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”.
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| 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.
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| 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.
