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Large numbers of densely distributed Rhizocorallium provide a window to understanding the behavioural and functional changes of the trace maker after extreme environmental events. This study focuses on the analysis of Rhizocorallium from the Middle Triassic (Anisian) limestones in the Boyun section, Yunnan Province, Southwest China to investigate the ecological strategies and ethological signatures of its producers before and after storm events. Based on its morphological features, the studied Rhizocorallium is attributed to two ichnospecies Rhizocorallium jenense and R. commune, the latter showing two varieties R. commune var. auriforme and R. commune var. irregulare. In addition to Rhizocorallium, the trace fossil assemblage consists of Arenicolites, Palaeophycus, Planolites and Thalassinoides, which allows assignation to the Cruziana ichnofacies associated with the shallow marine environment. The studied ichnofossil-bearing intervals in the Boyun section are usually high bioturbated, with BPBI (bedding plane bioturbation index) around 5. The ichnoassemblage has a relatively low ichnodiversity and shows high density of Rhizocorallium, which is interpreted as the result of the opportunist strategy when the Rhizocorallium trace makers exploited the quiet, nutrient-rich substrates after storm events. Crosscutting relationships between multiple levels of Rhizocorallium indicate that earlier burrows were reworked by later ones due to successive storm events and erosion of the shallow-tier burrows. Orientation of the marginal tubes of Rhizocorallium jenense shows a bidirectional pattern at 295°NW and 160°SE, which may record the changes of the main flow direction. Diameter measurements support the total recovery of the macrobenthic trace makers during the Middle Triassic (Anisian).
Changes in diversity of trace fossils through time provide information about evolutionary innovations in animal‐substrate interactions. Global ichnodiversity changes at ichnogeneric rank are useful to capture major trends, but may be insufficient to reveal minor behavioural innovations. A quantitative analysis of ichnodiversity trajectories at ichnospecies rank for bioturbation structures has been performed for the first time at a global scale to evaluate how innovations in animal‐substrate interactions may be reflected at this ichnotaxonomic hierarchy. The timing of diversification at ichnospecific rank is highly variable, but mostly linked to the Cambrian Explosion (CE) and the Mesozoic Marine Revolution (MMR). Nearly all top‐heavy ichnogenera diversified during the MMR, and most bottom‐heavy ichnogenera illustrate innovations during the CE. Timing of diversification at ichnospecies rank refines characterization of the ichnological equivalents of the Cambrian, Palaeozoic and Modern evolutionary fauna. Evaluating the timing of ichnospecific diversification within ichnogenera is a valuable tool in evolutionary palaeoecology.
Dictyodora has been generally considered an ichnogenus with a restricted temporal range from the Cambrian to the Mississippian. Until now, there have been no reports of Dictyodora after the Mississippian. Here we report on well-preserved and abundant Dictyodora from deep-sea sediments of the Lopingian Maomaolong Formation in West Qinling, central China, which is hitherto the youngest occurrence of Dictyodora and extends the stratigraphic range of Dictyodora to the Late Permian. Two ichnospecies of Dictyodora are distinguished: D. zimmermanni and D. cf. scotica. Unlike the small-sized Dictyodora in the pre-Pennsylvanian, the three-dimensional morphology of Dictyodora studied here shows larger amplitudes and longer limbs with clearly inclined walls and basal burrows, which are interpreted to have been made by a soft-bodied worm-like organism with a snorkel-like organ as similarly documented in previous studies. Detailed systematic ichnological features and SEM and EDS results indicate that Dictyodora here is not simply a respiration structure made during the locomotion of the trace maker, but resulted from active filling by the trace maker. Dictyodora is probably not a good indicator of the deep-marine environment in the Paleozoic. From our dataset, Dictyodora originated from the shallow-marine environment in the early Cambrian, then migrated into the deep sea in the Ordovician and the marginal-marine setting in the Wenlock, and was finally restricted to the deep-marine environment in the Carboniferous and Permian. Both Psammichnites and Dictyodora are structures made by organisms with snorkel-like organs, and both were eliminated during the end-Permian mass extinction.
