Small shelly fossils

Small shelly fossils (SSF), or small shelly fauna, are tiny fossils, many only a few millimetres long. They have a record from the latest stages of the Ediacaran to the early Cambrian period. Almost all are from earlier rocks than more familiar fossils such as trilobites. 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 skeletons of small organisms, including the mysterious "Cloudina" and some snail-like molluscs. However, the bulk of the fossils are fragments or parts of larger organisms, including sponges, molluscs, slug-like halkieriids, brachiopods, echinoderms, and onychophoran-like organisms that may have been close to the ancestors of arthropods.
One of the early explanations for the appearance of the SSFs – and the evolution of mineralized skeletons – 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 invertebrates evolved, and for the pace and pattern of evolution in the Cambrian explosion. They include the earliest known representatives of some modern phyla, and they have the great advantage of presenting a nearly continuous record of early Cambrian organisms whose bodies include hard parts.