The Cambrian Period

The Cambrian is a geologic period that began around 542 million years ago and ended about 490 million years ago. The Cambrian Period is the earliest period in whose rocks are found numerous large, distinctly-fossilizable multicellular organisms more complex than sponges or medusoids. During this time, roughly fifty separate major groups of organisms or "phyla" (including almost all the basic body plans of modern animals) emerged suddenly, in most cases without evident precursors. This radiation of animal phyla is referred to as the Cambrian explosion.

Cambria is the Roman name for Wales, a place of extensive Cambrian-age rocks investigated by Adam Sedgwick in the 1830s. Eventually as the stratigraphic series was filled out, the youngest 'Cambrian' came to overlap the oldest parts of the 'Silurian' sequence of strata that had been identified by Sir Roderick Murchison. In 1879, Charles Lapworth defined an 'Ordovician' Period that included the overlapping beds.