Small shelly fossil

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Small shelly fossils is a name for an assemblage of fossils preserved in phosphate. These are especially common in the Lower Cambrian all over the world. Many different groups of animals are found in these: Molluscs, brachiopods, coelosclerithophorans (Chancellorids and Sachitids), spicules, arthropods among others.

Usually it is a steinkern of the fossil that is preserved. Sometimes extremely delicate preservation is observed like the Orsten fauna from Scandinavia or the Doushantuo embryos from Guizhou, China. The easiest way to collect small, shelly fossils is to dissolve a limestone in weak organic acid and carefully sieve the residues to collect the fossils which are typically less than a millimeter in length.

A Steinkern is an Internal Mold of an organism. Something that is similar to a cast is a steinkern but the interior space within the shell, where the tissue was located fills with sediment and becomes lithified. Then the shell is lost or removed leaving behind a casting of the interior called an internal mold. Pelecypods and gastropods commonly form steinkerns, namely, the large Silurian Pelecypod, Megalomus canadensis. The brachiopod, Pentamerites, from the Silurian of Iowa has been found as steinkerns. This is unusual in that brachiopods produce calcite which is stable but here the calcite shell surrounded dolomite mud on the inside. Groundwater dissolved the calcite but did not remove the dolomite that filled the shell cavity.

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(Partially taken from the University of North Carolina at Wilmington. http://people.uncw.edu/dockal/gly312/fossils/fossils.htm)