Biotic Recovery Research Articles

Exceptional fossil assemblages confirm the existence of complex Early Triassic ecosystems during the early Spathian

The mass extinction characterizing the Permian/Triassic boundary (PTB; ~ 252 Ma) corresponds to a major faunal shift between the Palaeozoic and the Modern evolutionary fauna. The temporal, spatial, environmental, and ecological dynamics of the associated biotic recovery remain highly debated, partly due to the scarce, or poorly-known, Early Triassic fossil record. Recently, an exceptionally complex ecosystem dated from immediately after the Smithian/Spathian boundary (~ 3 myr after the PTB) was reported: the Paris Biota (Idaho, USA). However, the spatiotemporal representativeness of this unique assemblage remained questionable as it was hitherto only reported from a single site. Here we describe three new exceptionally diverse assemblages of the same age as the Paris Biota, and a fourth younger one. They are located in Idaho and Nevada, and are taxonomic subsets of the Paris Biota. We show that the latter covered a region-wide area and persisted at least partially throughout the Spathian. The presence of a well-established marine fauna such as the Paris Biota, as soon as the early Spathian, indicates that the post-PTB biotic recovery and the installation of complex ecosystems probably took place earlier than often assumed, at least at a regional scale.

Marine productivity variations and environmental perturbations across the early Triassic Smithian-Spathian boundary: Insights from zinc and carbon isotopes

The biotic recovery following the end Permian mass extinction (EPME) was unstable with repeated environmental crises, each of which was followed by a comparably more hospitable interval for life. The early Triassic Smithian-Spathian boundary (SSB) event represents a conspicuous turnover in the environment, climate, and biotic communities. To explore the causes and consequences of this event we measured zinc and carbon isotopes along with elemental paleoenvironment proxies through the West Pingdingshan section, Chaohu, South China. An abrupt decrease of δ66Zn values, along with the highly negative δ13C values occur in the latest Smithian in Beds 47–50 of the section, are coincident with the bio-crisis, and are ascribed to weak marine productivity and extremely hot climatic conditions. Rapid positive shifts in δ13Corg and δ13Ccarb values and high contents of TOC in Beds 51–52, demonstrate elevated marine productivity associated with a surface seawater cooling event, but also resulted in oxygen-starved conditions lethal to organisms. Marine and terrestrial ecosystems recovered in the early Spathian in Beds 53–57, as evidenced by high values of δ66Zn, δ13C and the absence of framboid pyrite, and low Al content. These characteristics indicate a return to normal marine productivity and oxygenated seawater, and decreased terrestrial sediment input. Our findings propose that marine productivity played an important role in affecting the marine Zn cycling across the SSB and provide a scenario of environmental perturbations in response to temperature changes during the SSB transition.

Magnetostratigraphy of lower Paleocene strata in the Ferris Formation, Hanna Basin, Wyoming, with refined resolution of the Pu1–Pu2 interval-zone boundary of the Puercan North American Land Mammal Age

ABSTRACT Recent studies of early Paleocene stratigraphic sections across the U.S. Western Interior are refining our understanding of the biotic recovery in the aftermath of the Cretaceous–Paleogene (K–Pg) mass extinction event. Herein we present magnetostratigraphic data from an approximately 600-m-thick section of strata of the Ferris Formation in the Hanna Basin, south-central Wyoming, that spans the K–Pg boundary and includes in conformable superposition the three subdivisions of the earliest Paleocene Puercan North American Land Mammal Age: interval zones Pu1, Pu2, and Pu3. Prior studies on early Paleocene stratigraphic sections in Montana to the north of Hanna Basin and in Colorado and New Mexico to the south have been only marginally successful in defining the temporal boundaries of these divisions. This earlier work typically restricted Pu1 entirely within reversed magnetochron C29r, Pu2 entirely within normal magnetochron C29n, and at least the early part of Pu3 in C29n, as well. Results of the present study confirm what has been only tentatively suggested previously: that interval zone Pu2 begins in the youngest part of C29r, with later Pu2 fauna extending into C29n. Although Pu3 is known to begin in C29n, its younger limit remains unknown in our Hanna Basin section, because the top of the local Puercan is well above the stratigraphic level of our sampling for this project. We estimate a date for the Pu1–Pu2 boundary in the Hanna Basin section of approximately 65.82 Ma—nearly 70 k.y. earlier than prior estimates. This boundary marks the transition from the Pu1 survival fauna to later phases of the mammalian recovery characterized by a significant increase in diversity, appearance of considerably larger-bodied mammals, and greater dental and dietary specializations.

