Small shelly fauna

The small shelly fauna, small shelly fossils (SSF), or early skeletal fossils (ESF) are mineralized fossils, many only a few millimetres long, with a nearly continuous record from the latest stages of the Ediacaran to the end of the Early Cambrian period. 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 trilobites. Since most SSFs were preserved by being covered quickly with phosphate 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 skeletons of small organisms, including the mysterious Cloudina and some snail-like molluscs. However, the bulk of the fossils are fragments or disarticulated remains 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 therefore the evolution of mineralized skeletons – suggested a sudden increase in the ocean's concentration of calcium. However, many SSFs are constructed of other minerals, such as silica. Because the first SSFs appear around the same time as organisms first started burrowing to avoid predation, it is more likely that they represent early steps in an evolutionary arms race 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 invertebrates evolved, and particularly for the pace and pattern of evolution in the Cambrian explosion. Besides including the earliest known representatives of some modern phyla, they have the great advantage of presenting a nearly continuous record of early Cambrian organisms whose bodies include hard parts.

History of discovery

The term "small shelly fossils" was coined by Samuel Matthews and V. V. Missarzhevsky in 1975. The term is often abbreviated to "small shellies" or "SSF". It is quite a misnomer since, as Stefan Bengtson says, "they are not always small, they are commonly not shelly". Paleontologists have been unable to invent a better term, and have vented their frustration in parodies such as "small silly fossils" and "small smellies". The great majority of all the morphological features of later shelled organisms appear among the SSFs. No-one has attempted a formal definition of "small shelly fauna", "small shelly fossils" or other similar phrases. 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. Unfortunately the papers that described these discoveries were in Russian, and the 1975 paper by Matthews and Missarzhevsky first brought the SSFs to the serious attention of the non-Russian-reading world. There was already a vigorous debate about the early evolution of animals. Preston Cloud argued in 1948 and 1968 that the process was "explosive", and in the early 1970s Niles Eldredge and Stephen Jay Gould developed their theory of punctuated equilibrium, which views evolution as long intervals of near-stasis "punctuated" by short periods of rapid change. 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.

Occurrence 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. There are different views about the time range of the SSFs. The Russian discoveries of the late 1960s were assigned to the Tommotian age of the Cambrian period, and for some time the term "small shelly fauna" was applied only to that age. On the other hand, Bengston includes in "SSF" Ediacaran fossils like Cloudina and post-Tommotian fossils like Microdictyon from the lagerstätte of the Maotianshan Shales. SSFs have been found in layers that also contain fossil trilobites. 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.

Mode of preservation Small shelly fossils are typically, although not always, preserved in phosphate. Whilst some shellies were originally phosphatic, in most cases the phosphate represents a replacement of the original calcite. They are usually extracted from limestone by placing the limestone in a weak acid, typically acetic acid; the phosphatized fossils remain after the rock is dissolved away. Preservation of microfossils by phosphate seems to have become less common after the early Cambrian, perhaps as a result of increased disturbance of sea-floors by burrowing animals. Without this fossil-forming mode, many small shelly fossils may not have been preserved – or been impossible to extract from the rock; hence the animals that produced these fossils may have lived beyond the early Cambrian – the apparent extinction of most SSFs by the end of the Cambrian may be an illusion. For decades it was thought that halkieriids, whose "armor plates" are a common type of SSF, perished in the end-Botomian mass extinction; but in 2004 halkieriid armor plates were reported from Mid Cambrian rocks in Australia, a good 10 million years more recent than that.

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