Small shelly fauna

The small shelly fauna, small shelly fossils (SSF), or early skeletal fossils (ESF) are <a href="/facts/Biomineralization/m5mGPaTM">mineralized</a> <a href="/facts/Fossil/by7NzIA1">fossils</a>, many only a few millimetres long, with a nearly continuous record from the latest stages of the <a href="/facts/Ediacaran/z8jSUITT">Ediacaran</a> to the end of the Early <a href="/facts/Cambrian/jIfzKtod">Cambrian</a> <a href="/facts/Period_(geology)/6LBR11yQ">Period</a>. They are very diverse, and there is no formal definition of "small shelly fauna" or "small shelly fossils". Almost all are from earlier rocks than more familiar fossils such as <a href="/facts/Trilobite/Tk8ppaKc">trilobites</a>. Since most SSFs were preserved by being covered quickly with <a href="/facts/Phosphate/FDGVCxUG">phosphate</a> and this method of preservation is mainly limited to the late Ediacaran and early Cambrian periods, the animals that made them may actually have arisen earlier and persisted after this time span.
Some of the fossils represent the entire <a href="/facts/Skeleton/FXc9Y2lF">skeletons</a> of small organisms, including the mysterious <a href="/facts/Cloudina/ujnTmI8x">Cloudina</a> and some snail-like <a href="/facts/Mollusc/1elgknsQ">molluscs</a>. However, the bulk of the fossils are fragments or disarticulated remains of larger organisms, including <a href="/facts/Sponge/E9T2WFWA">sponges</a>, molluscs, slug-like <a href="/facts/Halkieriid/g6sHTj1V">halkieriids</a>, <a href="/facts/Brachiopod/XVBnEPj1">brachiopods</a>, <a href="/facts/Echinoderm/j2rd8qwt">echinoderms</a>, and <a href="/facts/Onychophora/CVn4wdwx">onychophoran</a>-like organisms that may have been close to the ancestors of <a href="/facts/Arthropod/JKJLTFzp">arthropods</a>.
One of the early explanations for the appearance of the SSFs – and therefore the evolution of mineralized skeletons – suggested a sudden increase in the ocean's concentration of <a href="/facts/Calcium/hf252CC0">calcium</a>. However, many SSFs are constructed of other minerals, such as <a href="/facts/Silica/A3WILp6J">silica</a>. Because the first SSFs appear around the same time as organisms first started burrowing to avoid <a href="/facts/Predation/JEGdrX3f">predation</a>, it is more likely that they represent early steps in an <a href="/facts/Evolutionary_arms_race/7uUfRu88">evolutionary arms race</a> between predators and increasingly well-defended prey. On the other hand, mineralized skeletons may have evolved simply because they are stronger and take less energy to produce than all-organic skeletons like those of insects. Nevertheless, it is still true that the animals used minerals that were most easily accessible.
Although the small size and often fragmentary nature of SSFs makes it difficult to identify and classify them, they provide very important evidence for how the main groups of marine <a href="/facts/Invertebrate/3CyqCVnN">invertebrates</a> evolved, and particularly for the pace and pattern of evolution in the <a href="/facts/Cambrian_explosion/Wg29M7PN">Cambrian explosion</a>. Besides including the earliest known representatives of some modern <a href="/facts/Phylum/ELT3Jyax">phyla</a>, they have the great advantage of presenting a nearly continuous record of early Cambrian organisms whose bodies include hard parts.

Small shelly fauna open-in-new

Small shelly fauna