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1. The trace fossil record of the Nama Group, Namibia: Exploring the terminal Ediacaran roots of the Cambrian explosion

   https://doi.org/https://doi.org/10.1016/j.earscirev.2020.103435  

   Darroch, S.A. ( January 2020 , Earthscience reviews)

   null (Ed.)

   The Ediacaran–Cambrian transition marks one of the most important geobiological revolutions in Earth History, including multiple waves of evolutionary radiation and successive episodes of apparent mass extinction. Among the proposed drivers of these events (in particular the extinction of the latest Neoproterozoic ‘Ediacara biota’) is the emergence of complex metazoans and their associated behaviors. Many metazoans are thought to have crucial geobiological impacts on both resource availability and the character of the physical environment – ‘ecosystem engineering’ – biological processes best preserved in the geological record as trace fossils. Here, we review this model using the trace fossil record of the Ediacaran to Cambrian Nama Group of southern Namibia, combining previous published accounts with the results of our own field investigations. We produce a revised ichnostratigraphy for the Nama Group that catalogues new forms, eliminates others, and brings the trace fossil record of the Nama into much closer alignment with what is known from other Ediacaran sections worldwide. We provide evidence for a link between sequence stratigraphy, oxygen, and the emergence of more complex bilaterian behaviors. Lastly, we show that observed patterns of extinction and survival over pulses of Ediacaran extinction are hard to ally with any one specific source of ecological stress associated with bioturbation, and thus a biologically-driven extinction of the Ediacara biota, if it occurred, was more likely to have been driven by some combination of these factors, rather than any single one. 

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2. The Temporal and Environmental Context of Early Animal Evolution: Considering All the Ingredients of an “Explosion”

   https://doi.org/https://doi.org/10.1093/icb/icy088  

   Sperling, E.A. ; Stockey, R.G. ( January 2018 , Integrative and comparative biology)

   Animals originated and evolved during a unique time in Earth history—the Neoproterozoic Era. This paper aims to discuss (1) when landmark events in early animal evolution occurred, and (2) the environmental context of these evolutionary milestones, and how such factors may have affected ecosystems and body plans. With respect to timing, molecular clock studies—utilizing a diversity of methodologies—agree that animal multicellularity had arisen by ∼800 million years ago (Ma) (Tonian period), the bilaterian body plan by ∼650 Ma (Cryogenian), and divergences between sister phyla occurred ∼560–540 Ma (late Ediacaran). Most purported Tonian and Cryogenian animal body fossils are unlikely to be correctly identified, but independent support for the presence of pre-Ediacaran animals is recorded by organic geochemical biomarkers produced by demosponges. This view of animal origins contrasts with data from the fossil record, and the taphonomic question of why animals were not preserved (if present) remains unresolved. Neoproterozoic environments demanding small, thin, body plans, and lower abundance/rarity in populations may have played a role. Considering environmental conditions, geochemical data suggest that animals evolved in a relatively low-oxygen ocean. Here, we present new analyses of sedimentary total organic carbon contents in shales suggesting that the Neoproterozoic ocean may also have had lower primary productivity—or at least lower quantities of organic carbon reaching the seafloor—compared with the Phanerozoic. Indeed, recent modeling efforts suggest that low primary productivity is an expected corollary of a low-O2 world. Combined with an inability to inhabit productive regions in a low-O2 ocean, earliest animal communities would likely have been more food limited than generally appreciated, impacting both ecosystem structure and organismal behavior. In light of this, we propose the “fire triangle” metaphor for environmental influences on early animal evolution. Moving toward consideration of all environmental aspects of the Cambrian radiation (fuel, heat, and oxidant) will ultimately lead to a more holistic view of the event. 

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3. 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. 