Evidence for the expansion of anoxia during the Smithian from a quantitative interpretation of paired C-isotopes

The Smithian negative C-isotope excursion was one of the major perturbations of the global carbon cycle following the end-Permian mass extinction event. Profound oceanographic and biological changes occurred during the Smithian, but the mechanism driving the negative C-isotope excursion and its links with marine environmental and biotic changes remain poorly constrained. Here, we use high-resolution paired C-isotope records from the early Smithian to the early Spathian in the Jiarong section, South China, to investigate the origin of the carbon cycle perturbation and its relationship with contemporaneous environmental changes. The paired C-isotope data reveal parallel negative excursions in both δ13Ccarb and δ13Corg during the Smithian that can be correlated globally. The global Smithian negative C-isotope excursion shows a good first-order correspondence with global transgression and ocean anoxia. The results of numerical box modeling suggest that the Smithian negative C-isotope excursion could have been generated by increased organic carbon oxidation in response to the upward movement of anoxic bottom waters during transgression. The oxidative decay of organic carbon is primarily fueled by the consumption of oxygen and sulfate in the atmosphere–ocean system. Hence, we use the model to quantify the oxidants consumption rate that could be required to simulate the global Smithian negative C-isotope excursion through organic carbon oxidation. The modeling results show that the organic carbon oxidation during the Smithian could have generated a high demand for oxidants in the ocean, leading to the expansion of anoxia. Our study provides quantitative constraints on the causal link between the Smithian negative C-isotope excursion and widespread anoxia. The nadir of the Smithian negative C-isotope excursion coincided with the severe loss of biodiversity, suggesting that the expansion of anoxia in response to the transgressive upward movement of anoxic waters may have driven the late Smithian extinction. Hence, it could be one of the causes of the prolonged Early Triassic biotic recovery.

Environmental changes in the Middle Triassic lacustrine basin (Ordos, North China): Implication for biotic recovery of freshwater ecosystem following the Permian-Triassic mass extinction

Increasing evidence shows that the recovery of marine ecosystems after the Permian/Triassic mass extinction (PTME) was coupled with environmental amelioration. Recently, abundant insects and complete trophic structures were unraveled from the Ladinian (late Middle Triassic) in the Ordos basin, central China, deciphering recovery of lacustrine ecosystem from the PTME. Coeval environmental settings, however, remain unclear. In the Ordos basin, the Ladinian succession is dominated by black organic-matter-rich shales of the Chang 7 Member of the Yanchang Formation, and their depositional environments and palaeo-productivity are reconstructed based on major and trace element concentrations, pyrite framboid size and morphology, and total organic carbon (TOC) contents. High values of TOC and phosphorous (P) concentrations, as well as more enrichment of trace-metal elements such as Copper (Cu) and zinc (Zn) all indicate a rather high primary productivity. Such a high productivity during the deposition of the Chang 7 3 sub-member is also reinforced by and the presence of abundant organic-matter-enrich laminae in black shales and the diverse chrysophytes, as well as intensive occurrence of tuff layers and possible hydrothermal fluids. Analyses of framboidal pyrite size and morphology as well as C org /P ratio and trace-metal elements such as molybdenum (Mo) and uranium (U), indicate the dysoxic to oxic conditions in water columns of the Ordos lake during the late Middle Triassic. The Ladinian-Carnian cooling likely promoted the occurrence of dissolved oxygens in the lake. As a result, although the massive black shales of the Chang 7 3 sub-member were deposited and suggest a rather high primary productivity, they also indicate slightly oxygen-poor to oxygenated conditions of water columns, which are inductive for lacustrine biota to inhabit, and also offer hospitable habitats for freshwater lake ecosystems to recover from the PTME. • A rather high primary productivity occurred in a Ladinian freshwater lake. • Slightly oxygen-poor to oxygenated water conditions developed in a Ladinian lake. • Ordos lake offered hospitable habitats for freshwater ecosystems to recover.

