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The Beishan Orogen, located in the southernmost Central Asian Orogenic Belt, is a key area for research on accretionary orogens. The formation of the Beishan Orogen is closely related to the evolution of the Paleo-Asian Ocean, but the closure time of a significant branch of the Paleo-Asian Ocean in the Beishan area, i.e., the Hongliuhe–Niujuanzi–Xichangjing (HNX) Ocean, remains controversial. This study investigates the sedimentary sequences of Baihu Basin adjacent to the HNX suture zone and presents new detrital zircon U Pb geochronology and whole-rock geochemistry data to reveal depositional ages, provenance variations, paleogeography changes and attributes of Baihu Basin. Detrital zircon ages indicate that deposition began in the Permian and lasted until the Triassic. Two coarse sandstones in Member 1 and one siltstone in Member 2 contain abundant detrital zircons older than 700 Ma, in addition to the typical age peak that characterises the Gongpoquan Arc at ca. 450 Ma, while two sandstones in Member 3 contain rare detrital zircons older than 700 Ma. The whole-rock geochemistry shows that the lower part of the Hongyanjing Formation possesses numerous old components and higher chemical index of alteration values. The sediment in the lower part of the Hongyanjing Formation was derived from both the Gongpoquan Arc, mainly comprising Palaeozoic arc-related magmatic rocks, and the Shuangyingshan Arc, including large quantities of Precambrian meta-sedimentary rocks. The upper part originated solely from the juvenile Gongpoquan Arc. During the collision, the Shuangyingshan Arc played a role similar to a craton. Craton-derived source rocks and fining-upward sequences in the lower part indicate uplift of the Shuangyingshan Arc, which may represent the development of a forebulge and foredeep. In the upper part, the source rocks were derived from the Gongpoquan Arc and accretionary wedges, with coarsening-upward sequences reflecting the springback of the Shuangyingshan Arc and disappearance of the forebulge. Based on the sedimentary sequences, provenance variations, and paleogeographic changes, we suggest that Baihu Basin is a foreland basin associated with the collision between the Gongpoquan Arc and Shuangyingshan Arc, marking the final closure of the HNX Ocean from the Late Permian to Early Triassic. We investigate the closure of the HNX Ocean and reconstruct the evolution of the central Beishan Orogen in the youngest Palaeozoic. • Baihu Basin adjacent to the Niujuanzi suture zone is proved to be a foreland basin. • Baihu foreland basin began to form in the Permian and lasted until the Triassic. • In the early stage, sediments derived from two arcs and accretionary wedge. • In the late stage, the source areas were mainly the Gongpoquan Arc and wedge. [ABSTRACT FROM AUTHOR]

The process of marine ecosystem collapse during the Permian–Triassic mass extinction may have varied in different paleogeographic settings and the intensity of oxygen depletion changed with different water depths. It remains largely unknown about the detailed biotic community collapse and its relation to paleo-redox condition in the deep-water basin. At the Kejiao section in the Nanpanjiang Basin of South China, the continuous sedimentation sequences and complete fossil records from the Changhsingian of Permian to the lowest Triassic provide a unique opportunity to study the progressive biotic crisis associated with oxygen deficiency in a deep-water setting. Here, we document high-resolution records of both the biodiversity dataset of different taxa and the redox data from the Kejiao section to achieve a new understanding of ecosystem deterioration associated with changes of redox conditions. The redox evolution revealed by changes in pyrite framboid size and morphology shows a change trend from lower dysoxic to oxygenated, and then to dysoxic interrupted by oxygenated and anoxic conditions in the late Changhsingian, and ending with euxinic conditions in the earliest Triassic. 60 species in 41 genera from radiolarians, bivalves, brachiopods and cephalopods were identified in the section and their occurrences show two pulses of extinction. The first pulse corresponds to the reduction in species richness of planktonic radiolarians and nektonic cephalopods, and the second pulse is documented by the wipe out of benthic brachiopods and remnant nektonic cephalopods and the decline in the species richness of benthic bivalves. This indicates the collapse of plankton and most nekton communities is ahead of the counterpart of benthos in the deepwater setting, supporting what observed in other sections in South China. The plankton and nekton communities inhabited water columns and the benthic community dwelled in the bottom water. Thus, the stagewise collapse of communities might be contributed by the expansion/migration, in particular, the downward expansion of Oxygen Minimum Zone (OMZ) documented by the information from pyrite framboids and the sedimentation. • New records on the largely unknown deep-water palaeocommunity collapse near the PTB. • Radiolarian, bivalve, brachiopod and cephalopod species were examined at Kejiao. • The collapse of plankton and most nekton communities is ahead of that for benthos. • Changes in pyrite framboid sizes show a fluctuated, deficient oxygen condition. • The stagewise collapse of communities recalled the expansion of OMZ. [ABSTRACT FROM AUTHOR]

The morphology and position of elements in the apparatus are keys to resolving the taxonomy, homology, evolutionary relationships, structure, function and feeding patterns among conodont taxa. Fused clusters preserving natural associations between elements provide direct information on element morphology, the positions of elements within the apparatus, and even their original three-dimensional arrangement. Here, we report 41 fused conodont clusters from Member II of the Guanling Formation in Luoping County, Yunnan Province, southwestern China, which provide a basis for inferring the multielement composition of the apparatus of the early Middle Triassic Nicoraella. The apparatus is composed of 15 elements (a single S 0 element, two pairs of S 1–4 , M and P 1–2 elements) like other apparatuses in Gondolellidae, i.e. the genera Novispathodus and Neogondolella. These Luoping Biota clusters are significant because (a) they permit a positional homology-based comparison of multielement Novispathodus with form genera such as Cypridodella (S 1), Enantiognathus (S 2), and Hindeodella (S 3 and S 4), (b) they facilitate a review of apparatus composition within superfamily Gondolelloidea, (c) they provide direct insight into apparatus architecture currently interpreted largely in light of distantly related Carboniferous polygnathacean ozarkodinins, and (d) these clusters, along with collections of discrete conodont elements, provide a model for inferring the multielement composition of closely related species known only from discrete element collections. • Exceptionally preserved Middle Triassic conodont clusters from Luoping, Yunnan, southwest China; • conodont multielement apparatus of Nicoraella were reconstructed; • The new 15 element conodont apparatus highlights the importance to revise other apparatus in family Gondolelloidea; [ABSTRACT FROM AUTHOR]