Small Shelly Fauna - Evolutionary Significance

Evolutionary Significance

In some locations, up to 20% of Cloudina fossils show borings, holes that are thought to have been made by predators. The very similar shelly fossil Sinotubulites, which is often found in the same locations, was not affected by borings. In addition, the distribution of borings in Cloudina suggests selection for size – the largest holes appear in the largest shells. This evidence of selective attacks by predators suggests that new species may have arisen in response to predation, which is often presented as a potential cause of the rapid diversification of animals in the Early Cambrian.

The small shellies provide a relatively continuous record throughout the early Cambrian, and thus provide a more useful insight into the Cambrian explosion than instances of exceptional preservation. 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. Such an assessment shows that the earliest small shellies are the most basal. As time goes on, they can be placed in the stem group to an ever smaller clade. In other words, the earliest (Ediacaran) small shellies can be tentatively considered diploblastic, in other words made of two main tissue layers. Later shellies are more convincingly triploblastic, as all "higher" animals are. Subsequently the Helcionellids are the first shelly fossils that can be placed in the stem group to a phylum (mollusca). As one looks at more recent SSFs, the arguments for stem group placements become stronger, and by the Atdabanian, some SSFs can be assigned to the crown group of a modern phylum, echinoderms. 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", and thus reveals the true pace of the Cambrian explosion.