| 1. |
- Babcock, Loren E., et al.
(författare)
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Cambrian trilobite biostratigraphy and its role in developing an integrated history of the Earth system
- 2017
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Ingår i: Lethaia. - : Scandinavian University Press / Universitetsforlaget AS. - 0024-1164. ; 50:3, s. 381-399
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Tidskriftsartikel (refereegranskat)abstract
- One of the principal biostratigraphical tools used in the lower Palaeozoic, and especially the Cambrian System, is trilobites. Historically, both polymerids and agnostoids have commonly been included as 'trilobite' groups, although currently the question of whether they comprise a monophyletic group or are polyphyletic is unsettled. Beginning in the late 1800s, the base of the Cambrian System was marked by the lowest occurrence of olenelline trilobites. Discovery of a rich pre-trilobite metazoan record in the mid-1900s led to significant modification of that concept, but polymerids and especially agnostoids still rank among the leading biostratigraphical and chronostratigraphical guides through much of the Cambrian. Chronostratigraphical sub-division of the trilobite-bearing part of the Cambrian System derives largely from biostratigraphical, ecological and evolutionary concepts about agnostoids and polymerids introduced in the 1940s to 1970s by A.H. Westergård, A.R. Palmer and R.A. Robison. Westergård introduced a zonation for Scandinavia that was largely based on agnostoids. Palmer explained the distribution of Cambrian trilobite faunas in terms of restricted and unrestricted access to open oceans. Together, these ideas coalesced in Robison's recognition of separate zonation schemes for restricted-shelf polymerids, open-shelf polymerids and open-shelf (cosmopolitan) agnostoids. Palmer also introduced the concept of biomeres, which placed sharp limits on biostratigraphical intervals recognizable from trilobites. Global correlation in the upper half of the Cambrian today depends to a large extent on the ranges of agnostoids and some polymerids characteristic of open-shelf to slope areas, ones that facilitate identification of precise intercontinental tie points. Agnostoid and polymerid biostratigraphy is now being integrated with information about coastal onlap and eustatic sea-level history, geochemical cycling and other data to provide a more complete understanding of the early Palaeozoic biosphere and its complex physico-chemical context.
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| 4. |
- Babcock, Loren, et al.
(författare)
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Proposed reassessment of the Cambrian GSSP
- 2014
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Ingår i: Journal of African Earth Sciences. - : Elsevier BV. - 1464-343X. ; 98, s. 3-10
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Tidskriftsartikel (refereegranskat)abstract
- Since the time of its ratification in 1992, the Cambrian GSSP, 23 m above the base of Member 2A (Quaco Road Member) of the Chapel Island Formation, Fortune Head section, Newfoundland, Canada, has been challenged as posing an ambiguous correlation level. Difficulties have been encountered in precisely correlating the horizon coinciding with the GSSP to strata on most paleocontinents, but especially to Siberia and South China (Gondwana). The GSSP point, which was intended to coincide with the base of the Treptichnus pedum (formerly Phycodes pedum or Trichophycus pedum) Ichnozone at the FAD of the trace fossil T. pedum, occurs above the first appearance of the trace in the stratotype section. Trace fossils of other forms in the stratotype provide a means of bracketing the position of the GSSP, but are imprecise guides for correlation globally. Other chronostratigraphic guides are unavailable at the stratotype because of a dominance of coarse siliciclastic lithologies and metamorphic overprint. To facilitate global correlation of the Cambrian base and ensure nomenclatural stability to the extent possible, discussion of the merits and perceived weaknesses of the Cambrian GSSP is sought. Possible solutions to the problems surrounding the current GSSP definition are addressed in hopes that the global scientific community will actively participate in developing a well-reasoned, practical solution that will stand the test of time. (C) 2014 Elsevier Ltd. All rights reserved.
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| 5. |
- Landing, Ed, et al.
(författare)
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Global standard names for the Lowermost Cambrian Series and Stage
- 2007
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Ingår i: Episodes. - 0705-3797. ; 30:4, s. 287-289
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Tidskriftsartikel (refereegranskat)abstract
- The GSSP marking the base of the Cambrian System was ratified by the IUGS in 1992. Ratification of the GSSP point at the base of the Trichophycus pedum Ichnozone in the Fortune Head section, eastern Newfoundland, Canada, automatically defined the conterminant base of the lowermost series and stage of the Cambrian although names for those subdivisions were not proposed at the time of the decision. In 2007, the IUGS ratified the names Terreneuvian Series and Fortunian Stage for these previously unnamed chronostratigraphic subdivisions. The Terreneuvian Series replaces the provisional name "Series 1," and the Fortunian Stage replaces the provisional name "Stage 1," of the Cambrian System.
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| 6. |
- Peng, Shanchi, et al.
