Continent and Life Evolution
ISSN: 3008-0851 (Print)
ISSN: 3008-086X (Online)
CODEN: CLEABK
Trilobite biofacies reveal a ‘dual-slope’ paleogeographic pattern of South China during the Early–Middle Ordovician
Trilobite biofacies are often indicative of particular paleoecological settings and can reveal spatial paleogeographic changes. Multivariate analyses (CA, PCoA and NMDS) were performed using the Raup-Crick coefficient on a dataset of 62 trilobite genera and subgenera from 15 localities across South China during the Early–Middle Ordovician (Floian–Dapingian). Seven trilobite biofacies are distinguished from platform to slope environments, showing prominent ecological differentiation along bathymetric gradients and across different substrate conditions. Benthic and endemic trilobites generally occurred on the shallow-water platform, whereas the mesopelagic and atheloptic forms were associated with graptolite-dominated slope facies. The Olenid Biofacies and the Cyclopygid Biofacies, identified from deep-water environments, support the existence of the Jiangbei Slope (Northern Slope) and the Jiangnan Slope (Southern Slope) in South China at least during the Early–Middle Ordovician.
Healing and regeneration in horseshoe crab, with possible application to trilobite injuries
Trilobites exhibiting injuries are ideal palaeontological archives for Palaeozoic predator-prey interactions. However, the current methods for diagnosing trilobite injuries primarily depend on the various morphological features at the edges of the injured exoskeleton, without evaluating the process and timing of morphological changes. Here we performed an amputation experiment utilizing the extant horseshoe crab as a proxy of trilobites to study the morphological changes after injury. The results show that: the wound closure was accomplished by forming hemolymph clotting on the wound surface in a short time. And then, a thin cuticle layer was growing on the inner side of wounded exoskeleton, with the retention of a sharp outline of the wound. Particularly, the newly-formed exoskeleton only regenerates and modifies the shape of the wound subsequent to moulting. After rechecking 409 abnormal trilobites documented in the published literature, we recognized that the injuries with sharp exoskeleton edge are very rare in the fossil record, and 95% of injured trilobites with clearly identifiable scars had undergone at least one subsequent ecdysis. This indicates that the majority of the injuries were not identified, as the sharp wounds were indistinguishable from post-mortem breakage. These unidentified injuries may encompass successful predation events, lethal injuries, and damage incurred during the adult stage, none of which exhibit signs of moulting. Although we are currently unable to deal with this recognition bias, through injury experiments on living arthropods, our understanding of the damage types and repair processes of trilobites will be further enriched.
Research on nonlinear rheology perspective of loess mudflows based on large amplitude oscillatory shear (LAOS)
Loess mudflows are typical geological hazards that undergo a dynamic solid-to-liquid transition, yet the yielding process remains poorly constrained. This study investigates the nonlinear rheological behavior of loess slurries with water contents of 28%, 32%, and 36% using large amplitude oscillatory shear (LAOS) and Fourier-transform rheology at frequencies from 0.1 to 10 Hz. Results show that the rheological response evolves from linear behavior to solid-like nonlinearity with a G″ overshoot, and finally to fluid-like behavior. Increasing frequency lowers the linear limit and nonlinear transition strain but raises the nonlinear limit and solid–liquid transition strain. Higher water content can increase the strain required for complete liquefaction; at 36% water content and 5-10 Hz, G′ remains above G″ and no solid–liquid transition occurs. The nonlinear coefficient Q remains nearly constant at small strains and decays with increasing strain and sensitive to strain, water content, and frequency, serving as an early indicator of structural change. SEM observations reveal that higher water content disperses the soil structure and weakens interparticle interactions, explaining the weaker nonlinearity and delayed fluidization. These findings provide a strain-resolved characterization of the solid–liquid transition in loess mudflows and offer theoretical support for hazard assessment and early warning.
