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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.

Stromatolites archive critical information on Precambrian marine environments, but their geochemical signals are often obscured using complex diagenetic processes. Tonian stromatolites from the Weiji Formation, North China, show selective dolomitization in dark stromatolitic laminae, forming zoned dolomites. The zoned dolomite crystals with well‐preserved growth zoning record the evolution of diagenetic fluid, yet their genesis remains controversial. To constrain selective dolomitization processes and zoned dolomite formation in stromatolites, and further assess their reliability as paleoceanographic proxies, we use LA‐ICP‐TOF‐MS elemental mapping to reveal crystallization mechanisms and early diagenetic controls in stromatolitic carbonates. Results show distinct geochemical distribution patterns between dark and light stromatolitic laminae, reflecting complex microbial‐induced diagenetic reactions during early diagenesis. Fe is preferentially concentrated in dark stromatolitic layers, while Mn anomalously accumulates in light layers (with Mn/Fe ratio up to ∼2). Meanwhile, high terrigenous‐indicative elemental contents (Al‐Si‐K) are observed in dark laminae and the adjacent clay‐rich matrix. The anomalous Fe‐Mn distribution is attributed to the redox oscillations and terrigenous pulses (ROTP) model. Selective dolomitization proceeds through an Ion‐Exchange Motor mechanism during penecontemporaneous to early diagenesis, where Mg2+ derived from seawater undergoes dehydration via clay mineral adsorption and/or microbial mediation. Within ferroan zoned dolomite, elemental zoning exhibits alternating Mn‐enriched bands and Fe‐rich zones, indicating redox‐controlled diagenesis with preferential Mn(IV) reduction prior to Fe(II) mobilization. While Precambrian stromatolites remain valuable proxies for paleo‐ocean chemistry, our results emphasize the critical need for in situ analytical approaches to distinguish primary signals from diagenetic overprints in Precambrian carbonate systems.

Detrital zircon U–Pb ages from samples of the Neoproterozoic Visingsö Group, Sweden, yield a maximum depositional age of ≤ 886±9 Ma (2σ). A minimum depositional age is established biochronologically using organic-walled and vase-shaped microfossils present in the upper formation of the Visingsö Group; the upper formation correlates with the Kwagunt Formation of the 780–740 Ma Chuar Group in Arizona, USA, and the lower Mount Harper Group, Yukon, Canada, that is older than 740 Ma. Mineralized scale microfossils of the type recorded from the upper Fifteenmile Group, Yukon, Canada, where they occur in a narrow stratigraphic range and are younger than 788 Ma, are recognized for the first time outside Laurentia. The mineralized scale microfossils in the upper formation of the Visingsö Group seem to have a wider stratigraphic range, and are older than c. 740 Ma. The inferred age range of mineralized scale microfossils is 788–740 Ma. This time interval coincides with the vase-shaped microfossil range because both microfossil groups co-occur. The combined isotopic and biochronologic ages constrain the Visingsö Group to between ≤ 886 and 740 Ma, thus Tonian in age. This is the first robust age determination for the Visingsö Group, which preserves a rich microfossil assemblage of worldwide distribution. The organic and mineralized microorganisms preserved in the Visingsö Group and coeval successions elsewhere document global evolutionary events of auto- and heterotrophic protist radiations that are crucial to the reconstruction of eukaryotic phylogeny based on the fossil record and are useful for the Neoproterozoic chronostratigraphic subdivision.

The diversification of protists and multicellular microorganisms is recorded in numerous worldwide Tonian age successions, including the Visingsö Group in Sweden. The Visingsö Group contains a taxonomically rich assemblage of cyanobacteria, stromatolites, algal phytoplankton and vase-shaped microfossils. A new record of organic-walled microfossils from the Visingsö 1 drillcore reveals the high taxonomic diversity. Several species are reported for the first time from the Visingsö Group, and one new species Leiosphaeridia gorda n. sp. is described. They are in gross phycoma-like cysts of the prasinophycean algae Pterospermopsimorpha, Pterospermella, Simia, Macroptycha and Dictyotidium. Morphologically similar to zygotic cysts of chlorophycean algae are Leiosphaeridia gorda n. sp., Cerebrosphaera, Culcitulisphaera and Lanulatisphaera. Schizofusa may represent the earliest yellow-green algae of the Eustigmatiphyte among Stramenopiles. The recorded biodiversity documents the global trend in the evolution of eukaryotic protists during the Tonian Period and the increased radiation of numerous, presumably photoautotrophic biotas, representing various algal lineages.