Ocean redox changes from the latest Permian to Early Triassic recorded by chromium isotopes

Multiple observations have shown that widespread oceanic anoxia may have played an important role in the end-Permian mass extinction and delayed biotic recovery in the Early Triassic. However, it has not been reached a consensus on the temporal and spatial changes in oceanic redox conditions at that time. Chromium (Cr) isotopes have been widely used to trace redox processes in surface environments. Here, we present the Cr isotope compositions (δ53Cr) of the Upper Permian–Lower Triassic carbonate ramp from the Xiakou section in South China (eastern Paleotethys). We observed largely fractionated δ53Cr values (1.15‰ to 2.16‰) in the latest Permian, which are higher than most values among Phanerozoic sedimentary carbonates. Such high values may result from Cr reduction facilitated by the development of a reducing environment during the latest Permian. The high Cr isotope ratios were followed by an abrupt negative excursion of δ53Cr values (from average 1.78‰ to average −0.13‰) across the extinction horizon, which may be attributed to a rapid expansion of oceanic anoxia in the eastern Paleotethys. Persistent negative δ53Cr values covering the Early Triassic indicate that the ocean may not have recovered from anoxia for an extended period. Our Cr isotope results support a stepwise deterioration of oceanic redox environments, which has been previously proposed as a possible cause of the most severe biotic catastrophe in the Phanerozoic.

Calibrating the late Smithian (Early Triassic) crisis: New insights from the Nanpanjiang Basin, South China

Abstract The biotic recovery following the Permian/Triassic boundary mass extinction was influenced by several secondary extinctions during the Early Triassic, of which the late Smithian crisis is the most severe known for some nekto-pelagic organisms such as ammonoids. The Smithian-Spathian transition is characterized by successive global biotic and environmental changes, including a dramatic positive carbon isotopic excursion, oceanic anoxia and a cooling event beginning in the late Smithian. However, the tempo, modalities and the causal relationships among these various events remain poorly constrained. Here we synthesized paleontological, sedimentological, and geochemical data from three sections (Motianling, Nafang, and Shanggang) within the Nanpanjiang Basin, representing platform slop to basinal deposits spanning the Smithian/Spathian boundary. High-resolution analyses of thin sections show a sudden decrease in richness and abundance of fossil grains at the middle/upper Smithian boundary in all three sections, coinciding with the onset of a positive shift in δ13Ccarb. It also slightly precedes the beginning of the late Smithian cooling. At Shanggang, a change in dominance from bivalve-rich to ostracod-rich strata is also recorded at the middle/upper Smithian boundary. Overall, our results therefore indicate that a first major biotic crisis and turnover happened during the beginning of the late Smithian in the Nanpanjiang Basin, rather than around the Smithian/Spathian boundary. Complementary analyses on pyrite framboid size indicate that these observed biotic changes are concurrent with oxygen depletion in the studied sections, supporting the hypothesis that oceanic anoxia also played an important role in the late Smithian crisis in combination with climate cooling and oceanic acidification.

