Small shelly fauna

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 calcium.

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 invertebrates evolved, and particularly for the pace and pattern of evolution in the Cambrian explosion.

[10] Specimens and sometimes quite rich collections of these fossils were discovered between 1872 and 1967, but no-one drew the conclusion that the Early Cambrian contained a diverse range of animals in addition to the traditionally recognized trilobites, archaeocyathans, etc.

In the late 1960s Soviet paleontologists discovered even richer collections of SSFs in beds below and therefore earlier than those containing Cambrian trilobites.

[12] On the other hand, around the same time Wyatt Durham and Martin Glaessner both argued that the animal kingdom had a long Proterozoic history that was hidden by the lack of fossils.

[2][13] Rich collections have been found in China, Russia, Mongolia, Kazakhstan, Australia, and Antarctica; and moderately diverse ones in India, Pakistan, Iran, Europe and North America.

[14] The mass extinction at the end of the Cambrian period's Botomian age was thought to have wiped out most of the SSF, with the exception of the halkieriids, wiwaxiids and Pojetaia.

[19] A recently discovered modern gastropod that lives near deep-sea hydrothermal vents illustrates the influence of both earlier and contemporary local chemical environments: its shell is made of aragonite, which is found in the earliest fossil molluscs; but it also has armor plates on the sides of its foot, and these are mineralized with the iron sulfides pyrite and greigite, which had never previously been found in any metazoan but whose ingredients are emitted in large quantities by the vents.

Hypotheses to explain the evolution of biomineralization include physiological adaptation to changing chemistry of the oceans, defense against predators and the opportunity to grow larger.

[2] The idea that biomineralization was a response to changes in ocean chemistry is also undermined by the fact that small shelly fossils made of calcite, aragonite, calcium phosphate and silica appeared virtually simultaneously in a range of environments.

[17] He cited as another example of hardened defenses from this time the fact that the earliest protective "skeletons" included glued-together collections of inorganic objects — for example the early Cambrian worm Onuphionella built a tube covered with mica flakes.

[25] Such a strategy required both anatomical adaptations that allowed organisms to collect and glue objects and also moderately sophisticated nervous systems to co-ordinate this behavior.

Mineral-organic composites are both stronger and take less energy to build than all-organic skeletons, and these two advantages would have made it possible for animals to grow larger and, in some cases, more muscular.

A similar pattern is visible in living marine animals, since biomineralized skeletons are rarer and more fragile in polar waters than in the tropics.

[30] Although most of the SSFs are difficult to identify, those assigned positions in modern taxa, or in their stem groups of evolutionary "aunts" or "cousins", enable scientists to assess the pattern and speed of animal evolution on the strength of the small shelly evidence.

[30] This gives the impression that the first SSF animals, from the late Ediacaran, were basal members of later clades, with the phyla subsequently appearing in a "rapid, but nevertheless resolvable and orderly" fashion, rather than as a "sudden jumble",[30]: 163  and thus reveals the true pace of the Cambrian explosion.

[30] The few collections of SSF from the Ediacaran period have a limited range of forms[2] Fully and partially mineralized tubes are common and form a really mixed collection: the structures and compositions of their walls vary widely; specimens have been classified as members of a wide range of clades including foraminiferans, cnidarians, polychaete and pogonophoran annelids, sipunculids and others.

Many have been attributed to well-known groups such as molluscs, slug-like halkieriids, brachiopods, echinoderms, and onychophoran-like organisms that may have been close to the ancestors of arthropods.

Most of the Cambrian SSF consists of sclerites, fragments that once made up the external armor of early animals, such as Halkieria[38] or "scale worms".

[40] Many sclerites are of the type called "coelosclerites", which have a mineralized shell around a space originally filled with organic tissue and which show no evidence of accretionary growth.

[2] Halkieriids produced scale- or spine-shaped coelosclerites, and complete specimens show that the animals were slug-shaped, and had cap-shaped shell plates at both ends in addition to the sclerites.

[2] Tommotiids have a wide range of sclerite shapes and internal structures, and may in fact represent a polyphyletic set of lineages, in other words they may have independently developed phosphatic scleritomes rather than inheriting them from a common ancestor.

[44] Fossils have been found that resemble the opercula ("lids") used by snails to close the openings in their armor, and are attributed to hyoliths, small animals that had conical shells and may have been molluscs or worm-like Sipuncula.