It appears that these mutations often may be small changes in amino acid sequence or gene regulation. The rapid development of insect resistance to pesticides illustrates the point that genetic change may be rapid. This point may help to explain how such a large number of species could be produced in a relatively short time. Another point illustrated is that much genetic change is decoupled from morphological change. Calibrating rates of Early Cambrian evolution.
Cambrian rocks contain the first abundant record of shelly fossils.
Most extant phyla and classes of animals appear in Cambrian sediments. The appearance of so many new body types in only 16 million years has been much discussed in the literature. New radiometric dates are now interpreted as indicating the period of time was even shorter. Zircon crystals taken from rocks at the base of the Cambrian in Siberia gave uranium-lead dates of 544 million years. The sudden appearance of a diversity of higher taxa, each represented by a small number of species, seems to be a common feature of the fossil record.