If this hole is later filled with other minerals, it is a cast.
An endocast or internal mold is formed when sediments or minerals fill the internal cavity of an organism, such as the inside of a bivalve or snail or the hollow of a skull. If the chemistry is right, the organism (or fragment of organism) can act as a nucleus for the precipitation of minerals such as siderite, resulting in a nodule forming around it.
If this happens rapidly before significant decay to the organic tissue, very fine three-dimensional morphological detail can be preserved.
Nodules from the Carboniferous Mazon Creek fossil beds of Illinois, USA, are among the best documented examples of such mineralization.
Compression fossils, such as those of fossil ferns, are the result of chemical reduction of the complex organic molecules composing the organism's tissues.Some fossils are biochemical and are called chemofossils or biosignatures.Permineralization is a process of fossilization that occurs when an organism is buried.Some fossils consist only of skeletal remains or teeth; other fossils contain traces of skin, feathers or even soft tissues. In some cases the original remains of the organism completely dissolve or are otherwise destroyed.The remaining organism-shaped hole in the rock is called an external mold.This process can occur in very small spaces, such as within the cell wall of a plant cell.