Stage 2 of the
Cambrian is the unnamed upper
stage of the
Terreneuvian Series. It lies atop the
Fortunian and below
Stage 3 of the Cambrian. It is commonly referred to as the Tommotian, after the Cambrian stratigraphy of Siberia.[2][3] Neither the upper nor lower boundary has yet been defined by the
International Commission on Stratigraphy.[4][5]
The preferred definitions for the lower boundary are the
first appearance of the
molluscsWatsonella crosbyi or Aldanella attleborensis around 529 million years ago.[6][7] The correlation between the lowest occurrences of A. attleborensis and W. crosbyi with the Zhujiaqing positive carbon isotope excursion (ZHUCE) has led to the conclusion that the combination of these two markers are a reliable indicator of the boundary between the Fortunian and Stage 2.[8] The proposed upper boundary might be the first appearance of
trilobites around 521 million years ago.[4]
Naming
The name "Laolinian" after the Laolin village in
Yunnan, China, was proposed for the Stage 2 in 2012.[9]
Stratotype
Possible candidates for a
GSSP include the first appearance of Watsonella crosbyi in the Zhujiaqing Formation (朱家庆组) in Yunnan, China or the Pestrotsvet Formation near the
Aldan River on the
Siberian Platform.[10]
References
^"Chart/Time Scale". stratigraphy.org. International Commission on Stratigraphy.
^Kouchinsky, Artem; Bengtson, Stefan; Missarzhevsky, Vladimir V.; Pelechaty, Shane; Torssander, Peter; Val'kov, Anatolij K. (24 September 2001). "Carbon isotope stratigraphy and the problem of a pre-Tommotian stage in Siberia". Geological Magazine. 138 (4): 387–396.
Bibcode:
2001GeoM..138..387K.
doi:
10.1017/S0016756801005684.
S2CID3884375.
^Li, Guoxiang; Zhao, Xin; Gubanov, Alexander; Zhu, Maoyan; Na, Lin (1 April 2011). "Early Cambrian Mollusc Watsonella crosbyi: A Potential GSSP Index Fossil for the Base of the Cambrian Stage 2". Acta Geologica Sinica - English Edition. 85 (2): 309–319.
Bibcode:
2011AcGlS..85..309L.
doi:
10.1111/j.1755-6724.2011.00400.x.
S2CID129506892.