Minor changes in biomarker assemblages in the aftermath of the Cretaceous-Paleogene mass extinction event at the Agost distal section (Spain)

The Cretaceous/Palaeogene Boundary (KPgB) represents one of the five major mass extinctions in Earth's history, and determining the nature of associated environmental change and biotic recovery is critical for understanding the history of life on our planet. To explore that, we examined the distributions of selected biomarkers (n-alkanes, acyclic isoprenoids, steranes and hopanes), organic and carbonate δ13C values, total organic carbon contents and major and trace elements in a distal section spanning the KPgB (Agost, SE Spain). The studied Agost section is an ~32 cm-thick bathyal sequence of marlstones, clays and marly limestones, and high sedimentation rates allow high (cm) resolution analysis. These analyses exhibit sample-to-sample variability, but there are few significant differences between pre- and post-KPgB biomarker assemblages, suggesting a rapid recovery of the non-fossilizing phytoplankton community after the KPgB. Despite the persistence of life, the organic matter assemblage is rather variable through the first 10 kyr after the impact event. This interval is associated with changing terrigenous and petrogenic inputs as well as varying redox conditions as reflected by the enrichment factor of uranium (UEF) vs that of molybdenum (MoEF) as well as biomarker indices (gammacerane and homohopane indices). Moreover, sterane distributions do differ between pre- and post-KPgB sediments. Thus, the KPgB impact did affect environmental conditions and non-fossilizing algal and bacterial communities even in distal sites, but these organisms appear to have rapidly recovered, within 10 kyr after the KPgB.

Spatiotemporal variations of sedimentary carbon and nitrogen isotopic compositions in the Yangtze Shelf Sea across the Ordovician-Silurian boundary

The Hirnantian Glaciation resulted in a perturbation in global carbon cycling, which is recognized as a positive excursion in the δ13C values of both carbonates and organic matter. To investigate the factors controlling the carbon and nitrogen cycles from the late Ordovician to the early Silurian, the C and N isotopic and geochemical compositions of marine sediments deposited on the Yangtze Platform were measured. The majority of rocks in the Wukemuchang section exhibit high δ15Nbulk values, reflecting aerobic nitrogen cycling in the inner shelf. The global spatial variation in δ15Nbulk values likely reflects the occurrence of nitrate gradients in seawater, implying that nitrate was limited in offshore environments that were characterized by oxic conditions. More eukaryotes may be restricted to the inner shelf environments, where large amounts of bioavailable nitrate were accessible. The limited availability of nitrate may have affected the fossil distributions and the complexity of animal ecosystems, thereby influencing the late Ordovician extinction and early Silurian biotic recovery. The δ13Corg profiles from the Yangtze Platform are consistent with earlier reports of Hirnantian shelf gradients in sedimentary δ13CDIC values. We suggest that the global variation in δ13Corg values during the late Ordovician was related to the amount of nitrate because productivity is limited when bioavailable N is scarce. In addition to intense carbonate weathering, the high level of photosynthesis in the inner shelf caused by abundant nitrate and nutrients may have enhanced the dissolved inorganic carbon gradient on shelves globally, driving sedimentary δ13Corg values even higher than those of sediments deposited in the outer shelf where carbon exchange with the surface ocean was less restricted

Earliest Palaeocene purgatoriids and the initial radiation of stem primates.

Plesiadapiform mammals, as stem primates, are key to understanding the evolutionary and ecological origins of Pan-Primates and Euarchonta. The Purgatoriidae, as the geologically oldest and most primitive known plesiadapiforms and one of the oldest known placental groups, are also central to the evolutionary radiation of placentals and the Cretaceous-Palaeogene biotic recovery on land. Here, we report new dental fossils of Purgatorius from early Palaeocene (early Puercan) age deposits in northeastern Montana that represent the earliest dated occurrences of plesiadapiforms. We constrain the age of these earliest purgatoriids to magnetochron C29R and most likely to within 105–139 thousand years post-K/Pg boundary. Given the occurrence of at least two species, Purgatorius janisae and a new species, at the locality, we provide the strongest support to date that purgatoriids and, by extension, Pan-Primates, Euarchonta and Placentalia probably originated by the Late Cretaceous. Within 1 million years of their arrival in northeastern Montana, plesiadapiforms outstripped archaic ungulates in numerical abundance and dominated the arboreal omnivore–frugivore niche in mammalian local faunas.