Proterozoic eukaryotic macroalgae are difficult to interpret because morphological details required for proper phylogenetic studies are rarely preserved. This is especially true of morphologically simple organisms consisting of tubes, ribbons, or spheres that are commonly found in a wide array of bacteria, plants, and even animals. Previous reports of exceptionally preserved Tonian (ca. 950–900 Ma) fossils from the Dolores Creek Formation of Northwestern Canada feature enough morphological evidence to support a green macroalgal affinity. However, the affinities of two additional forms identified on the basis of the size distribution of available specimens remain undetermined, while the presence of three unique algal forms supports other reports of increasing algal diversity in the early Neoproterozoic. Archaeochaeta guncho new genus new species is described as a green macroalga on the basis of its well-preserved morphology consisting of an unbranching, uniseriate thallus with uniform width throughout and possessing an elliptical to globose anchoring holdfast. A larger size class of ribbon-like forms is interpreted as Vendotaenia sp. A third size class is significantly smaller than Archaeochaeta n. gen. and Vendotaenia, but in the absence of clear morphological characters, it remains difficult to assign. As Archaeochaeta n. gen. and Vendotaenia represent photoautotrophic taxa, these findings support the hypothesis of increasing morphological complexity and phyletic diversification of macroalgae during the Tonian, leading to dramatic changes within benthic marine ecosystems before the evolution of animals.

Brittle reactivation of plastic shear zones is frequently observed in geologically old terranes. To better understand such deformation zones, we have studied the >700 Ma long structural history of the Himdalen–Ørje Deformation Zone (HØDZ) in SE Norway by K–Ar and 40Ar–39Ar geochronology, and structural characterization. Several generations of mylonites make up the ductile part of HØDZ, the Ørje Shear Zone. A 40Ar–39Ar white mica plateau age of 908.6 ± 7.0 Ma constrains the timing of extensional reactivation of the Ørje mylonite. The mylonite is extensively reworked during brittle deformation events by the Himdalen Fault. 40Ar–39Ar plateau ages of 375.0 ± 22.7 Ma and 351.7 ± 4.4 Ma from pseudotachylite veins and K–Ar ages of authigenic illite in fault gouge at c. 380 Ma are interpreted to date initial brittle deformation, possibly associated with the Variscan orogeny. Major brittle deformation during the Early–Mid Permian Oslo Rift is documented by a 40Ar–39Ar pseudotachylite plateau age of 294.6 ± 5.2 Ma and a K–Ar fault gouge age of c. 270 Ma. The last datable faulting event is constrained by the finest size fraction in three separate gouges at c. 200 Ma. The study demonstrates that multiple geologically significant K–Ar ages can be constrained from fault gouges within the same fault core by combining careful field sampling, structural characterization, detailed mineralogy and illite crystallinity analysis. We suggest that initial localization of brittle strain along plastic shear zones is controlled by mechanical anisotropy of parallel-oriented, throughgoing phyllosilicate-rich foliation planes within the mylonitic fabric.

Strongly peraluminous (SP) leucogranite typical of Himalayan-type collision orogen is reported from Kottayam, the western Madurai block, Southern Granulite Terrane (SGT). Low concentration of mafics (average ≈ 5%), consistently high A/CNK (≥1.1), and invariable presence of garnet and normative corundum (1.24–2.12 wt.%) qualify Kottayam granite (KG) as SP leucogranite. Geochemically, KG is ferroan to magnesian (F* = 0.79–0.90), alkali-calcic to calc-alkali (MALI = 6.64–7.54), peraluminous (ASI = 1.19–1.26) and plot in peraluminous leucogranite field. Further, all the KG samples fall within the ‘SP granite quadrilateral,’ defined by the Al 2 O 3 /TiO 2 and CaO/Na 2 O ratios of SP granites from orogens around the world. The REE pattern is akin to SP granites from the Himalayas and the Hercynides. Various tectonic discrimination criteria and plots indicate that KG formed in a Himalayan-type collisional orogeny. Source discrimination norms, together with zircon ε Hf values (−6.62 to −4.02), strongly point to a crustal source comprising an admixture of metapelites and meta-greywacke. U–Pb analyses of zircons define a concordia age of 741 ± 12 Ma, which coincides with the timing of the breakup of the Rodinia supercontinent. Mineralogical, geochemical, and isotopic evidence, thus, demonstrate that KG exemplifies crustal recycling during the Tonian period. Research highlights Strongly peraluminous (SP) leucogranite distinctive of Himalayan-type collision orogens is reported from Kottayam, Kerala, which forms part of the western Madurai block, Southern granulite terrane (SGT), southern India. Mineralogy, geochemistry, and isotope data substantiate that this SP leucogranite formed in Himalayan-type collision orogeny. Geochemical and isotope indices show that present SP leucogranite formed from a pelite-psammite mixed source, signifying crustal recycling, during the Tonian period in SGT. Chemical proxies and geochronology data suggest that this SP leucogranite is probably associated with the post-collisional phase of collision orogeny, contemporaneous with Rodinia rifting.

