A selection of opalised animal fossil specimens from Early Cretaceous rocks of Lightning Ridge, New South Wales. Skeletal hard parts, such as bones, shells and teeth, are often completely replaced by silica (SiO2) in the form of opal. These examples include a plesiosaur tooth (top left), three mollusc shells (two bivalves and one gastropod, top right), turtle and crocodile vertebrae (middle row) and dinosaur and other reptile toe-bones (bottom row).