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1. Revising the late Cambrian time scale and the duration of the SPICE event using a novel Bayesian age modeling approach

   https://doi.org/10.1130/B37919.1  

   Farrell, TP; Cothren, HR; Sundberg, FA; Schmitz, MD; Dehler, CM; Landing, E; Karlstrom, KE; Crossey, LJ; Hagadorn, JW (February 2025, Geological Society of America Bulletin)

   We present a refined global Furongian (late Cambrian) time scale derived through the application of Bayesian age modeling, using an integrative assemblage of conditioning likelihoods (age constraints) including U-Pb zircon maximum depositional ages in the Steptoean positive isotopic carbon excursion (SPICE) reference section in Smithfield Canyon (Utah, USA) and nearby McPherson Canyon (Idaho, USA); Re-Os geochronology from the SPICE-bearing interval of the Andrarum-3 core (Scania, Sweden); and new high-precision chemical abrasion−isotope dilution−thermal ionization mass spectrometry U-Pb zircon tuff ages from Avalonian Wales. We embed these radioisotopic ages within a novel probabilistic treatment of biozones to establish temporal constraints on rock accumulation rates in the Great Basin (USA), the duration of the SPICE event, and Laurentian trilobite biozones correlated to the global Cambrian time scale. Results reveal a beginning of 494.5 (+0.7/−0.6) Ma and an end of 487.3 ± 0.08 Ma for the Furongian Epoch, representing a reduction of the traditional late Cambrian by ∼30% and an extension of the Ordovician by nearly half a million years. Furthermore, the SPICE is confined to a duration of 2.6 (+0.9/−0.8) m.y. Our new approach to integrating faunal succession into Bayesian age modeling can help to constrain rock accumulation rates and possible hiatuses in sections with limited radioisotopic ages. Additionally, it offers a robust calibration tool for further refining the numerical calibration of the geologic time scale, for testing hypotheses about the rates of trilobite evolution and extinction, for evaluating causes of the SPICE, and for constraining paleoclimatic conditions including atmospheric O2 levels. 

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2. Redefining the Tonto Group of Grand Canyon and recalibrating the Cambrian time scale

   https://doi.org/10.1130/G46755.1  

   Karlstrom, K.E.; Mohr, M.T.; Schmitz, M.D.; Sundberg, F.A.; Rowland, S.M.; Blakey, R.; Foster, J.R.; Crossey, L.J.; Dehler, C.M.; Hagadorn, J.W. (February 2020, Geology)

   null (Ed.)

   Abstract We applied tandem U-Pb dating of detrital zircon (DZ) to redefine the Tonto Group in the Grand Canyon region (Arizona, USA) and to modify the Cambrian time scale. Maximum depositional ages (MDAs) based upon youngest isotope-dilution DZ ages for the Tapeats Sandstone are ≤508.19 ± 0.39 Ma in eastern Grand Canyon, ≤507.68 ± 0.36 Ma in Nevada, and ≤506.64 ± 0.32 Ma in central Arizona. The Sixtymile Formation, locally conformable below the Tapeats Sandstone, has a similar MDA (≤508.6 ± 0.8 Ma) and is here added to the Tonto Group. We combined these precise MDAs with biostratigraphy of trilobite biozones in the Tonto Group. The Tapeats Sandstone is ca. 508–507 Ma; the Bright Angel Formation contains Olenellus, Glossopleura, and Ehmaniella biozones and is ca. 507–502 Ma; and the Muav Formation contains Bolaspidella and Cedaria biozones and is ca. 502–499 Ma. The Frenchman Mountain Dolostone is conformable above the Muav Formation and part of the same transgression; it replaces McKee’s Undifferentiated Dolomite as part of the Tonto Group; it contains the Crepicephalus Biozone and is 498–497 Ma. The Tonto Group thickens east to west, from 250 m to 830 m, due to ∼300 m of westward thickening of carbonates plus ∼300 m of eastward beveling beneath the sub-Devonian disconformity. The trilobite genus Olenellus occurs in western but not eastern Grand Canyon; it has its last appearance datum (LAD) in the Bright Angel Formation ∼45 m above the ≤507.68 Ma horizon. This extinction event is estimated to be ca. 506.5 Ma and is two biozones below the Series 2–Miaolingian Epoch boundary, which we estimate to be ca. 506 Ma. Continued tandem dating of detrital grains in stratigraphic context, combined with trilobite biostratigraphy, offers rich potential to recalibrate the tempo and dynamics of Cambrian Earth systems. 

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3. A Glyptagnostus reticulatus trilobite faunule from the Cambrian of the Northern Qilian Mountains, northwest China, and its paleogeographical implications

   https://doi.org/10.1017/jpa.2021.117  

   Zhu, Xuejian; Hughes, Nigel C.; Zhou, Zhiqiang (July 2022, Journal of Paleontology)

   Abstract AGlyptagnostus reticulatus(Angelin, 1851)–bearing trilobite assemblage has been found from an unnamed Cambrian formation in the Northern Qilian Mountains area, of which geographical placement in the Cambrian is contested.Glyptagnostus reticulatusis a biostratigraphic indicator of the Furongian Series and Paibian Stage, and three agnostoid and six polymerid taxa from the Changgou section, Daliang area are described herein, along with conspecific forms from the nearby Chuancigou section. This well-preserved assemblage shows strong taxonomic affinity with northwestern Queensland, Australia, and western Hunan–eastern Guizhou, China, and likely comes from deep outer-shelf to slope setting associated with the Northern Qilian arc. It is consistent with other arguments that during the Cambrian, the Northern Qilian arc, along with the Hexi Corridor of the Alxa terrane, were more closely allied to South China than to North China. 

