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| 2. |
- Ahn, Soo Yeun, et al.
(författare)
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Microorganism-mediated preservation of Planolites, a common trace fossil from the Harkless Formation, Cambrian of Nevada, USA
- 2012
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Ingår i: Sedimentary Geology. - : Elsevier BV. - 0037-0738. ; 263, s. 30-35
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Tidskriftsartikel (refereegranskat)abstract
- Specimens of Planolites, a tubular ichnofossil, from the Harkless Formation (Cambrian of Nevada) are commonly rimmed by thin layers of limonite, a probable pseudomorph after pyrite, suggesting that bacterially induced biomineralization played a key role in the preservation of the burrows. Organic material secreted by a vermiform, infaunal tracemaker is inferred to have formed the substrate for a biofilm that facilitated iron sulfide biomineralization by bacteria under localized anaerobic or dysaerobic conditions. Rapid, microbially mediated biomineralization of mucus-lined burrow walls is inferred to have promoted three-dimensional preservation of burrows by strengthening them against sediment compaction, and possibly providing sufficient support for burrows to allow time for sediment infill. Decomposition of organic materials in Planolites also left evidence in sediment layers immediately above the burrows: undulose sediment surfaces and small, round, pimpleform gas escape structures. (C) 2011 Elsevier B.V. All rights reserved.
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| 3. |
- Álvaro, J. Javier, et al.
(författare)
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Global Cambrian trilobite palaeobiogeography assessed using parsimony analysis of endemicity
- 2013
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Ingår i: Early Palaeozoic Biogeography and Palaeogeography. - 0435-4052. - 9781862393738 ; Memoir 38:38, s. 273-296
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Bokkapitel (refereegranskat)abstract
- Palaeobiogeographical data on Cambrian trilobites obtained during the twentieth century are combined in this paper to evaluate palaeoceanographic links through c. 30 myr, once these arthropods biomineralized. Worldwide major tectonostratigraphic units are characterized at series intervals of Cambrian time and datasets of trilobite genera (629 for Cambrian Series 2, 965 for Cambrian Series 3, and 866 for the Furongian Series) are analysed using parsimony analysis of endemicity. Special attention is given to the biogeographical observations made in microcontinents and exotic terranes. The same is done for platform-basinal transects of well-known continental margins. The parsimony analysis of endemicity analysis resulted in distinct palaeogeographical area groupings among the tectonostratigraphic units. With these groupings, several palaeobiogeographical units are distinguished, which do not necessarily fit the previously proposed biogeographical realms and provinces. Their development and spatial distributions are broadly controlled by Cambrian palaeoclimates, palaeogeographical conditions (e.g. carbonate productivity and anoxic conditions) and ocean current circulation.
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| 4. |
- Babcock, Loren, et al.
(författare)
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Cambrian (Guzhangian Stage) trilobites from Ohio, USA, and modification of the Cedaria Zone as used in Laurentia
- 2014
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Ingår i: GFF. - : Informa UK Limited. - 2000-0863 .- 1103-5897. ; 136:1, s. 6-15
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Tidskriftsartikel (refereegranskat)abstract
- Two Cambrian trilobites, Olenoides? sp. from the Mount Simon Sandstone and Cedaria woosteri from the Eau Claire Formation, are described from the subsurface of western Ohio, USA. The definition of the Cedaria Zone is modified to reflect differing interpretations of the zone as used historically in restricted-shelf and open-shelf lithofacies of the Laurentian palaeocontinent. The Cedaria prolifica Zone is proposed for use primarily in open-shelf lithofacies, and the C. woosteri Zone is proposed for use in more restricted, inner-shelf lithofacies. Olenoides? sp. from the Mount Simon Formation is interpreted as representing the C. prolifica Zone, and C. woosteri is the eponymous species of the C. woosteri Zone. Both polymerid trilobite biozones are in the Guzhangian Stage (Cambrian, provisional Series 3). Based on trilobite zonation, the Mount Simon Sandstone and most of the Eau Claire Formation of western Ohio are interpreted as being Guzhangian in age.
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| 5. |
- 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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| 6. |
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| 7. |
- Babcock, Loren E., et al.
(författare)
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Palaeobiology, taphonomy and stratigraphic significance of the trilobite BUENELLUS from the Sirius Passet Biota, Cambrian of North Greenland
- 2007
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Ingår i: Memoirs of the Association of Australasian Palaeontologists. - 0810-8889. ; 34, s. 401-418
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Tidskriftsartikel (refereegranskat)abstract
- The Sirius Passet Biota, collected from the lower Buen Formation (Cambrian Series 2, Stage 3), North Greenland, contains one species of trilobite, the nevadiid Buenellus higginsi Blaker, 1988. The species shows considerable morphological variation, especially in exoskeletal width. Many specimens of B. higginsi show some form of exceptionally preserved, non-biomineralised tissue. Structures interpreted as alimentary tracts and probable digestive glands are commonly preserved by silica and limonite. Antennae are rarely preserved, apparently through replication in clay minerals. Tendinous bars in the axial region also seem to be preserved by replication in clay minerals. Mineral-filled gut tracts in B. higginsi suggest they were fluid-filled at the time of burial, and that the species was a non-durophagous predator. Healed injuries, some of which are the result of unsuccessful predaceous attacks, are uncommon in B. higginsi. Other exoskeletons show evidence of post-mortem disruption, perhaps scavenging. Buenellus higginsi, one of the earliest known trilobites from Laurentia, seems to have played an important role in the Sirius Passet ecosystem, serving both as predator on, and prey for, contemporary animals.
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| 8. |
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| 9. |
- Babcock, Loren, et al.
(författare)
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Global climate, sea level cycles, and biotic events in the Cambrian Period
- 2015
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Ingår i: Palaeoworld. - : Elsevier BV. - 1875-5887 .- 1871-174X. ; 24:1-2, s. 5-15
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Tidskriftsartikel (refereegranskat)abstract
- The developing high-resolution chronostratigraphy of the Cambrian provides an updated age model for various geologic and biotic events that occurred during this critical period of Earth history. Broad, time-specific patterns of lithofacies, such as organic-rich deposits, and biofacies appear to be consistent across all Cambrian paleocontinents. Records of important evolutionary events including first appearances of certain metazoan taxa, migrations, and extinctions, tend to coincide with changes in eustatic sea level, as do the positions of many Konservat-Lagerstatten, concretion horizons, agnostoid-rich beds, and other sedimentary features. Most of these events or horizons also show a relationship to perturbations in the global carbon cycle. The positions of organic-rich deposits bear strong relationship to both paleogeographic position and sea level history. Cambrian strata show evidence of cyclicity at multiple scales. Synchronous or near-synchronous global cyclicity is inferred to be associated with oceanographic and climatic cycles characteristic of glacial expansion and deglaciation. (C) 2015 Published by Elsevier B.V. on behalf of Nanjing Institute of Geology and Palaeontology, CAS.
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| 10. |
- 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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