Early in the Earth history, at the beginning of the Cambrian period, the first assemblage of multicellular organisms with hard skeletal parts appeared. In the Lower Cambrian rocks, paleontologists all over the world have discovered minute shells and sclerites which are together called “small shelly fauna”.
|Aldanella kunda from Europeana - image is under CC BY-SA of Museum of Geology, University of Tartu.|
Small shelly fauna is an artificial designation of various skeletal parts of various groups of animals. This fauna appeared in fact just before the beginning of the Cambrian period, but its greatest diversity is achieved in the early Cambrian.
|Detail of Aldanella from Europeana - image is under CC BY-SA of Museum of Geology, University of Tartu.|
Shape of sclerites varied from conical shells, spines to small plates. Some of these fossils look familiar, for instance Aldanella, which clearly is a mollusk. Others look strange and could represent fragments of larger organisms like sponges, velvet worms related animals, brachiopods, etc.
Fossils of the small shelly fauna are usually composed of calcium phosphate, calcium carbonate or silica. These minerals might not have originally built the shell; they rather replace the original material during fossilization. It is assumed that the invention of the shell in some of these organisms is a consequence of the onset of first predators.