Figure 1 — Examples of fossil-bearing concretions from the 300-million-year-old Mazon Creek site. All scale bars are 3 cm. (a) A concretion viewed from outside. Note the non-descript appearance. (b, c) The same concretion cracked open, revealing the preserved remains of Bellinurus danae, a horseshoe crab. (d) Another concretion viewed from the exterior. (e, f), The same concretion cracked open, exposing the partial fossil of the millipede Acantherpestes major.