Mazon Creek

Everything here arrives sealed inside a rock. The Mazon Creek fossils sit in ironstone concretions, lumpy nodules that formed about 309 million years ago in the Francis Creek Shale, and the animal or plant is in the middle of the lump rather than on the surface of a slab.

The chemistry that made them explains why soft bodies survived at all. Remains were buried fast in a muddy bay that lay within about ten degrees of the equator. Bacteria decomposing them released carbon dioxide, which combined with iron dissolved in the groundwater to precipitate siderite in a shell around the body. The nodule set before the corpse rotted away, so the fine structure survives as a cast. Palaeontologists call the process authigenic mineralisation, and the site a konservat-lagerstatte, a deposit that keeps the things which normally leave no trace.

Two different worlds come out of this ground, and the old strip mines separate them. The northern pits, numbers one through eight, yield the Braidwood biota, mostly land plants and freshwater animals. Pit 11, southwest of Braidwood, yields the Essex biota with far more marine species. Pit 11 is now Braidwood State Fish and Wildlife Area, an Illinois state park, and fossil collecting is allowed there with a permit.

The beds were declared a National Historic Landmark in 1997 and they are still producing science. In 2021 a team led by Arjan Mann described Joermungandr bolti, a small recumbirostran from Mazon Creek preserved well enough to show the pattern of scales across its whole body, which let them trace how the skin of early tetrapods changed. The flora alone runs to more than 400 species from at least 130 genera. Its quality was recognised as early as the mid-nineteenth century, when a writer marvelled that plant fragments of even the softest texture had come through whole.