Anoxia may delay biotic recovery from the Late Ordovician mass extinction

Anoxia may delay biotic recovery from the Late Ordovician mass extinction

The stem-archosaur evolutionary radiation in South America

The oldest archosauromorphs (dinosaurs, birds, crocodiles, and their stem-taxa) are recorded in middle−upper Permian rocks, but it was not after the Permo−Triassic mass extinction that the group shows a substantially high taxonomic richness and ecomorphological disparity. The early evolutionary history of the Archosauromorpha during the Early and Middle Triassic is mainly based on fossils recovered from rocks in southern Africa, Europe and Asia, whereas South America possesses a more complete fossil record of the group only in the Late Triassic. Here we revisit, discuss, and reanalyse the non-archosaurian archosauromorph fossil record of the current-day South America. The Early Triassic archosauromorph record in this continent is still scarce, but it documents the early evolution of the group in western Pangaea and is crucial to understand more globally the biotic recovery after the Permo−Triassic mass extinction. The Middle Triassic record is extremely scarce, but the Late Triassic archosauromorph assemblage of South America is among the most diverse and abundant worldwide. The last decade has witnessed a considerable improvement in our knowledge of the record, taxonomy, phylogeny, and macroevolution of the group with the input from the South American fossils. Nevertheless, a considerable amount of research is needed and ideally should be focused on some particular aspects of the Triassic evolutionary radiation of Archosauromorpha. Among them, the Early Triassic record should be expanded, more numerous and more complete Middle Triassic archosauromorph specimens are crucial to have a more complete picture of the evolution of the group, and the taxonomy of groups like proterochampsids and hyperodapedontine rhynchosaurs should be clarified through detailed anatomical work.

Geochemical markers of paleoenvironments, weathering, and provenance in Permian–Triassic terrestrial sediments

ABSTRACTThis study compares several geochemical factors (major and minor rare earth elements, Ti/Al ratios, and chemical index of alteration, CIA, values) in the Early Triassic Cañizar Formation (Fm) (Buntsandtein facies) of E Iberia with those of adjacent Middle Permian and Middle Triassic units (Alcotas and Eslida fms, respectively). According to significant differences detected, it seems that most geochemical perturbation occurred during the Early Triassic. Variations in Ti/Al ratios suggest changes in source areas between the studied units and even within the Cañizar Fm. These provenance changes correlate with successive tectonic pulses during the opening and development of the Iberian Basin, as they can be linked to major sedimentary surfaces and unconformities, as well as major sedimentological variations. Ti enrichment in the lower and middle part of the Cañizar Fm, together with high Sr and P concentrations, may be indicative of environmental alterations related to acid meteoric waters. Moreover, this acid alteration took place under arid conditions as reflected by CIA values, indicating that during the deposition of the Cañizar Fm, variable but predominantly physical weathering prevailed in contrast to the chemical weathering that took place when the Alcotas and Eslida formations were deposited. Our data along with the known fossil record of the study area indicate that during the Middle–Late Permian and Early Triassic, conditions in this tectonically active area changed from humid to arid-acid, hampering biotic recovery. Then, during late Early–Middle Triassic times, the return of more humid and less acid environments promoted biotic development. Geochemical markers emerged as useful tools complementary to sedimentological, paleontological, and tectonic data for unveiling paleoenvironmental events, especially in a setting of significant regional change.

Early Middle Triassic (Anisian) trace fossils, ichnofabrics, and substrate types from the southeastern Germanic Basin (Wellenkalk facies) of Upper Silesia, southern Poland: Implications for biotic recovery following the Permian/Triassic mass extinction