(författare)
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A potential GSSP for the base of the uppermost Cambrian stage, coinciding with the first appearance of Lotagnostus americanus at Wa'ergang, Hunan, China
- 2014
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Ingår i: GFF. - : Informa UK Limited. - 2000-0863 .- 1103-5897. ; 136:1, s. 208-213
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Tidskriftsartikel (refereegranskat)abstract
- The first appearance datum (FAD) of the cosmopolitan agnostoid trilobite Lotagnostus americanus is a widely used marker for defining the base of the uppermost stage of the Cambrian System. Detailed field study on the L. americanus-bearing interval of the Wa'ergang section, Hunan, China, reveals the FAD of L. americanus to be 29.65m above the base of the Shengjiawan Formation. This horizon is close to the lowest known occurrences of the intercontinentally distributed polymerid trilobites Hedinaspis regalis and Charchaqia norini. The section has strong potential as a candidate for defining a global standard stratotype-section and point marking the base of provisional Stage 10.
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| 7. |
- Peng, Shanchi, et al.
(författare)
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Continuing progress on chronostratigraphic subdivision of the Cambrian System
- 2011
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Ingår i: Bulletin of Geosciences. - : Czech Geological Survey. - 1214-1119 .- 1802-8225. ; 86:3, s. 391-396
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Tidskriftsartikel (refereegranskat)abstract
- This paper is a review of the chief accomplishments toward defining Cambrian stage- and series-level GSSPs since the founding of the International Subcommission on Cambrian Stratigraphy (ISCS) in 1961, and is an assessment of the Subcommission's progress toward defining the bases of remaining provisional stages and series.
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| 8. |
- Peng, Shanchi, et al.
(författare)
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Global Standard Stratotype-Section and Point (GSSP) for the Base of the Jiangshanian Stage (Cambrian: Furongian) at Duibian, Jiangshan, Zhejiang, Southeast China
- 2012
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Ingår i: Episodes. - 0705-3797. ; 35:4, s. 462-477
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Tidskriftsartikel (refereegranskat)abstract
- The International Commission on Stratigraphy and the IUGS Executive Committee have recently approved a Global Standard Stratotype-section and Point (GSSP) defining the base of the second stage of the Furongian Series, Cambrian System. This stage is named the Jiangshanian Stage, after Jiangshan City, western Zhejiang Province, China, where the GSSP is located. The GSSP is exposed in a natural outcrop near Duibian Village. It is defined at the base of a limestone (wackestone) layer 108.12 m above the base of the Huayansi Formation in the Duibian B section, coinciding with the first appearance of the cosmopolitan agnostoid trilobite Agnostotes orientalis (base of the A. orientalis Zone). The GSSP is at a position of 28 degrees 48.977'N latitude and 118 degrees 36.887 'E longitude. Secondary global markers at or near the base of the stage include the first appearance of the cosmopolitan polymerid trilobite Irvingella angustilimbata, which coincides with the FAD of the primary marker in the stratotype section, and near the end of a large positive carbon isotopic excursion (SPICE excursion). Faunal turnovers close to the base of the Jiangshanian Stage have been recognized as being at the base of the Iverian Stage in Australia, the Gonggrian Stage in Korea, and the Agnostotes orientalis Irvingella perfecta Zone in Siberia, and near the base of the Aksayan Stage in Kazakhstan, the Sunwaptan Stage in Laurentia, and the Parabolina brevispina Zone in Baltica.
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| 9. |
- Peng, Shanchi, et al.
(författare)
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Intraspecific variation and taphonomic alteration in the Cambrian (Furongian) agnostoid Lotagnostus amerkanus: new information from China
- 2015
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Ingår i: Bulletin of Geosciences. - : Czech Geological Survey. - 1214-1119 .- 1802-8225. ; 90:2, s. 281-306
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Tidskriftsartikel (refereegranskat)abstract
- The concept of the agnostoid arthropod species Lotagnostus americanus (Billings, 1860), which has been reported from numerous localities in the upper Furongian Series (Cambrian) of Laurentia, Gondwana, Baltica, Avalonia, and Siberia, is reviewed with emphasis on morphologic and taphonomic information afforded by large collections from Hunan in South China, Xinjiang in Northwest China, and Zhejiang in Southeast China. Comparisons are made with type and topotype material from Quebec, Canada, as well as material from elsewhere in Canada, the USA, the United Kingdom, Sweden, Russia, and Kazakhstan. The new information clarifies the limits of morphologic variability in L. americanus owing to ontogenetic changes and variation within holaspides, including inferred microevolution. It also provides details on apparent variation of taphonomic origin. The Chinese collections demonstrate a moderately wide variation in L. americanus, indicating that arguments favoring restriction of Lotagnostus species to narrowly defined, geographically restricted forms are unwarranted. Species described as L. trisectus (Salter, 1864), L. asiaticus Troedsson, 1937, and L. punctatus Lu, 1964, for example, fall within the range of variation observed in L. americanus, and are regarded as junior synonyms. The effaced form Lotagnosilis obscurus Palmer, 1955 is removed from synonymy with L. americanus. A review of the stratigraphic distribution of L. americanus as construed here shows that the earliest occurrences of the species in all regions of the world are nearly synchronous.
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