Geochronology and geochemistry of mafic rocks in the Wutai area, North China: constraints on the latest Neoarchean–Paleoproterozoic tectonic setting of the Trans-North China Orogen and the onset of plate tectonics
The Neoarchean-Paleoproterozoic is a key period for the North China Craton due to the formation of continental crust and the beginning of oceanic subduction. The amalgamation of the Eastern and Western blocks of the North China Craton remains debated, particularly regarding the timing, directionality (eastward vs. westward subduction), and number of collisional phases. Previous studies disproportionately focus on felsic lithologies, creating a critical mafic blind spot that obscures insights into mantle dynamics and crust-mantle interactions. Systematic investigation of Late Archean to Paleoproterozoic mafic suites was essential to reconcile conflicting tectonic models and refine the craton’s amalgamation history. In order to constrain the complex geological processes more clearly, we present new whole-rock geochemistry and zircon U-Pb geochronology for mafic rocks in the Wutai Complex. The Late Archean (2.56~2.54 Ga) gabbros are characterized by relatively depleted Nb–Ta and Zr–Hf anomalies, slightly positive Eu* anomalies, low K₂O concentrations, enrichments of LILE (Ba, Cs, Th, and Rb), differentiated HFSE patterns and moderate Mg# values (43–53). They show positive εNd (t) values (+4.9–+6.1) and high (⁸⁷Sr/⁸⁶Sr)i ratios (0.70063–0.70091). Additionally, these gabbros have high Nb concentration (13.3–23.3) and display enrichments in light rare earth elements (LREE) ((La/Yb)N = 7.00–8.96), high-field strength elements (HFSE, e.g., Nb, Ta, Zr), and high Nb/U and Nb/La ratios, suggesting a derivation from an arc-like mantle source. The gabbro melts were generated by a low degree of 4%–5% partial melting of garnet-spinel lherzolite mantle. The Paleoproterozoic (2.16~2.08 Ga) amphibolites also display depleted Nb-Ta and Zr-Hf anomalies, enriched light rare earth elements (LREEs), but show lower Eu* anomalies, εNd(t) values (+0.9–+1.2), and (⁸⁷Sr/⁸⁶Sr)i ratios (0.69770–0.69930). The amphibolites exhibit a geochemical signature marked by LREE enrichment, negative HFSE anomalies, and distinct Sm-Nd isotopic composition, suggesting a subduction-related magma source. The enrichment of Cs, Rb, Ba, and the depletion of Nb, Ta, P, and Ti, imply that their magma source was significantly modified by subducted crustal materials. The trace element ratios, such as K/Rb, Rb/Y, Nb/Y, Th/Zr, and so on, further indicate that the gabbros were derived from a mantle substantially altered by siliceous slab-derived melts, whereas the amphibolites originated from a mantle influenced by slab-derived melts and fluids. The amphibolites were generated by the 15% partial melting of garnet-spinel lherzolite and the 15% melting of spinel lherzolite at a shallower mantle source. In combination with the previously published data of mafic rocks in the Wutai Complex, we infer that the Late Archean gabbros suggest their derivation in a subduction-related setting, whereas the Paleoproterozoic amphibolites formed in a back-arc basin setting. These findings underscore a tectonic transition from Late Archean oceanic subduction to Paleoproterozoic lithospheric extension in the North China Craton, indicating that plate tectonics at least partly happened most likely in the latest Neoarchean.
Stratigraphic refinement unveils high-resolution shallow marine biotic dynamics immediately preceding the Late Ordovician Mass Extinction in South China
Exceptionally preserved Late Ordovician successions in South China offer a globally significant archive for investigating both the Great Ordovician Biodiversification Event (GOBE) and the Late Ordovician Mass Extinction (LOME). In contrast to the graptolitic facies that dominate much of the region, the late Katian carbonate deposits in the Jiangshan–Changshan–Yushan (JCY) area of East China (South China paleoplate), formerly referred to as the Sanqushan Formation (or its equivalents), are virtually the only strata in South China preserving diverse shallow marine biotas, providing rare ecological snapshots of the final biodiversity peak of the GOBE immediately preceding the LOME. Despite their importance, the lithostratigraphic framework and age constraints of these fossiliferous rocks remain debated. The prevailing view interprets the Xiazhen Formation as a nearshore equivalent of the Sanqushan Formation, with both units broadly assigned to a generalized late Katian age. Based on a critical review of integrated sedimentologic and paleontologic data, supplemented by new field observations, we support the interpretation that the Xiazhen Formation represents the upper portion of the ‘Sanqushan Formation’, and propose to elevate the latter to group rank to represent platform facies of this entire interval. The revised Sanqushan Group comprises, in ascending order, the Yaojiakeng, the Jitoushan, and the Xiazhen formations. The Yaojiakeng and the Jitoushan formations correlate more precisely with the Dicellograptus complexus Biozone, while the Xiazhen Formation aligns with the Paraorthograptus pacificus Biozone. This refined stratigraphic framework enables high-resolution reconstruction of biotic evolution prior to the LOME, offering new insights into regional ecological dynamics and their broader global significance.
Material cycling across Earth’s spheres and triggers of Cambrian explosion: a tectonic perspective
Earth is the only known planet in the solar system that can sustain and provide a habitable environment for life, largely due to the presence of stable but evolved oceans, continents, magnetic field, atmosphere, surface temperature, as well as exchange of elements and energy between different spheres of Earth that are driven by plate tectonics and mantle plume. Particularly, the Cambrian life Explosion is a milestone in Earth history, which overlapped with the Neoproterozoic oxygenation event (NOE), Gondwana assembly (~680–430 Ma) and the Snowball Earth events. Moreover, modern plate tectonics may have been established during this period as well. However, the tectonic factors that drove the evolution of surficial systems in this period are less well constrained. Here, a review of observation patterns shows that the chains of reactions initiated with the breakup of the Rodinia supercontinent, which triggered the Snowball Earth and Neoproterozoic oxygenation events. Meanwhile, Snowball Earth also contributed to continental weathering and oxidation event. The subsequent Gondwana assembly, within a modern plate tectonic regime, generated orogens that lasted for more than 250 million years. The topographic relief might have reached high level during this period, which can provide ecological space for ocean animal system. These super orogens served as a continuous driving force for the changes of Earth’s surficial environs. This, together with low-latitude orogenic belts, caused extensive weathering and supplied huge continental sediments to the ocean, which profoundly changed ocean composition and increased ocean productivity, and sustained the NOE. In addition, Earth had an extremely weak magnetic field at E-C boundary, possibly related to the formation of the Earth’s inner core and establishment of modern plate tectonics. Overall, tectonic processes provided the first order contribution to changes of environment that triggered the Cambrian life Explosion.
