Researchers in Britain say evidence from 180 million years ago confirms the likelihood of risks to modern marine life from global warming. Higher temperatures and lower oxygen levels during the ancient Jurassic era caused drastic changes to marine communities, they said, and while the Jurassic seas eventually recovered from the effects of global warming, the marine ecosystems that returned were noticeably different from before. "Our study of fossil marine ecosystems shows that if global warming is severe enough and lasts long enough it may cause the extinction of marine life, which irreversibly changes the composition of marine ecosystems," Richard Twitchett of the University of Plymouth said. Twitchett and his colleagues undertook fieldwork along the North Yorkshire coast studying the different sedimentary rocks and the marine fossils they contained, which yielded information about the environmental conditions on the sea floor at the time the rocks were laid down. The researchers said they discovered a "dead zone" recorded in the rock, which showed virtually no signs of life and contained no fossils. This was followed by evidence of a return to life, but with new species. "Patterns of change suffered by these Jurassic ecosystems closely mirror the changes that happen when modern marine communities are exposed to declining levels of oxygen," Twitchett said.