Trilobite zone

Trilobites are used as index fossils to subdivide the Cambrian period. Assemblages of trilobites define trilobite zones. The Olenellus-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–Elrathina-zone (contemporaneous with the Burgess Shale) 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 -zone (e.g. Spence Shale) and overlain (perhaps not directly) by the Ptychagnostus punctuosus-zone.

The lower Middle CambrianGlossopleura-zone (Spence Shale) is above the Albertella-zone.

The Elvinia-zone is upper Cambrian.

Recently, it has been proposed to subdivide the Olenellus-zone. The following zones have been proposed to replace the Upper Olenellus-zone. Each lower boundary is defined by the first occurrence of the naming species. Each upper boundary is defined by the first occurrence of the naming species of the overlying zone. In case of the youngest zone, this is Eokochaspis nodosa, that also marks the base of the Middle Cambrian Stage 5 (Delamaran).

Species: Nephrolenellus multinodus (lower half), Mesonacis fremonti, Olenellus terminatus s.l., Olenellus puertoblancoensis s.l., Olenellus fowleri s.l., Olenellus gilberti, Bolbolenellus brevispinus (not the lower part), Olenellus chiefensis (upper half), Olenellus sp.1 (upper half), Nephrolenellus geniculatus (upper part), Olenellus sp.2 (upper part), Olenellus howelli (very uppermost part).

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