What is Small shelly fossils?
[[File:Cloudina.svg|thumb|right|150px|''Cloudina'', a tiny animal, part of the smally shelly fauna]] '''Small shelly fossils''' ('''SSF'''), or '''small shelly fauna''', are tiny [[fossil]]s, many only a few millimetres long. They have a record from the latest stages of the [[Ediacaran]] to the early [[Cambrian]] [[period (geology)|period]]. Almost all are from earlier rocks than more familiar fossils such as [[trilobite]]s. Most SSFs were preserved by being covered quickly with [[phosphate]]. This method of preservation is mainly limited to the later Ediacaran and early Cambrian periods, so the animals that made them may have arisen earlier and continued after this time. Some of the fossils represent the entire [[skeleton]]s of small organisms, including the mysterious ''Cloudina'' and some snail-like [[mollusc]]s. However, the bulk of the fossils are fragments or parts of larger organisms, including [[sponge]]s, [[mollusc]]s, slug-like [[halkieriid]]s, [[brachiopod]]s, [[echinoderm]]s, and [[onychophora]]n-like organisms that may have been close to the ancestors of [[arthropod]]s. One of the early explanations for the appearance of the SSFs – and the evolution of mineralized [[skeleton]]s – is a sudden increase in the ocean's concentration of [[calcium]]. However, many SSFs are made of other minerals, such as [[silica]]. The first SSFs appear around the same time as organisms first started [[burrowing]] to avoid [[predation]], so it is likely that they represent early steps in an [[evolutionary arms race]] between predators and increasingly well-defended prey. Nevertheless, it is still true that the animals used minerals that were most easily accessible. The small size and (usually) broken nature of SSFs makes them difficult to identify and classify. However, they provide evidence for how the main groups of marine [[invertebrate]]s evolved, and for the pace and pattern of evolution in the [[Cambrian explosion]]. They include the earliest known representatives of some modern [[phylum|phyla]], and they have t