Widespread oceanic anoxia has been implicated as a killing mechanism during the end-Permian mass extinction (EPME). A high-resolution investigation of pyrite framboids and bioturbation structures in the Shangsi section of South China has revealed repeated and frequent fluctuations of coupled anoxic/euxinic to dysoxic/oxic episodes in the latest Permian, immediately preceding the EPME. Anoxic bottom waters and interstitial water conditions are documented by the smallest pyrite framboids (3-5 μm) and the absence of bioturbation, and dysoxic bottom waters and anoxic interstitial waters are inferred from larger pyrite framboids (6-10 μm) and the occurrence of Zoophycos. In comparison, oxic conditions for both bottom waters and interstitial waters are revealed by a high bioturbation index (BI = 4) and the occurrence of mid-tier Thalassinoides. Fluctuating pyrite framboid sizes and bioturbation structures suggest that bottom and interstitial oxygen levels were neither fully, nor persistently, anoxic/euxinic before the EPME, and instead recorded frequent periodic fluctuations between anoxic/euxinic and dysoxic/oxic conditions. The paleoceanic redox variation appears to correlate with the 405-ka eccentricity cycles in the anhysteretic remanent magnetization (ARM) series, suggesting that fluctuations might have been controlled by Milankovitch cycles. The role of fluctuations between anoxic/euxinic and dysoxic/oxic episodes in the latest Permian to the EPME is unclear, but it seems plausible that they represent a prelude to the highly stressed conditions that are characteristic of the EPME.
In this paper, we review the biotic and environmental events by examining the trace fossil records in association with the 'Big 5' mass extinctions in Phanerozoic, including the transitions of Ordovician-Silurian, Frasnian-Famennian, Permian-Triassic, Triassic-Jurassic and Cretaceous-Paleogene. The benthic fauna reflected in the trace fossil records show a negative response to all of the 'Big 5' events, documented by the decreases in the ichnodiversity, burrow size, bioturbation depth, and ichnofabric tier. The behavior and food-feeding strategy of benthic fauna is found to vary among the 'Big 5' mass extinctions. The deposit-feeding strategy dominated after the mass extinctions during Ordovician-Silurian and Cretaceous-Paleogene transitions, whilst the suspension-feeding strategy dominated after the mass extinction during Triassic-Jurassic transition. Opportunistic trace fossils, such as Planolites, and both the deposit- and suspension-feeding strategies dominated after the mass extinctions during Frasnian-Famennian and Permian-Triassic transitions. The benthic ecosystems, reflected in the trace fossil records after the mass extinctions during Frasnian-Famennian and Permian-Triassic transitions, changed from simple to complex pattern, from two-dimensional to three-dimensional ecospace.
Trace fossils are important evidence of benthic activity and proxies of bottom and interstitial water oxygen content, making them a promising tool to understand the oceanic redox evolution and extinction patterns during the end-Permian mass extinction (EPME). Detailed bed-by-bed ichnologic studies were performed at a high-resolution scale from two uppermost Permian sections (Shangsi and Dongpan) in South China, documenting the presence of Chondrites targionii, Chondrites isp., Nereites isp., Planolites isp. A, Planolites isp. B, Palaeophycus isp., Phycosiphon incertum, Thalassinoides isp., and Zoophycos isp. The uppermost Permian strata at the Shangsi section mainly comprise siliceous limestone interbedded with illite-montmorillonite claystone and shale, recording a deep shelf setting. The uppermost Permian strata at the Dongpan section consist of mixed chert, mudstone and claystone, formed in a slope environment. Bioturbation index in uppermost Permian strata is mostly 1–2, but with thin intervals reaching 3–4. Fluctuating bioturbation intensities, ichnofaunal composition (Thalassinoides, Planolites, Zoophycos) and ethologies, as well as the ratio of highly reactive iron to total iron (FeHr/FeT) and the proportion of the highly reactive iron pool in the form of pyrite (Fepy/FeHR) suggest that bottom and interstitial oxygen levels were not permanently anoxic/euxinic, but were interrupted by brief periods of aerobic conditions during the latest Permian. Three bioturbation phases in the EPME are identified. Phase I is characterized by the absence of typical pascichnia ichnotaxa (e.g. Nereites isp.) and relatively large fodinichnia burrow (e.g., Planolites isp. B; Zoophycos isp.). Phase II mainly consists of smaller Planolites isp. A, Thalassinoides isp. and Chondrites isp., recording a transition from softground to firmground. Only occasional smaller Planolites isp. A occur in Phase III without any associated vertical burrows. Ichnodiversity and bioturbation index from Phase I to Phase III show a step by step decline, corresponding to the two-stage extinction patterns. Long-term environmental stress (e.g., oceanic anoxia, volcanic winter, global warming, ocean acidification, hypercapnia) may have played major role in the collapse of the infaunal ecospace and only facies-crossing, highly tolerant, opportunistic organisms may have persisted in otherwise vacant ecological niches in the aftermath of the extinction.
