Ben Yang

The Zavkhan terrane (western Mongolia) sits in the core of the Central Asian Orogenic Belt and holds the key to understanding the evolution of the belt. However, the Neoproterozoic position of the Zavkhan terrane has remained contentious for a long time. Current models propose either a Siberian affinity, a peri-Siberian location, or an isolated paleogeographic position in the Paleo-Asian Ocean. These competing interpretations rely predominantly on tectonic and sedimentary evidence, with a notable lack of paleontological constraints. Here, we report Ediacaran large acanthomorphic acritarchs (LAAs) from the Shuurgat Formation at three sections in the Duruvlgin and Aldarkhaan areas of the northern Zavkhan terrane. Integrated with carbon isotope data from the same formation, these acritarchs support a stratigraphic correlation with the lower Doushantuo (Yangtze block) and Krol A (Lesser Himalaya) formations, indicating an early Ediacaran age. The Shuurgat LAA assemblage, comprising 24 species, shows high similarity to those of Yangtze block (South China) and Lesser Himalaya (northern India), and it shares a number of taxa that are common to, or even restricted to, these two blocks. Quantitative bioprovincial analysis of a global Ediacaran LAA database further confirms their inclusion within a single bioprovince. Together with comparable detrital zircon age spectra and stratigraphic sequences, these results support a close paleogeographic affinity of the Zavkhan terrane with the Yangtze block and Lesser Himalaya in the Ediacaran, challenging models invoking a Siberian connection.

With special tubular and skeletal bauplan, Ediacaran tubular fossils are one unique metazoan group that has irreplaceable significance for our understanding of the early metazoan evoution. However, their biological affinities have long been disputed because of their limited morphological features and ambiguous primary composition. In all the recorded taxa, this problem typically existed with Cloudina and Sinotubulites, the two most important tubular fossils which have stratigraphic and evolutionary significances. Here we carried out a detailed study on their microstructure, mineralogical composition and taphonomic background, for the purposes of revealing their taphonomic characteristics and biomineralogical potentials which may shed light on the understanding of their phylogenetic affinities. Phosphatic and calcitic specimens from the Yangtze Block, Namibia, and Paraguay were studied using integrative approaches including EBSD, Raman spectroscopy, and cathodoluminescence. It was revealed that, excluding the influence of diagenesis, crystal orientations in all studied Cloudina and Sinotubulites lack an ordered alignment which are commonly seen in biomineralization processes. In most specimens, the luminescence between the surrounded rocks and the fossils showed no diagenetic related differentiation, indicating no traces of original carbonatization. Besides, Raman spectroscopy analysis showed intense carbon within the parallel laminae of tube walls in contrast to the surrounding rocks among both phosphatic and carbonatic taphono-facies preserved which may be residue of original organic matters. These analyses altogether suggest that the original tube-wall composition of both Cloudina and Sinotubulites was likely organic with no detectable traces of primary biomineralization.

Tubular fossils are a unique metazoan group emerging in the late Ediacaran Period and demonstrating early skeletogenesis and an increase in the diversity of early biocommunities. Among the known records, Sinotubulites is widely distributed and distinct in morphology and ultrastructure, holding important evolutionary and stratigraphic significance comparable to the well-known Cloudina. However, its biological affinity remains unclear until now. Among various reasons, taphonomic bias is one of the major factors responsible for this, as it not only altered the primary morphology but also modified the ultrastructure and composition of the fossil. Thus, a further study on its taphonomic process would help to decode the biological affinity of Sinotubulites. For this purpose, we conducted a taphonomic study on Sinotubulites from the top of the Shibantan Member of the Dengying Formation at the Zhongling section in the Yangtze Gorges area (Hubei Province, China). We applied electron backscatter diffraction (EBSD) and cathodoluminescence (CL) to reveal its mineralogical features. EBSD and CL analyses demonstrate that both the fossils and matrix are composed of unoriented calcite, and the matrix shows slight dolomitization with sporadic dolomite grains. The calcite crystals within the Sinotubulites tubes are significantly larger than those in the matrix, indicating that the tubular structure provided sufficient space for crystal growth. The absence of lamellar structures in the tubular walls further suggests that the original biogenic material may have been dissolved during diagenetic calcification. The absence of dolomitization in the fossils indicates that this process may have been inhibited by either their large calcite crystals or the enclosed space confined by the outer shell. The identical non-luminescent features of the matrix and fossils suggest that their calcification likely occurred during the same stage. This study demonstrates that taphonomic biases must be accounted for when analyzing the original structure and composition. Additionally, this research documents the occurrence of Sinotubulites in the Shibantan Member, representing its lowest stratigraphic horizon in the Yangtze Block.