Abstract The late Olenekian to early Ladinian marine carbonates of the epicontinental European (Germanic) Basin accumulated during the several-million-year-long biotic recovery following the Permian/Triassic (P/Tr) mass extinction. In this paper, we discuss the environmental and evolutionary controls on the temporal ichnodiversity and ichnofabric changes within Anisian open-marine limestones (called Muschelkalk) deposited in the south-eastern margin of the basin, close to the Tethys Ocean, and their implications for the timing of biotic recovery. The focus is on the lithologically similar transgressive systems tracts (TST) of three consecutive 3rd-order depositional sequences, each of which start with high-energy shoal-sand packstones-grainstones overlain by deeper-platform lime mudstones with skeletal and conglomeratic tempestites. In general, these facies are characterized by a high degree of bioturbation, high ichnotaxonomical diversity (20 softground ichnotaxa, 2 firmground ichnotaxa, 3 hardground ichnotaxa, and undetermined mottles and spots in a soupground suite), and high ethological diversity (domichnia, fodinichnia, cubichnia, repichnia, pascichnia, praedichnia). Despite the high overall diversity, monoichnospecific and almost monoichnospecific assemblages of Rhizocorallium, Oravaichnium, and Thalassinoides prevail. With these assemblages dominance changes stratigraphically, from Rhizocorallium dominated assemblages in TST1, to Oravaichnium dominated in TST2, and Thalassinoides dominated in TST3. Concurrently, the width of Thalassinoides burrows increases, from 1 to 2 cm in TST1 and TST2, to 3–7 cm in the TST3. The two secular changes may represent a gradual recovery of burrowing organisms following the P/Tr biotic crisis, given that environmental conditions remained more-or-less stable through time, as evidenced by facies analysis and stable carbon and oxygen isotope data. The occurrence of large Rhizocorallium and medium-sized Thalassinoides represents the penultimate stage in recovery, whereas the widespread appearance of large Thalassinoides – comparable in size with typical pre-extinction Permian forms – marks the final recovery of trace-maker communities.

Factors controlling organic-matter accumulation in the Upper Ordovician-Lower Silurian organic-rich shale on the northeast margin of the Upper Yangtze platform: Evidence from petrographic and geochemical proxies

Organic-rich shales are widespread in the Upper Ordovician-Lower Silurian Wufeng-Longmaxi Formations on the northeastern margin of the Upper Yangtze platform, and these shales have been recognized to be the hydrocarbon source rocks and shale gas reservoirs with the greatest development potential in China. However, organic matter accumulation in these shales needs further research, especially in the areas which experienced an extensive upwelling and volcanism. Here, this work integrates detailed petrographic and geochemical proxies, namely redox-sensitive trace elements (RSTEs) and ratios of Si/Al & Ti/Al, from two Ordovician-Silurian sections on the northeast margin of Upper Yangtze platform, in order to understand the depositional environment and identify the major factors controlling the organic matter accumulation during the Ordovician-Silurian transition. Data of Si/Al and Ti/Al ratios indicate that the deposition of black-gray siliceous mudstones in the Lower Wufeng Formation commenced with a transgression triggered by the Boda warming event which led to a reduced detrital input. Based on RSTEs, the black carbonaceous shales of the Upper Wufeng Formation are organic-rich owing to an elevated primary productivity caused by seasonal upwelling driven by the vigorous thermohaline circulation. Abundant nutrients supplied by the post-glacial transgression and a mass biotic recovery of marine microorganisms stimulated productivity and further strengthened the depletion of oxygen in the water column, which lead to high concentrations of organic matter in Lower Longmaxi Formation. Although the water column conditions were not conducive to the accumulation of organic matter, it was possible that the increased productivity from upwelling resulted in relatively high organic matter contents in the middle Longmaxi Formation. The RSTEs (especially Cd/Mo, Co × Mn) reveal that the organic matter accumulation of the Wufeng-Longmaxi Formation during Ordovician-Silurian transition may have been primarily controlled by the preservation conditions and also influenced by the marine primary productivity and the detrital input.