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4. Detrital zircon geochronology and provenance of the Middle to Late Jurassic Paradox Basin and Central Colorado trough: Paleogeographic implications for southwestern Laurentia

   https://doi.org/10.1130/GES02264.1  

   Ejembi, John I. ; Potter-McIntyre, Sally L. ; Sharman, Glenn R. ; Smith, Tyson M. ; Saylor, Joel E. ; Hatfield, Kendall ; Ferré, Eric C. ( July 2021 , Geosphere)

   Abstract Middle to Upper Jurassic strata in the Paradox Basin and Central Colorado trough (CCT; southwestern United States) record a pronounced change in sediment dispersal from dominantly aeolian deposition with an Appalachian source (Entrada Sandstone) to dominantly fluvial deposition with a source in the Mogollon and/or Sevier orogenic highlands (Salt Wash Member of the Morrison Formation). An enigmatic abundance of Cambrian (ca. 527–519 Ma) grains at this provenance transition in the CCT at Escalante Canyon, Colorado, was recently suggested to reflect a local sediment source from the Ancestral Front Range, despite previous interpretations that local basement uplifts were largely buried by Middle to Late Jurassic time. This study aims to delineate spatial and temporal patterns in provenance of these Jurassic sandstones containing Cambrian grains within the Paradox Basin and CCT using sandstone petrography, detrital zircon U-Pb geochronology, and detrital zircon trace elemental and rare-earth elemental (REE) geochemistry. We report 7887 new U-Pb detrital zircon analyses from 31 sandstone samples collected within seven transects in western Colorado and eastern Utah. Three clusters of zircon ages are consistently present (1.53–1.3 Ga, 1.3–0.9 Ga, and 500–300 Ma) that are interpreted to reflect sources associated with the Appalachian orogen in southeastern Laurentia (mid-continent, Grenville, Appalachian, and peri-Gondwanan terranes). Ca. 540–500 Ma zircon grains are anomalously abundant locally in the uppermost Entrada Sandstone and Wanakah Formation but are either lacking or present in small fractions in the overlying Salt Wash and Tidwell Members of the Morrison Formation. A comparison of zircon REE geochemistry between Cambrian detrital zircon and igneous zircon from potential sources shows that these 540–500 Ma detrital zircon are primarily magmatic. Although variability in both detrital and igneous REE concentrations precludes definitive identification of provenance, several considerations suggest that distal sources from the Cambrian granitic and rhyolitic provinces of the Southern Oklahoma aulacogen is also likely, in addition to a proximal source identified in the McClure Mountain syenite of the Wet Mountains, Colorado. The abundance of Cambrian grains in samples from the central CCT, particularly in the Entrada Sandstone and Wanakah Formation, suggests northwesterly sediment transport within the CCT, with sediment sourced from Ancestral Rocky Mountains uplifts of the southern Wet Mountains and/or Amarillo-Wichita Mountains in southwestern Oklahoma. The lack of Cambrian grains within the Paradox Basin suggests that the Uncompahgre uplift (southwestern Colorado) acted as a barrier to sediment transport from the CCT. 

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5. The Ediacaran−Cambrian transition in the southern Great Basin, United States

   https://doi.org/10.1130/B36401.1  

   Smith, Emily F. ; Nelson, Lyle L. ; O’Connell, Nizhoni ; Eyster, Athena ; Lonsdale, Mary C. ( September 2022 , GSA Bulletin)

   The Ediacaran−Cambrian boundary strata in the Great Basin of the southwestern United States record biological, geochemical, and tectonic change during the transformative interval of Earth history in which metazoans diversified. Here, we integrate new and compiled chemostratigraphic, paleontological, sedimentological, and stratigraphic data sets from the Death Valley region, the White-Inyo Ranges, and Esmeralda County in Nevada and California and evaluate these data within a regional geologic framework. A large negative carbon isotope (δ13C) excursion—also known as the Basal Cambrian Excursion, or BACE—is regionally reproducible, despite lateral changes in sedimentary facies and dolomitization across ∼250 km, consistent with a primary marine origin for this perturbation. Across the southern Great Basin, Ediacaran body fossils are preserved in a variety of taphonomic modes, including cast and mold preservation, two-dimensional compressional preservation, two-dimensional and three-dimensional pyritization, and calcification. The stratigraphic framework of these occurrences is used to consider the relationships among taphonomic modes for fossil preservation and paleoenvironmental settings within this basin. In this region, Ediacaran-type fossils occur below the nadir of the BACE, while Cambrian-type trace fossils occur above. Sedimentological features that include giant ooids, stromatolites, and textured organic surfaces are widespread and abundant within the interval that records biotic turnover and coincide with basaltic volcanism and the BACE. We hypothesize that the prevalence of these sedimentological features, the BACE, and the disappearance of some Ediacaran clades were caused by environmental perturbation at the Ediacaran-Cambrian boundary. 

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