More Like this

1. Late Neoproterozoic – early Paleozoic basin evolution in the Coal Creek inlier of Yukon, Canada: implications for the tectonic evolution of northwestern Laurentia

   https://doi.org/10.1139/cjes-2020-0132  

   Busch, James F.; Rooney, Alan D.; Meyer, Edward E.; Town, Caleb F.; Moynihan, David P.; Strauss, Justin V. (April 2021, Canadian Journal of Earth Sciences)

   null (Ed.)

   The age and nature of the Neoproterozoic – early Paleozoic rift–drift transition has been interpreted differently along the length of the North American Cordillera. The Ediacaran “upper” group (herein elevated to the Rackla Group) of the Coal Creek inlier, Yukon, Canada, represents a key succession to reconstruct the sedimentation history of northwestern Laurentia across the Precambrian–Cambrian boundary and elucidate the timing of active tectonism during the protracted breakup of the supercontinent Rodinia. These previously undifferentiated late Neoproterozoic – early Paleozoic map units in the Coal Creek inlier are herein formally defined as the Lone, Cliff Creek, Mount Ina, Last Chance, Shade, and Shell Creek formations. New sedimentological and stratigraphic data from these units is used to reconstruct the depositional setting. In the Last Chance Formation, chemostratigraphic observations indicate a ca. 5‰ δ 13 C carb gradient coincident with the globally recognized ca. 574–567 Ma Shuram carbon isotope excursion. Map and stratigraphic relationships in the overlying Shell Creek Formation provide evidence for latest Ediacaran – middle Cambrian tilting and rift-related sedimentation. This provides evidence for active extension through the Cambrian Miaolingian Series in northwestern Canada, supporting arguments for a multiphase and protracted breakup of Rodinia. 

   more » « less
2. Small shelly fossils and carbon isotopes from the early Cambrian (Stages 3–4) Mural Formation of western Laurentia

   https://doi.org/10.1002/spp2.1313  

   Skovsted, Christian B.; Balthasar, Uwe; Vinther, Jakob; Sperling, Erik A.; Álvaro, ed., Javier (June 2020, Papers in Palaeontology)

   Abstract The extraordinary window of phosphatized and phosphatic small shelly fossils (SSF) during the early and middle Cambrian is an important testament to the radiation of biomineralizing metazoans. WhileSSFare well known from most Cambrian palaeocontinents during this time interval, western Laurentia has relatively fewSSFfaunas. Here we describe a diverseSSFfauna from the early Cambrian (Stages 3–4) Mural Formation at three localities in Alberta and British Columbia, Canada, complemented by carbon isotope measurements to aid in a potential future bio‐chemostratigraphic framework. The fauna expands the recordedSSFassemblage diversity in western Laurentia and includes several brachiopods, four bradoriids, three chancelloriids, two hyoliths, a tommotiid and a helcionellid mollusc as well as echinoderm ossicles and specimens ofMicrodictyon,VolborthellaandHyolithellus. New taxa include the tommotiid genusCanadiellagen. nov., the new bradoriid speciesHipponicharion perforatasp. nov. andPseudobeyrichona tauratasp. nov. Compared with contemporaneous faunas from western Laurentia, the fauna is relatively diverse, particularly in taxa with originally phosphatic shells, which appear to be associated with archaeocyathid build‐ups. This suggests that the generally low faunal diversity in western Laurentia may be at least partly a consequence of poor sampling of suitable archaeocyathan reef environments. In addition, the tommotiidCanadiella filigranaappears to be of biostratigraphical significance in Cambrian Stage 3 strata of western Laurentia, and the unexpected high diversity of bradoriid arthropods in the fauna also suggests that this group may prove useful for biostratigraphical resolution in the region. 

   more » « less
3. Asynchronous trilobite extinctions at the early to middle Cambrian transition

   https://doi.org/10.1130/G46913.1  

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

   null (Ed.)

