| 1. |
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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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| 11. |
- Butler, Aodhán D., 1986-, et al.
(författare)
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Exceptionally-preserved Mickwitzia from the Indian Springs Lagerstätte.
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Ingår i: Journal of Paleontology. - 0022-3360 .- 1937-2337.
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- A new assemblage of the early Cambrian stem group brachiopod Mickwitzia is described from the Indian Springs Lagerstätte possessing exceptionally preserved mantle setae. Critical analysis of shell structure and mantle setae from these specimens with those from additional sites with variable diagenetic history reveals the extent of taphonomic alteration and further sheds light on the phylogenetic position of the mickwitziids. A morphometric approach to shell outline and growth landmarks within these specimens reveals a clear species level discriminant signal of Nevada Mickwitzia in comparison to M. monlifera from Sweden. Detailed electron micrographs allow revision of the genus diagnosis for Mickwitzia based on presence of inward pointing phosphatic cones and tangential setae bearing tubes. We also conclude the inward pointing cone structures are not consistent with setal bearing structures as previously thought, but rather represent an endopunctae-like structure. A tommotiid-like shell architecture and presence of acrotretid columns in the dorsal juvenile shell of M. cf. occidens further strengthens the proposed close relationship between stem-group brachiopods and tommotiids.
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| 12. |
- Butler, Aodhán D., et al.
(författare)
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Exceptionally preserved Mickwitzia from the Indian Springs Lagerstätte (Cambrian Stage 3), Nevada
- 2015
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Ingår i: Journal of Paleontology. - : Cambridge University Press (CUP). - 0022-3360 .- 1937-2337. ; 89:6, s. 933-955
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Tidskriftsartikel (refereegranskat)abstract
- ABSTRACT Exceptionally preserved specimens of the Cambrian stem-group brachiopod Mickwitzia occidens Walcott, 1908 are described in detail from the Indian Springs LagerstÀtte in Nevada, USA. Shell structure and preserved mantle setae from these specimens reveal a variable diagenetic (taphonomic) history and provide insight into the phylogenetic position of mickwitziids. Morphologic and morphometric comparison to M. monilifera (Linnarsson, 1869) from Sweden and M. muralensis Walcott, 1913 from British Columbia, Canada reveals clear species-level distinctions. Scanning electron microscopic analysis allows revision of the generic diagnosis. The Mickwitzia shell is characterized by the presence of inwardly pointing phosphatic cones and tangential setae-bearing tubes. The inwardly pointing cone structures are not consistent with setal bearing structures as previously thought, but rather represent endopunctae-like structures. Acrotretid-like shell structures and shell-penetrating setae in M. occidens strengthen the previously proposed close relationship between stem-group brachiopods and tommotiids, a group of small shelly fossils.
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| 13. |
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| 14. |
- Cederstrom, Peter, et al.
(författare)
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Morphology, ontogeny and distribution of the Cambrian Series 2 ellipsocephalid trilobite Strenuaeva spinosa from Scandinavia
- 2012
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Ingår i: GFF. - : Informa UK Limited. - 2000-0863 .- 1103-5897. ; 134:3, s. 157-171
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Tidskriftsartikel (refereegranskat)abstract
- The ellipsocephalid trilobite Strenuaeva spinosa occurs widely in Scandinavia in the lower part of the Ornamentaspis? linnarssoni Zone (Cambrian Series 2). Its first appearance is a readily recognisable, widespread biohorizon in Scandinavia. During ontogeny, intergenal spines are lost and the librigenae develop from a generative zone in the genal region. The palpebro-ocular ridges are distinct and connected to the posterior part of the frontal glabellar lobe in the early meraspid period, but eye ridges become separate from the palpebral lobes and the glabella in the late meraspid period. S. spinosa is characterised by large genal spines, distinct axial spines and falcate pleural tips. The spines may have functioned in predation resistance. S. spinosa also has a small pointed projection at the anterolateral corners of some thoracic pleurae, the function of which is unknown. The presence of five furrows in the cephalic axial lobe of S. spinosa, best expressed in meraspides, suggests that the head of this ellipsocephalid trilobite was composed of six fused primary segments.
