Trilobites are used as index fossil

Index fossil

Index fossils are fossils used to define and identify geologic periods . They work on the premise that, although different sediments may look different depending on the conditions under which they were laid down, they may include the remains of the same species of fossil...

s to subdivide the Cambrian period. Assemblages of trilobites define trilobite zones. The Olenellus

Olenellus

Olenellus is a genus of trilobite that lived during the Early Cambrian. It is commonly found in areas of Europe and North America...

zone has traditionally marked the top of the Lower Cambrian, and is followed by the Eokochaspis zone.

The last two zones of the Middle Cambrian are the Bathyuriscus

Bathyuriscus

Bathyuriscus is an extinct genus of Cambrian trilobite. It was a nektobenthic predatory carnivore. Specimens have been found in the Marjumian of the USA and in the Middle Cambrian of Australia, Canada , Greenland, Mexico, and the USA...

–Elrathina zone (contemporaneous with the Burgess Shale

Burgess Shale

The Burgess Shale Formation, located in the Canadian Rockies of British Columbia, is one of the world's most celebrated fossil fields, and the best of its kind. It is famous for the exceptional preservation of the soft parts of its fossils...

) and the subsequent Bolaspidella zone (starting at the base of the Drumian stage). These are overlain by the lowermost Upper Cambrian Cedaria zone.

Alternative zoning names place the Burgess Shale in the Peronopsis bonnerensis zone, which is underlain by the Peronopsis bonnerensis zone (e.g. Spence Shale) and overlain (perhaps not directly) by the Ptychagnostus punctuosus zone.

The lower Middle Cambrian Glossopleura zone (Spence Shale) is above the Albertella zone.

The Elvinia zone is upper Cambrian.

Series	Stage	Trilobite zone
Furongian (Upper Cambrian)	|- | style="background-color:;" | Stage 9 |- | rowspan=2 style="background-color:;" | Paibian	>-	Cedaria
	Series 3 (Middle Cambrian)	| rowspan=2 | Bolaspidella (>/ Ptychagnostus Ptychagnostus Ptychagnostus is an extinct genus of blind trilobite that lived during the Cambrian period, part of the order Agnostida. Ptychagnostidae generally do not exceed ten millimetres in length. Their remains are sometimes found in the otherwise empty tubes of the polychaete worm Selkirkia. It probably...  praecurrens ?? ). |- | style="background-color:;" | Drumian |- | rowspan=2 style="background-color:;" | Stage 5	Bathyuriscus Bathyuriscus Bathyuriscus is an extinct genus of Cambrian trilobite. It was a nektobenthic predatory carnivore. Specimens have been found in the Marjumian of the USA and in the Middle Cambrian of Australia, Canada , Greenland, Mexico, and the USA... –Elrathina (?) >- |Eokochaspis
		Series 2	Stage 4	Olenellus Olenellus Olenellus is a genus of trilobite that lived during the Early Cambrian. It is commonly found in areas of Europe and North America... |- | rowspan=2 style="background-color:;" | Stage 3 |> | Nevadella Nevadella Nevadella is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the late Atdabanian stage, which lasted from 530 to 524 million years ago during the early part of the Cambrian Period....
			Series 1	Stage 2	| style="background-color:;" | Sta>e 1