From New Scientist, 27 October 2012:
VAST swathes of Earth were literally too hot for life 249 million years ago, at the start of the Triassic period. So extreme was the heat that it killed off many tropical species.
Paul Wignall at the University of Leeds, UK, and his colleagues studied the ratio of oxygen isotopes in marine fossils called conodonts. (A fossilised conodont element is shown in the picture). The ratio is linked to climate, allowing the team to deduce that sea temperatures hit 40 °C in the early Triassic.
Such lethal heat may have been one legacy of the huge end-Permian extinction a few million years earlier. It left Earth with few plants to remove atmospheric carbon dioxide. The resulting Triassic warming wiped out tropical fish and marine reptiles (Science, doi.org/jj7).