Question

Animals that are readily classified into extant phyla, such as Mollusca and Arthropoda, appeared in the Cambrian without transitional forms that show how their distinctive body plans evolved. This “explosion” in fossil diversity had to come from somewhere. What are some of the best hypotheses explaining why animal fossils are not found before the Cambrian, despite molecular evidence suggesting divergence in the much more distant past than that?

Answer 1

Fosilisation is a rare event and most of the fossils formed are lost before observation due to erosion and metamorphosis. Thus it is difficult to obbtain fossils that are completely preserved. Further, only the parts of organisms that were already mineralised are usually preserved, such as the shells of molluscs. Since most animal species are soft-bodied, they decay before they can become fossilised. The Cambrian fossil record includes an unusually high number of lagerstätten, which preserve soft tissues. These allow paleontologists to examine the internal anatomy of animals, which in other sediments are only represented by shells, spines, claws, etc. – if they are preserved at all.