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@john-christian, hey John … Jack already gave you the correct definition. Here it is again with some more references: A mold preserves a negative imprint of the surface, while a cast preserves the external form of the organism (Taylor, Taylor & Krings, 2009, p. 22). Sometimes a shell can be filled with minerals and then dissolve away. The internal cast that remains is termed a steinkern, which in German means “stone cast” (Pothero, 1998, p. 7). Steinkerns are most often represented by the internal molds of mollusks. The pith cast of Calamites is the most common plant steinkern. As a Calamites tree matured the center of the stem (pith) became hollow, developing into a tube-shaped air cavity. The pith cast preserves an impression of the pith cavities outside surface, which represents the inside vascular and cortex tissue (Taylor, Taylor & Kring, 2009, p. 23). Most molds and casts do not contain the actual remains of an organism. Shells, bone, and wood often form as molds or casts. A specialty is the so called ‘Ersatzschale’ German for surrogate shale, when the hard shell dissolves (like with bivalves) and gets replaces (mostly Calcium Hydroxide that crystallizes into the hollow space – especially at the German Muschelkalk). The fossil shell then is a perfect replica of the original shell.

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Ronny