and to converse meaningfully. Biological correlations A number of fossil groups contain short-lived species; when the species occur in disparate rocks, it can be assumed that the rocks were deposited at the same time. Useful groups for this biostratigraphic approach include: Small shelly fossils There is an assemblage of small shelly fossils which was thought to be characteristic of the Cambrian. (The shelly fossils Cloudina and Namacalathus are known from terminal-Ediacaran sediments.) This assemblage has been thought to appear and diversify rapidly at the base of the Cambrian, but re-dating of some Russian samples has placed them long before , and shown that they are stratigraphically below beds containing 'Ediacaran' fossils. Acritarchs Trilobites Trace fossils Treptichnus pedum has been internationally recognised as marking the base of the Cambrian: it is thought to represent an onset of vertical burrowing, which was absent before the Cambrian period. It is also widespread and easily recognised. However, the burrows were later found below the GSSP which marks the base of the Cambrian, and their appearance is not related to any of the more significant biological events, such as the appearance of the small shelly fauna or the disappearance of the Ediacara biota. More detailed analysis of the fossils suggest that they might have been formed by extensive under-mat mining, rather than true vertical burrowing, which may go some way to explain this - they may not represent a fundamentally new biological innovation. Other trace fossils, such as the vertically spiralling Gyrolithes, are considered to be Cambrian phenomena only. Monomorphichnus, an arthropod trace, occurs 2½ m above Treptichnus pedum in the Newfoundland reference section, and is considered in concert with T. pedum to be a good marker of the base of the Cambrian elsewhere in the world. Radiometric correlations Radiometric dates usually have high error margins - but in some cases precise ages are available. Geochemical correlations Chemical signals in rocks can be used to identify biological changes such as extinction or diversification, so they probably provide the most reliable indicator of the dawn of the Cambrian. However, they are very difficult to reliably extract in the field; and their interpretation is sometimes subjective. Naming and divisions Because geological time does not separate into discrete packages, the delineation of the period into stages is controversial. One controversy is the system of defining the start of a stage. To provide an unambiguous reference level for the