The Larsemann Hills sector of East Antarctica preserves complexly deformed Mesoproterozoic to Neoproterozoic-aged composite gneisses (Sostrene orthogneiss) and interleaved sequence of metasedimentary rocks (Brattstrand paragneiss). The Easther Island porphyroblastic gneiss, a unit of erstwhile ‘Brattstrand paragneiss’, is investigated in this work. Easther Island porphyroblastic gneiss comprises garnet + biotite + quartz + plagioclase + K-feldspar + ilmenite + sillimanite and preserves thick plagioclase corona over coarse-grained garnet. Conventional thermometry and phase equilibria modelling indicate temperatures of 830°C defining peak attained during metamorphism followed by decompression at ~4 kbar pressure. Zircon U–Pb isotopic data yield Concordia ages of 725 ± 43 and 566 ± 15 Ma, respectively. Whereas chemical geochronological data of in-situ monazites provide a unimodal CHIME age of 510 ± 27 Ma with no early Neoproterozoic signature. The new data is discussed in light of the much-debated relict Upper Tonian to Lower Cryogenian (800–700 Ma) event registered by the Larsemann Hills sector, followed by an early Paleozoic orogenic event related to the final amalgamation of Gondwana.

Deciphering the sub-ice geology in the Wilkes Subglacial Basin region is important for understanding solid earth-ice sheet evolution and for assessing geological ties between East Antarctica and formerly contiguous Australia. We analyse marine sediment samples derived from drill site U1359 of Integrated Oceanic Drilling Program Expedition 318. Our study reports for the first time that the inland sediment source area comprises a complex mafic igneous terrain and a metamorphosed Precambrian subglacial basement. Pyroxene geochemical analyses confirm the presence of tholeiitic to calc-alkaline basalts. The high-grade part of the subglacial terrain contains upper amphibolite to granulite facies rocks that are comparable to Archaean to Palaeoproterozoic rocks exposed in the Terre Adélie Craton and the formerly adjacent Gawler Craton in Australia. Chemical Th-U-total Pb isochron method (CHIME) ages extracted from a subhedral monazite grain associated with the low-grade biotite-muscovite schist rock fragment provide a unimodal age of 799 ± 13 Ma. Rare occurrences of 800 Ma age in the Terre Adélie Craton and/or George V Coast provide evidence for the presence of at least one late Neoproterozoic magmato-metamorphic event in the interior of Wilkes Land. The affinity of the unexposed geological domains of Wilkes Land, East Antarctica, with their Australian counterparts is discussed in the context of the Rodinia supercontinent.

The origin of eclogite-bearing granitoid gneisses and metapelites of the Chuacús Complex is investigated. This complex represents the internal basement massif of the Guatemala Suture Zone, a part of the western North America–Caribbean plate boundary. LA-ICP-MS U-Pb and trace element zircon data are combined with whole-rock Sm-Nd and Lu-Hf isotopes to re-evaluate granitoid petrogenesis and inquire into the sedimentary record. New granitoid ages of ca. 1030–1010 Ma are reported, adding to those already known of ca. 1100, 990 and 225 Ma. Stenian A-type granitoids within the bimodal Cubulco unit formed by mixing of magmas derived from late Palaeoproterozoic crust and mantle-derived melts produced in an extensional setting during Rodinia assembly. During the Tonian, an extended (or later) period of extensional tectonics produced peraluminous granitoids (Pachajob gneiss) by anatexis of rejuvenated late Mesoproterozoic crust. After a hiatus encompassing most of the Neoproterozoic, marine sedimentation occurred between the Ediacaran and the early Palaeozoic as recorded by the Palibatz schist, a sequence formed by detritus sourced from peri-Gondwanan continental areas. No evidence of middle to late Palaeozoic magmatism or sedimentation was found in the studied area. Late Triassic granitoids (Agua Caliente unit) were produced by mixing melts from late Mesoproterozoic crust with enriched mantle magmas in response to post-collisional thinning during the western Pangea breakup. This extensional stage led to considerable thinning of the Chuacús crust and its evolution into a passive margin that would be prone to subduct during the Cretaceous.