Trilobites Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the At ...

are used as

index fossil Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them.Hine, Robert. “Biostratigraphy.” ''Oxford Reference: Dictionary of Bio ...

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

Olenellus ''Olenellus'' is an extinct genus of redlichiid trilobites, with species of average size (about long). It lived during the Botomian and Toyonian stages (''Olenellus''-zone), , in what is currently North-America, part of the palaeocontinent Lau ...

''-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 The Burgess Shale is a fossil-bearing deposit exposed in the Canadian Rockies of British Columbia, Canada. It is famous for the exceptional preservation of the soft parts of its fossils. At old (middle Cambrian), it is one of the earliest fo ...

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

Cedaria ''Cedaria'' is a small, rather flat trilobite with an oval outline, a headshield and tailshield of approximately the same size, 7 articulating segments in the middle part of the body and spines at the back edges of the headshield that reach halfl ...

''-zone. Alternative zoning names place the Burgess Shale in the ''Peronopsis bonnerensis''-zone, which is underlain by the ''

Oryctocephalus indicus ''Oryctocephalus indicus'' is a species of corynexochid trilobite from the Cambrian. Its first appearance is proposed for the lower boundary of the Wuliuan, which corresponds to the beginning of the Miaolingian (The other proposal was '' Ovator ...

''-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.

Subdivision of the ''Olenellus''-zone

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

Wuliuan The Wuliuan stage is the fifth stage of the Cambrian, and the first stage of the Miaolingian Series of the Cambrian. It was formally defined by the ICS in 2018. Its base is defined by the first appearance of the trilobite species '' Oryctoceph ...

. * ''Nephrolenellus multinodus''-zone (youngest). 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). * ''Bolbolenellus euryparia''-zone. Species: ''Bolbolenellus euryparia'' (lower half), ''Mesonacis fremonti'', '' Bristolia fragilis s.l.'' (lower half), ''Olenellus terminatus s.l.'', ''Olenellus fowleri s.l.'', ''Olenellus puertoblancoensis s.l.'', ''Olenellus gilberti'' (uppermost part), '' Biceratops nevadensis'' (uppermost part), '' Bristolia brachyomma'' (very uppermost part). * ''Peachella iddingsi''-zone. Species: '' Peachella iddingsi'' (lower half), ''Mesonacis fremonti'', '' Olenellus nevadensis'' (lower part), '' Bristolia anteros'' (lowest half), ''Bristolia fragilis s.l.'', ''Olenellus terminatus s.l.'', ''Paranephrolenellus besti'' (very short period in the late lower part), '' Peachella brevispina'' (middle part). * ''Bristolia insolens''-zone. Species: '' Bristolia insolens'' (lower half), ''Mesonacis fremonti'', ''Olenellus nevadensis'', '' Olenellus clarki'', ''Olenellus sp.3'', ''Paranephrolenellus klondykensis'' (lowest part), '' Bristolia harringtoni'' (middle part), '' Bristolia bristolensis'' (lower half), ''Bristolia anteros'' (not lowest part), ''Bristolia fragilis s.l.'' (upper half), ''Paranephrolenellus inflatus'' (very short interval in the middle), '' Eopeachella angustispina'' (uppermost part). * ''Bristolia mohavensis''-zone. Species: '' Bristolia mohavensis'' (lower half), ''Mesonacis fremonti'', ''Olenellus nevadensis'', ''Olenellus clarki'', ''Olenellus sp.3'', ''Bristolia harringtoni'' (middle part), ''Bristolia bristolensis'' (upper half). * ''Arcuolenellus arcuatus''-zone (oldest). Species: ''Arcuolenellus arcuatus'' (lowest part), ''Arcuolenellus aff. megafrontatis'' (lower half), '' Mesonacis cylindricus'' (not the highest part), ''Olenellus nevadensis'', ''Olenellus clarki'' (not lowest part), ''Mesonacis fremonti'' (upper half), ''Olenellus sp.3'' (upper part).

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Zone Zone or The Zone may refer to: Places Climate and altitude zones * Death zone (originally the lethal zone), altitudes above a certain point where the amount of oxygen is insufficient to sustain human life for an extended time span * Frigid zone, ...

Stratigraphy