The Longmendong section is one of the few Early Triassic successions containing fluvial to marine deposits in South China. No trace fossils have been recorded in the fluvial deposits, but a total of 26 ichnospecies are reported from the Lower Triassic Dongchuan, Feixianguan and Jialingjiang formations in this section. This ichnofauna represents brackish- water conditions during the late Dienerian to Smithian, and fully shallow-marine settings in the Spathian. Several ichnologic metrics, such as ichnodiversity, ichnodisparity, ichnoabundance, bioturbation intensity, burrow size, and depth of bioturbation, have been analyzed in order to evaluate the role of environmental and evolutionary factors. Although the upward increases in ichnodiversity, ichnodisparity, burrow size, and bioturbation intensity may be linked to the biotic recovery after the end-Permian mass extinction, environmental controls also play an important role in this case. Transgressiveestuarine successions typically show a vertical decrease in the salinity stress, showing the passage of brackish-water ichnofaunas to more diverse associations showing more marine affinities. However, the vertical increase in depth of bioturbation cannot be explained by environmental controls alone, instead most likely reflecting the phase of biotic recovery. Overall, ichnologic data suggest that the brackish-water ecosystem was less impacted by the end-Permian mass extinction than the fully marine realm. The shallow, fully marine benthos completely recovered in the Spathian as is the case for other areas in South China. This study underscores the importance of a careful evaluation of sedimentary facies and environmental conditions as a prerequisite for interpreting evolutionary mechanisms of biotic recovery.
The Longmendong section is one of the few Early Triassic successions containing fluvial to marine deposits in South China. No trace fossils have been recorded in the fluvial deposits , but a total of 26 ichnospecies are reported from the Lower Triassic Dongchuan, Feixi-anguan and Jialingjiang formations in this section. This ichnofauna represents brackish water conditions during the late Die-nerian to Smithian, and fully shallow-marine settings in the Spathian. Several ichnologic metrics, such as ichnodiversity, ichnodispar-ity, ichno abundance, bioturbation intensity, burrow size, and depth of bioturbation, have been analyzed in order to evaluate the role of environmental and evolutionary factors. Although the upward increases in ichno-diversity, ichnodisparity, burrow size, and bioturbation intensity may be linked to the biotic recovery after the end-Permian mass extinction, environmental controls also play an important role in this case. Transgressive-estuarine successions typically show a vertical decrease in the salinity stress, showing the passage of brackish-water ichnofaunas to more diverse associations showing more marine affinities. However, the vertical increase in depth of bioturbation cannot be explained by environmental controls alone, instead most likely reflecting the phase of biotic recovery. Overall, ichnologic data suggest that the brackish-water ecosystem was less im-pacted by the end-Permian mass extinction than the fully marine realm. The shallow, fully marine benthos completely recovered in the Spathian as is the case for other areas in South China. This study underscores the importance of a careful evaluation of sedimen-tary facies and environmental conditions as a prerequisite for interpreting evolutionary mechanisms of biotic recovery.
New ichnological data from the Lower Triassic (Induan) Dongchuan Formation (Longmendong, South China) record the recovery interval of marginal-marine communities following the end-Permian mass extinction. Here, we document six ichnogenera from the upper part of the Dongchuan Formation of Dienerian (Early Triassic) age in the Longmendong area, Sichuan Province, South China. These are Cylindrichnus, Diplocraterion, Palaeophycus, Planolites, Siphonichnus, and Teichichnus, illustrating a depauperate Cruziana Ichnofacies. Facies analysis suggests that the depauperate Cruziana Ichnofacies is present in deposits recording the transition from fluvial to tide-dominated estuarine settings. Compilation of worldwide brackish-water ichnofaunas from Permian (average alpha ichnodiversity = 6.2) to Triassic (average alpha ichnodiversity = 5.5) suggests that no significant ichnodiversity decrease took place in these settings as a result of the mass extinction and further implies that the impact of the mass extinction may have been less severe in marginal-marine settings. Ichnofaunas in pre- and post-extinction marginal-marine environments are remarkably similar and tend to be dominated by facies-crossing ichnotaxa (e.g., Diplocraterion, Palaeophycus, Planolites) produced by opportunistic faunas.