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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