Mazon Creek

Mazon Creek in Illinois is a fossil locality where animals from the age of the coal forests are preserved inside round nodules of iron carbonate. Iron carbonate nodules formed rapidly around carcasses in a brackish delta and sealed them before decay set in. The fauna includes Tullimonstrum, the Tully monster, whose place in the tree of life is still disputed after sixty years — read in turn as a vertebrate and as something else entirely. The site is one of the few places showing what the coal forests actually teemed with, rather than only which trees became coal.