Upper Toarcian (Lower Jurassic) marine gastropods from the Cleveland Basin, England: systematics, palaeobiogeography and contribution to biotic recovery from the early Toarcian extinction event

AbstractHere we describe a new upper Toarcian (Lower Jurassic) marine gastropod fauna from rocks of the Cleveland Basin exposed on the North Yorkshire coast of England. The fossil assemblage consists of 16 species, of which three are new:Katosira?bicarinatasp. nov.,Turritelloidea stephenisp. nov. andStriactaenonina eleganssp. nov. Four species are described in open nomenclature asTricarilda? sp.,Jurildasp.,Cylindrobullinasp. andCossmanninasp. The other species have previously been described:Coelodiscus minutus(SchüblerinZieten),Procerithium quadrilineatum(Römer),Pseudokatosira undulata(Benzinvon Zieten),Palaeorissoinaaff.acuminata(Gründel),Pietteia unicarinata(Hudleston),Globulariacf.canina(Hudleston),Striactaeoninacf.richterorumSchulbert & Nützel,Striactaenoninaaff.tenuistriata(Hudleston) andSulcoactaeon sedgvici(Phillips). Most of these species are the earliest records of their respective genera and show palaeobiogeographical connections with contemporary gastropod associations from other regions of Europe and South America. The taxonomic composition of the upper Toarcian Cleveland Basin gastropod assemblage differs substantially from the faunas of the upper Pliensbachian and lower ToarcianTenuicostatumZone, showing the strong effect of the early Toarcian mass extinction event on the marine gastropod communities in the basin. Only a few gastropod species are shared between the late Toarcian faunas and the much more diverse Aalenian gastropod faunas in the Cleveland Basin, suggesting that there was a facies control on gastropod occurrences at that time. This is also a potential explanation for the taxonomic differences between the late Toarcian gastropod faunas in the Cleveland Basin and those in France, and northern and southern Germany.

Trace fossils from the Lower Triassic of North China — a potential signature of the gradual recovery of a terrestrial ecosystem

The Permian–Triassic Mass Extinction (PTME) resulted in the most severe loss of life in the history of the earth, both marine and terrestrial ecosystems suffered heavy losses simultaneously, and the biotic recovery process was difficult for both. There are many studies on the timing and pattern of this mass extinction, however, the consistency of the recovery between the marine and terrestrial facies remains controversial. In particular, few metazoans survived and the deposits were dominated by the microbialites during the Early Triassic. The reappearance of the metazoans signified the beginning of the biotic recovery. In this paper, we measured the trace fossils of the Heshanggou Formation in western Henan, North China, which plays an important role in the transformation from the flourishing of the microorganisms to the recovery of metazoans. We quantitatively described the characteristics and measured the diameter, depth, abundance and diversity of the trace fossils. Based on the acquired data, the lower part of the Heshanggou Formation is confined to simple, small, vertical burrows, suggesting that the ecological environment was suitable for the survival of special species. In the middle part, the vertical burrows become deeper, which may be due to the adaptation of organisms to the harsh environment. Disturbance is more intense and the diversity clearly increases in the upper part. Through the increasing bioturbation and diversity of the trace fossils, we found a gradual biotic recovery pattern in the Early Triassic, and the recovery mode is similar to marine ichnofossils in South China.

New middle and late Smithian ammonoid faunas from the Utah/Arizona border: New evidence for calibrating Early Triassic transgressive-regressive trends and paleobiogeographical signals in the western USA basin

New Smithian (Early Triassic) ammonoid assemblages were sampled near the Utah/Arizona border. They provide several spatiotemporal constraints on the regional Sinbad Formation showing that the extent of the Smithian sea in the southwestern-most part of the western USA basin is larger than previously expected, reaching northern Arizona and an area just east of Kanab in Kane County, Utah. This southwestern-most excursion of the sea in the western USA basin is part of the third order Smithian transgression-regression cycle, which is well-documented worldwide. These new spatiotemporal constraints also indicate that the Sinbad Formation spans the late middle to early late Smithian time interval in the studied area. The observed transgressive trend corresponds to the warm temperatures of the middle-early late Smithian and the regressive trend to the cooling phase spanning the Smithian-Spathian transition. Thus, the Sinbad Formation and its marine deposits are the direct result of global climatic fluctuations and related sea-level changes that occurred from the late middle Smithian to the Smithian-Spathian transition. This highlights the importance of the Sinbad Formation in untangling local and global changes occurring around the late Smithian, and thus in overcoming a major obstacle to the understanding of the biotic recovery after the Permian/Triassic boundary mass extinction. This also indirectly confirms that the western USA basin is a key area containing essential information for the understanding of the Early Triassic events. The taxonomic richness of late middle Smithian assemblages is much lower in the studied area than in the more northern localities. However, the occurring taxa confirm the known regional to global distribution of Smithian ammonoids, witnessing the major global environmental changes from the middle Smithian to the early Spathian.