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| 15. |
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| 16. |
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| 17. |
- 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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| 18. |
- Lerosey-Aubril, Rudy, et al.
(författare)
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Arthropod appendages from the Weeks Formation Konservat-Lagerstatte: new occurences of anomalocaridids in the Cambrian of Utah, USA
- 2014
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Ingår i: Bulletin of Geosciences. - : Czech Geological Survey. - 1214-1119 .- 1802-8225. ; 89:2, s. 269-282
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Tidskriftsartikel (refereegranskat)abstract
- The Guzhangian Weeks Formation (house Range, Utah, USA) contains a virtually unstudied but diverse assemblage of "soft-bodied" organisms. This fauna includes several enigmatic appendages of arthropods that are described in this contribution. Six appendages (two isolated and four paired appendages) are interpreted as frontal appendages of a probably new species of Anomalocaris. They are characterized by a slender morphology, 14 podomeres, ventral spines alternating in size, up to three auxiliary spines per ventral spine, and only two dorsal spines. Another isolated appendage is also tentatively assigned to Anomalocaris, but it exhibits a more robust morphology, a stronger distal tapering, and apparently simple ventral spines, suggesting that it may represent a distinct taxon. These frontal appendages represent the youngest occurrence of anomalocaridids in Laurentia and demonstrate the persistence of older, Burgess Shale-type taxa in the Weeks Formation. An assemblage of four antenniform and six robust and heavily-armed appendages is also described. These are interpreted as the serially arranged, anterior appendages of a single individual of an undetermined arthropod species. This association of three pairs of robust, spiny appendages with two pairs of antenniform structures in a Cambrian arthropod is unique.
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| 19. |
- LoDuca, Steven T., et al.
(författare)
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Reexamination of Yuknessia from the Cambrian of China and first report of Fuxianospira from North America
- 2015
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Ingår i: Journal of Paleontology. - : Cambridge University Press (CUP). - 0022-3360 .- 1937-2337. ; 89:6, s. 899-911
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Tidskriftsartikel (refereegranskat)abstract
- Yuknessia Walcott, 1919 recently was transferred from the green algae to the Phylum Hemichordata on the basis of new details observed for the type species, Y. simplex, from the Burgess Shale Formation (Cambrian Stage 5) of British Columbia. This has prompted reexamination of material attributed to Yuknessia from various Cambrian localities in South China. Findings preclude both a Yuknessia and a hemichordate affinity for all of the Chinese study material, and most of this material is formally transferred to Fuxianospira Chen and Zhou, 1997, a taxon common in the Chengjiang biota. Comparable material from the Cambrian Marjum, Wheeler, and Burgess Shale formations of North America is also assigned to Fuxianospira, and this reassignment expands both the paleogeographic and stratigraphic range of this taxon. All aspects of the study specimens, including details obtained from scanning electron microscopy, are consistent with an algal affinity, as proposed in the original descriptions of the Chinese material.
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| 20. |
- 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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| 21. |
- 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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| 22. |
- 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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| 23. |
- 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
-
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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| 24. |
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| 25. |
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| 26. |
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| 27. |
- Peng, Shanchi, et al.
(författare)
-
The Cambrian Period
- 2020
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Ingår i: Geologic Time Scale. - 9780128243633 - 9780128243619 ; 2, s. 565-565
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Bokkapitel (refereegranskat)abstract
- Appearance of metazoans with mineralized skeletons, “explosion” in biotic diversity and disparity, infaunalization of the substrate, occurrence of metazoan Konservat Fossil-lagerstätten, establishment of most invertebrate phyla, strong faunal provincialism, dominance of trilobites, generally warm climate but with possibleglacial-interglacial cycles in the later part, opening of the Iapetus Ocean, progressive equatorial drift and separation of Laurentia, Baltica, Siberia, and Avalonia from Gondwana characterize the Cambrian Period.
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| 28. |
- Peng, Shanchi, et al.
