Diverse labechiid stromatoporoids from the Upper Ordovician Xiazhen Formation of South China and their paleobiogeographic implications

A diverse labechiid stromatoporoid assemblage that includes 16 species in 8 genera was found in the Upper Ordovician Xiazhen Formation (mid–late Katian) at Zhuzhai, Jiangxi Province of South China. The assemblage is characterized by a combination of (1) North China provincial species succeeding from their origination in the Darriwilian, including Pseudostylodictyon poshanense Ozaki, 1938, Labechia shanhsiensis Yabe and Sugiyama, 1930, Labechia variabilis Yabe and Sugiyama, 1930, and Labechiella regularis (Yabe and Sugiyama, 1930) and (2) South China endemic species, including three new species (Labechia zhuzhainus Jeon n. sp., Labechiella beluatus Jeon n. sp., Sinabeatricea luteolus Jeon n. gen. n. sp.), and four species in open nomenclature (Rosenella sp., Cystostroma sp., Pseudostylodictyon sp., and Labechia sp.). The finding of Labechiella gondwanense Jeon n. sp., Stylostroma bubsense Webby, 1991, Stylostroma ugbrookense Webby, 1991, and Thamnobeatricea gouldi Webby, 1991 in the formation indicates that Tasmania was closely related to South China and had a closer paleobiogeographical relation with peri-Gondwanan terranes than with Laurentia. In addition, the occurrences of Labechia altunensis Dong and Wang, 1984 and Stylostroma species support a close biogeographic link between Tarim and South China through the Middle to Late Ordovician interval, corresponding with the results from other fossil groups such as brachiopods, conodonts and chitinozoans. The diverse labechiids from the Xiazhen Formation improve our understanding of the diversity of Ordovician stromatoporoids in peri-Gondwanan terranes and the biogeographic affinities among Australia (especially Tasmania), Tarim, and South China. UUID: http://zoobank.org/4f46c91b-fa4c-4fe5-bea9-e409f1785677

Spatiotemporal Evolution and Genesis of the Late Ordovician—Early Silurian Marine Euxinia in Northeastern Upper Yangtze Basin, South China

Marine redox conditions and their dynamic changes were a major factor that controlled the formation of black shale and caused the late Ordovician marine extinction in the Upper Yangtze Basin (South China). However, the spatiotemporal variation and potential controlling factors of marine redox conditions in this area remain unclear. We analyzed whole-rock geochemistry and pyrite sulfur isotopes (δ34Spy) of 47 shale samples from the Late Katian to Rhuddanian in a shelf-to-slope (Qianjiang Shaba section and Wc-1 well) region of northeastern Upper Yangtze Basin, and reconstructed water column redox conditions during the Late Ordovician–Early Silurian Transition. The geochemical characteristics of shale, including the ratio of elements, discriminant function and ternary diagram location in the study area suggest a passive continental margin sedimentary environment, wherein the terrigenous detritus is mainly derived from felsic igneous rocks in the upper crust, showing characteristics of near-source deposition. The redox indices (Fe speciation, Corg/P, UEF, and MoEF) showed that the development of anoxic water, especially euxinia, has obvious spatiotemporal heterogeneity. Under conditions of high availability of active organic carbon and limited sulfate supply, high active Fe input and strong biological irrigation in the shallow water area may effectively remove H2S produced by microbial sulfate reduction, conducive to the prevalence of ferruginous water columns. However, for this deep water area, the rapid accumulation rate of organic matter, decrease in dissolved Fe (caused by upwelling in the open sea), and seawater stratification (caused by the rising of sea level) promoted the development of a euxinic water column. This inference is supported by the covariant relationship between organic carbon accumulation rate, chemical index of alteration, Co × Mn, and δ34Spy. Our study highlights the potential control effects of sea level change, continental weathering and upwelling on the development of euxinic water columns.

All post- Cambrian ichnospecies of Psammichnites Torell, 1870 belong to Olivellites Fenton and Fenton, 1937a

The ichnogenus Psammichnites herein restricted to Psammichnites gigas is based on comparison of morphology, feeding behaviour, contrast between the burrows and the host rock and possible producers. The record of siphonal activity as a “snorkel device” is discussed. The diagnosis of the ichnogenus Olivellites now is amended and includes all the records of Psammichnites in the post-Cambrian. Olivellites is now documented in successions other than the classical tidal flat deposits facies of the Carboniferous of the USA. We propose that the producer of Olivellites was an animal with capacity for displacement to different shallow infaunal levels for different feeding strategies. An interpretation of detritus feeding behavior with sediment displacement (pasichnia) is favoured here. The producer of Olivellites was likely to have been a bivalved mollusc that evolved after the Late Ordovician mass extinction. It was euryhaline and lived in a broad bathymetric range, and is recorded in temperate to glacially related successions. The material of Olivellites implexus from western Argentina is the youngest record of the ichnogegenus from Western Gondwana.