The Base of the Cambrian: Small Shelley fauna
The base of the Cambrian period is defined as the appearance of the
first animals with hard parts. This picture shows some of the fossils that
define the beginning of the Cambrian. They include archaeocyathids, sponge-like
early reef-formers, in the first two rows. Inarticulate brachiopods and
molluscs are on the bottom row.
Evolution of the Earth, 1994 edition.