   Abstract Trilobites appeared and diversified rapidly in the Cambrian, but it is debated as to whether their radiations and extinctions were globally synchronous or geographically restricted and diachronous. The end of the early Cambrian is a classic example—it has traditionally been defined by the extinction of olenellid and redlichiid trilobites and the appearance of paradoxidid trilobites. Here we integrate the global biostratigraphy of these three trilobite groups with high-precision tuff and tandem detrital zircon U-Pb age constraints to falsify prior models for global synchronicity of these events. For the first time, we demonstrate that olenellid trilobites in Laurentia went extinct at least 3 Ma after the first appearance of paradoxidids in Avalonia and West Gondwana (ca. 509 Ma). They also disappeared before the extinction of redlichiids and prior to the base of the Miaolingian at ca. 506 Ma in South China. This indicates that these three trilobite groups (paradoxidids, olenellids, and redlichiids) and their associated biotas overlapped in time for nearly 40% of Cambrian Epoch 2, Age 4. Implications of this chronological overlap are: (1) trilobite transitions were progressive and geographically mediated rather than globally synchronous; and (2) paleontological databases underestimate the diversity of the early Cambrian. This ∼3 Ma diachroneity, at a critical time in the early evolution of animals, also impacts chemostratigraphic and paleoclimatic data sets that are tied to trilobite biostratigraphy and that collectively underpin our understanding of the Cambrian Earth system. 

   more » « less
4. A link between rift-related volcanism and end-Ediacaran extinction? Integrated chemostratigraphy, biostratigraphy, and U-Pb geochronology from Sonora, Mexico

   https://doi.org/10.1130/G47972.1  

   Hodgin, Eben B.; Nelson, Lyle L.; Wall, Corey J.; Barrón-Díaz, Arturo J.; Webb, Lucy C.; Schmitz, Mark D.; Fike, David A.; Hagadorn, James W.; Smith, Emily F. (September 2020, Geology)

   null (Ed.)

   Abstract We present chemostratigraphy, biostratigraphy, and geochronology from a succession that spans the Ediacaran-Cambrian boundary in Sonora, Mexico. A sandy hematite-rich dolostone bed, which occurs 20 m above carbonates that record the nadir of the basal Cambrian carbon isotope excursion within the La Ciénega Formation, yielded a maximum depositional age of 539.40 ± 0.23 Ma using U-Pb chemical abrasion–isotope dilution–thermal ionization mass spectrometry on a population of sharply faceted volcanic zircon crystals. This bed, interpreted to contain reworked tuffaceous material, is above the last occurrences of late Ediacaran body fossils and below the first occurrence of the Cambrian trace fossil Treptichnus pedum, and so the age calibrates key markers of the Ediacaran-Cambrian boundary. The temporal coincidence of rift-related flood basalt volcanism in southern Laurentia (>250,000 km3 of basalt), a negative carbon isotope excursion, and biological turnover is consistent with a mechanistic link between the eruption of a large igneous province and end-Ediacaran extinction. 

   more » « less
5. Fortunian archaeocyath sponges acquired biomineralization in the beginning of the Cambrian explosion

   https://doi.org/10.1130/G53249.1  

   Wang, Qi; Dai, Qiaokun; Vayda, Prescott; Luo, Jinzhou; Shao, Tiequan; Liu, Yunhuan; Hua, Hong; Xiao, Shuhai (April 2025, Geology)

   Archaeocyaths are biocalcified sponges largely restricted to the early Cambrian Period. Their perforated cup-shaped body facilitated filter feeding. Many of them were clonal modular animals that formed the earliest metazoan skeletal reefs. In Siberia, archaeocyaths extend from late Age 2 to Age 4 of the early Cambrian, representing an ∼15 m.y. range (ca. 525−510 Ma). Elsewhere, archaeocyaths emerged later than in Siberia and, in places, survived to the middle-late Cambrian. The existing fossil record thus indicates an out-of-Siberia scenario and delayed biomineralization in archaeocyaths relative to many other animals, which acquired biomineralization in the Fortunian Age of the early Cambrian. Here we report two microscopic archaeocyath species—Primocyathus uniseriatus Wang and Xiao, gen. et sp. nov. and Sinocyathus biseriatus Wang and Xiao, gen. et sp. nov.—from the Fortunian Kuanchuanpu Formation (ca. 533 Ma) in South China. Preserved as phosphatized internal molds, they are interpreted to have had a biomineralized, two-walled, perforated, cup-shaped skeleton. They were likely filter feeders, but their solitary habit and millimetric body size indicate that they were unlikely reef framework builders. They substantially extend the stratigraphic range of archaeocyaths, challenge the out-of-Siberia hypothesis, support archaeocyath biomineralization in the beginning of the Cambrian explosion, and imply a Precambrian divergence of sponge classes. 

   more » « less