animal species plummeted. That ecological crisis has been classed as one of the Big Five mass extinctions. But paleontologists think the main problem was not that existing species died out but that new ones failed to form. And the new paper—published in the January issue of GSA Today—holds invasions responsible for that failure to speciate, at least in some marine animal groups.