Dynamics of the Largest Carbon Isotope Excursion During the Early Triassic Biotic Recovery

The dynamics of the carbon cycle across different timescales is crucial for understanding past and present global climate changes. Following the Permian–Triassic boundary mass extinction (PTBME), the carbon cycle changed profoundly during the following 5.4 Myr, with magnitudes of changes comparable to those of the Precambrian. In pace with the successive cycles of the carbon budget, the recovery of the marine nekton underwent several evolutionary diversification and extinction cycles accompanied by eustatic sea-level changes and profound ecological reorganization of land plants, all indicative of climatic changes. Additional eruptive bursts of the Siberian Large Igneous Province (SLIP) are traditionally called upon as a plausible trigger for these climatic oscillations but firm evidence for coeval SLIP volcanism is still lacking. Based on new precise and accurate U-Pb zircon ages, we establish a high-resolution temporal calibration of the biggest positive carbon isotope excursion (CIE) spanning about 600 kyr in the late Smithian. The age of the Smithian-Spathian boundary (SSB) is established between 249.29 ± 0.06 and 249.11 ± 0.09 Ma. Our oldest U-Pb zircon ages indicate no overlap in time between the middle Smithian onset of the thermal maximum and the youngest available U-Pb zircon ages from SLIP volcanism. The constructed time line also indicates a duration of the global unconformity at the SSB that is compatible with glacio-eustatism. Potential cooling mechanisms such as a volcanic winter, the biological pump and the cessation of volcanism are discussed in the light of this new time line. In the low latitudes, the onset of the positive CIE remarkably predates the temperature drop by some 100 to 125 kyr. However, as long as the magnitude of such offset – if any – is unknown for the high latitudes, relations between the CIE and the cooling will remain an open question associated with largest Triassic extinction of the nekton.

Millennial-scale ocean redox and δ13C changes across the Permian–Triassic transition at Meishan and implications for the biocrisis

The Permian–Triassic extinction was the largest Phanerozoic mass extinction event, yet its ultimate causes remain unresolved. Ocean redox conditions, global warming, and carbon cycle perturbations owing to massive volcanism and their inter-relationships are keys to resolve the origins of this event. We here assess such relationships using a method to obtain an instantaneous stratigraphic response from high-resolution (millennial-scale) records of carbonate-δ13C and pyrite framboid size distributions across the Permian–Triassic boundary (PTB) at Meishan, China. We report two gradual long-term negative δ13C shifts before and after the PTB. Late Permian ocean redox conditions varied through three successive oxic–dysoxic, dysoxic–euxinic, and high-frequency euxinic stages. Oxic and high-frequency euxinic events occurred in the early Griesbachian and middle Griesbachian, respectively. The short-term euxinia events were not associated with the negative δ13C excursions. Siberian volcanism could have produced large light-carbon inputs, global warming, and low-oxygen conditions at intermediate ocean depths, but it may not directly drive high-frequency euxinia across the PTB. Long-term dysoxia and occasional euxinia well before the PTB stressed mobile macroorganisms and caused gradual extinction before the major extinction event. High-frequency latest Permian euxinia reduced overall biotic adaptability, and caused the initial rapid mass extinction. Increasing temperatures and other deleterious environment factors then reinforced the extinction process. Oxic conditions contributed to biotic recovery immediately after the mass extinction in the early Griesbachian. Further millennial-scale dysoxic/euxinic alternations in the middle Griesbachian would have prolonged Early Triassic biotic recovery.