Cambrian

adjective

• 1(chiefly in names or geographical terms) Welsh:the Cambrian Railway

• 2 Geology relating to or denoting the first period in the Palaeozoic era, between the end of the Precambrian aeon and the beginning of the Ordovician period.
• (as noun the Cambrian) the Cambrian period or the system of rocks deposited during it.

The Cambrian lasted from about 570 to 510 million years ago and was a time of widespread seas. It is the earliest period in which fossils, notably trilobites, can be used in geological dating