(författare)
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Trilobites
- 2020
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Ingår i: Geologic Timescale 2020. - 9780128243626 - 9780128243619 ; 1, s. 36-36
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Bokkapitel (refereegranskat)abstract
- Trilobites are marine arthropods that appeared in Cambrian Epoch 2 and became extinct at the end of the Permian. In some Paleozoic deposits, they number among the most abundant macrofossils. Trilobites are important for early Paleozoic biostratigraphy, especially Cambrian.
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| 29. |
- Robison, Richard A, et al.
(författare)
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Systematics, paleobiology, and taphonomy of some exceptionally preserved trilobites from Cambrian Lagerstatten of Utah
- 2011
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Ingår i: Paleontological Contributions. - 1046-8390. ; 5, s. 1-47
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Tidskriftsartikel (refereegranskat)abstract
- Mid-Cambrian biotas of the Spence, Wheeler, Marjum, and Weeks formations in Utah are exceptionally diverse. Continued collecting has produced additional trilobites here assigned to one new genus, Meniscopsia, and 25 new species. The new species, all known from complete or nearly complete dorsal exoskeletons, are Amecephalus jamisoni, Athabaskia gladei, Bolaspidella jarrardi, Bolaspidella reesae, Burlingia halgedahlae, Coosella kieri, Ehmaniella whitei, Glossopleura campbelli, Glossopleura yatesi, Ithycephalus stricklandi, Kootenia randolphi, Kootenia youngorum, Meniscopsia beebei, Menomonia sahratiani, Modocia comforti, Modocia kohli, Modocia whiteleyi, Norwoodia boninoi, Olenoides skabelundi, Olenoides vali, Polypleuraspis cooperi, Ptychoparella lloydi, Ptychoparella mettae, Zacanthoides liddelli, and Zacanthoides marshalli. Descriptions of two other species, Triadaspis bigeneris and Zacanthoides typicalis, are expanded to include new information from complete dorsal exoskeletons. The ventral cephalic presence of a functional rostral-hypostomal suture in the corynexochoid Zacanthoides marshalli further demonstrates that a fused rostral-hypostomal plate is not a unique defining character of the order Corynexochida, adding to evidence that the Corynexochida may be a polyphyletic taxon. The ventral cephalic presence of a median suture, associated with a rostellum in Norwoodia boninoi, order Ptychopariida, is further evidence for a polyphyletic origin of the median suture, previously cited as a unique apomorphic character defining the order Asaphida, which needs revision. The presence of a mineralized alimentary tract and digestive glands in some specimens of Meniscopsia beebei and Coosella kieri suggests that the gut was fluid filled at the time of burial and that the species were predaceous. Collapsed glabellas are interpreted as further evidence of fluid-filled gut tracts in some of the new species. Many of the articulated trilobites preserved in mid-Cambrian Lagerstätten of Utah were probably buried under rapidly deposited sediment clouds on a storm-prone marine shelf. Occasionally, trilobite clusters were buried. Encrustation of fully articulated individuals with calcitic cone-in-cone deposits in the Wheeler and Marjum formations is a likely consequence of concretionary-type growth within a biofilm shortly after death.
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| 30. |
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| 31. |
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| 32. |
- Walker, J. D., et al.
(författare)
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The Geological Society of America Geologic Time Scale
- 2013
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Ingår i: Geological Society of America Bulletin. - 0016-7606. ; 125:3-4, s. 259-272
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Forskningsöversikt (refereegranskat)abstract
- The Geological Society of America has sponsored versions of the geologic time scale since 1983. Over the past 30 years, the Geological Society of America Geologic Time Scale has undergone substantial modifications, commensurate with major advances in our understanding of chronostratigraphy, geochronology, astrochronology, chemostratigraphy, and the geomagnetic polarity time scale. Today, many parts of the time scale can be calibrated with precisions approaching less than 0.05%. Some notable time intervals for which collaborative, multifaceted efforts have led to dramatic improvements in our understanding of the character and temporal resolution of key evolutionary events include the Triassic-Jurassic, Permian-Triassic, and Neoproterozoic-Phanerozoic boundaries (or transitions). In developing the current Geological Society of America Time Scale, we have strived to maintain a consistency with efforts by the International Commission on Stratigraphy to develop an international geologic time scale. Although current geologic time scales are vastly improved over the first geologic time scale, published by Arthur Holmes in 1913, we note that Holmes, using eight numerical ages to calibrate the Phanerozoic time scale, estimated the beginning of the Cambrian Period to within a few percent of the currently accepted value. Over the past 100 years, the confluence of process-based geological thought with observed and approximated geologic rates has led to coherent and quantitatively robust estimates of geologic time scales, reducing many uncertainties to the 0.1% level.