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4. Environmental and trilobite diversity changes during the middle-late Cambrian SPICE event

   https://doi.org/10.1130/B36421.1  

   Zhang, Lei; Algeo, Thomas J.; Zhao, Laishi; Dahl, Tais W.; Chen, Zhong-Qiang; Zhang, Zihu; Poulton, Simon W.; Hughes, Nigel C.; Gou, Xueqing; Li, Chao (May 2023, Geological Society of America Bulletin)

   The Steptoean Positive Carbon Isotope Excursion (SPICE) event at ca. 497−494 Ma was a major carbon-cycle perturbation of the late Cambrian that coincided with rapid diversity changes among trilobites. Several scenarios (e.g., climatic/oceanic cooling and seawater anoxia) have been proposed to account for an extinction of trilobites at the onset of SPICE, but the exact mechanism remains unclear. Here, we present a chemostratigraphic study of carbonate carbon and carbonate-associated sulfate sulfur isotopes (δ13Ccarb and δ34SCAS) and elemental redox proxies (UEF, MoEF, and Corg/P), augmented by secular trilobite diversity data, from both upper slope (Wangcun) and lower slope (Duibian) successions from the Jiangnan Slope, South China, spanning the Drumian to lower Jiangshanian. Redox data indicate locally/regionally well-oxygenated conditions throughout the SPICE event in both study sections except for low-oxygen (hypoxic) conditions within the rising limb of the SPICE (early-middle Paibian) at Duibian. As in coeval sections globally, the reported δ13Ccarb and δ34SCAS profiles exhibit first-order coupling throughout the SPICE event, reflecting co-burial of organic matter and pyrite controlled by globally integrated marine productivity, organic preservation rates, and shelf hypoxia. Increasing δ34SCAS in the “Early SPICE” interval (late Guzhangian) suggests that significant environmental change (e.g., global-oceanic hypoxia) was under way before the global carbon cycle was markedly affected. Assessment of trilobite range data within a high-resolution biostratigraphic framework for the middle-late Cambrian facilitated re-evaluation of the relationship of the SPICE to contemporaneous biodiversity changes. Trilobite diversity in South China declined during the Early SPICE (corresponding to the End-Marjuman Biomere Extinction, or EMBE, of Laurentia) and at the termination of the SPICE (corresponding to the End-Steptoean Biomere Extinction, or ESBE, of Laurentia), consistent with biotic patterns from other cratons. We infer that oxygen minimum zone and/or shelf hypoxia expanded as a result of locally enhanced productivity due to intensified upwelling following climatic cooling, and that expanded hypoxia played a major role in the EMBE at the onset of SPICE. During the SPICE event, global-ocean ventilation promoted marine biotic recovery, but termination of SPICE-related cooling in the late Paibian may have reduced global-ocean circulation, triggering further redox changes that precipitated the ESBE. Major changes in both marine environmental conditions and trilobite diversity during the late Guzhangian demonstrate that the SPICE event began earlier than the Guzhangian-Paibian boundary, as previously proposed. 

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5. Trilobites of Thailand's Cambrian–Ordovician Tarutao Group and their geological setting

   https://doi.org/10.1002/spp2.1516  

   Wernette, Shelly J.; Hughes, Nigel C.; Myrow, Paul M.; Sardsud, Apsorn (September 2023, Papers in Palaeontology)

   Abstract Tuff‐bearing upper Cambrian to lowermost Ordovician strata on Ko Tarutao island, Satun province, southernmost peninsular Thailand, contain a rich trilobite fauna relevant to global biostratigraphy, peri‐Gondwanan palaeogeography and shifting evolutionary mode. This area of Sibumasu, a lower Palaeozoic marginal Gondwanan terrane, is shown to have been closely associated with Australia, North China (Sino‐Korea) and other continental fragments from the supercontinent's northern equatorial sector, including South China at that time. Shared faunas also suggest a Kazakhstani and Laurentian association. Collections from eight sections yielded 10 newly discovered species and one new genus from ancient shoreface and inner shelf siliciclastic deposits. With the new taxa and revision of taxa known previously, we refine the age of the upper two formations of the Tarutao Group to the middle of Cambrian Stage 10, and lower–middle Tremadocian. Two biozones are erected for Sibumasu: theEosaukia buravasiZone, encompassing all Cambrian sections from Ko Tarutao, and theAsaphellus charoenmitiZone, encompassing the Tremadocian fauna discussed herein. The new genus isTarutaoiaand new species areTsinania sirindhornae,Pseudokoldinioidia maneekuti,Pagodia?uhleini,Asaphellus charoenmiti,Tarutaoia techawani,Jiia talowaois,Caznaia imsamuti,Anderssonella undulata,Lophosaukia nuchanongiandCorbinia perforata. Other taxa reported for the first time from Tarutao areMansuyia? sp.,Parakoldinioidia callosaQian,Pseudagnostussp.,Homagnostussp.,Haniwa mucronataShergold,Haniwa sosanensis? Kobayashi,Lichengia simplexShergold,Pacootasaukiasp.,Wuhuia? sp.,Plethopeltellasp.,Apatokephalussp.,Akoldinioidiasp. 1 andKoldinioidiasp. 

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