Uranium-lead dating has been used to place a date of 252.6 million years ago on the end of the [[Permian|Permian period]] and the beginning of the [[Triassic|Triassic period]] with a margin of error much smaller than other dating methods had given.<ref>http://www.physorg.com/news1197.html</ref> | Uranium-lead dating has been used to place a date of 252.6 million years ago on the end of the [[Permian|Permian period]] and the beginning of the [[Triassic|Triassic period]] with a margin of error much smaller than other dating methods had given.<ref>http://www.physorg.com/news1197.html</ref> |