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| 33. |
- Zhang, Xingliang, et al.
(författare)
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Challenges in defining the base of Cambrian Series 2 and Stage 3
- 2017
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Ingår i: Earth-Science Reviews. - : Elsevier BV. - 0012-8252. ; 172, s. 124-139
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Tidskriftsartikel (refereegranskat)abstract
- Formal subdivision of the Cambrian System into four series and ten stages is in progress. The base of Cambrian Stage 3 (provisional), which is conterminous with the base of Cambrian Series 2 (provisional), is expected to be placed at a horizon close to the first appearance of trilobites, which marks the onset of the largest phase of the Cambrian explosion. Conceptually, an ideal boundary position would be marked by a significant and globally recognizable bioevent that divides the lower part of the Cambrian System into a sub-trilobitic Terreneuvian Series and a trilobite-dominated Series 2. If the level is to be identified principally through biostratigraphic means, its position also needs to be recognizable using non-biostratigraphic means, and its correlation potential should be tested through detailed investigation of several continuous successions embracing the critical interval. Major difficulties in identifying a suitable horizon include strong biotic provincialism, a plethora of regional unconformities, and facies changes on different paleocontinents in the Cambrian Terreneuvian–Series 2 boundary interval. Levels that provide potential for intercontinental correlation include the first appearance position of trilobites and the approximately equivalent first appearance positions of certain small shelly fossil and acritarch species. In addition, a non-biostratigraphic marker, such as a stable carbon isotopic excursion, may be useful as a correlation level. Cambrian Series 2 and Stage 3 are provisional stratigraphic units, and their conterminous base was previously suggested to coincide to the appearance of trilobites. Compared to other potential chronostratigraphic indicators, the first appearance of trilobites still has advantages for recognizing and correlating the base of Stage 3, and should remain as the provisional primary marker for the boundary position.
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| 34. |
- Zhu, Maoyan, et al.
(författare)
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Reply to ‘Uppermost Cambrian carbon chemostratigraphy : The HERB and undocumented TOCE events are not synonymous’
- 2021
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Ingår i: Geological Magazine. - 0016-7568. ; 158:7, s. 1323-1326
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Tidskriftsartikel (refereegranskat)abstract
- In a recent communication on carbon isotope chemostratigraphy of the uppermost Cambrian strata, it was claimed that the Top of Cambrian Excursion (TOCE) is (1) an undocumented negative δ13Ccarb excursion; (2) ambiguously defined; (3) deliberately fictitious or, in the authors’ words, a ‘nihilartikel’; and (4) not synonymous with the Hellnmaria–Red Tops Boundary (HERB) Event. As the authors who have been involved in much of the discussion surrounding the TOCE since its introduction and in subsequent clarification, we wish to emphasize that the recent communication overlooks the fact that the TOCE is in fact a well-documented and clearly defined negative δ13Ccarb excursion, and that the term ‘HERB Event’ was originally used informally, without definition or reference data, for a negative δ13Ccarb peak, a peak later shown to occur within the TOCE excursion. Nearly a decade after the TOCE was named, the concept of the HERB Event was modified from a negative δ13Ccarb peak to a negative δ13Ccarb excursion, making it conceptually synonymous with the TOCE excursion. The recently published commentary is misleading and replete with misconceptions, as we discuss here.
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