Cap-shaped skeletal fossils are the earliest undisputed body fossils of mollusks appearing in the basal Cambrian. A study on the morphometry of cap-shaped fossils from the Nanjiang area (North Sichuan, China) is undertaken to understand the origin and evolution of the early mollusks. The distribution of these fossil cap-shaped mollusks indicates a stepwise increase in their diversity during the early Cambrian.MaikhanellaZhegallo in Voronin et al., 1982 co-occuring with the spinose sclerites of siphogonuchitids, is regarded as the earliest scleritized mollusk. It is followed by other maikhanellids, e.g.,PurellaMissarzhevsky, 1974 andYunnanopleuraYu, 1987, which co-occur with the earliest univalved helcionellids, e.g.,IgorellaMissarzhevsky in Rozanov et al., 1969. Cluster analysis of their morphometric characteristics shows that theMaikhanellagroup is similar to thePurellaandYunnanopleuragroups, but is less comparable with univalved helcionellids. The maikhanellids are interpreted as representatives of the stem group Aculifera, although it remains uncertain if one or two larger cap-shaped shell plates were present on the elongate slug-like body, comparable to those ofHalkieriaPoulsen, 1967 orOrthrozanclusConway Morris and Caron, 2007. Maikhanellids are characterized by the prominent protrusions or scales on the cap-shaped shell plates arranged in a concentric pattern around the shell apex. Evolutionarily, the protrusions or scales are reduced in younger strata, whereas subsequently a typically concentric ornament developed, the cap-shaped shell plates developed higher profiles, and the apical region became increasingly bare of scales. Meanwhile, the cap-shaped shell plates gradually evolved into a helcionellid-like appearance with an anteroposteriorly inclined apex. The morphological evolution of the earliest sclerotized mollusks reflects biotic evolution and environmental adaption among the stem-group mollusks during the early Cambrian.

The Ediacaran tubular fossils Cloudina, Sinotubulites, and Conotubus are taxonomically revised with type materials. It is proposed that Aulophycus lucianoi Beurlen and Sommer, 1957, is not a senior synonym of Cloudina hartmannae Germs, 1972. Instead, most of its syntypes may be assigned to Sinotubulites or other taxa. Lectotypes of Sinotubulites baimatuoensis Chen et al., 1981, and Conotubus hemiannulatus Zhang and Lin in Lin et al., 1986, are designated from rediscovered syntypes. Sinotubulites baimatuoensis Chen et al., 1981, is reported from the Mooifontein Member of Nama Group at Aar Farm, Namibia. Cloudina waldei Hahn and Pflug, 1985, is assigned to Sinotubulites baimatuoensis, and thus its occurrence range is extended to Brazil. The lectotype of Conotubus hemiannulatus shows corrugations and annulations on the surface distinguishing it from Cloudina and other collared Ediacaran tubular fossils. Based on the taxonomic revision, we propose a Cloudina hartmannae Interval Zone for the terminal Ediacaran with the upper boundary defined by the first appearance datum of Protohertzina anabarica (i.e., the index fossil of the early Cambrian Anabarites trisulcatus-Protohertzina anabarica Assemblage Zone).

The Central Asian Orogenic Belt is complex and little understood in its geotectonic history. Here, we apply a new approach to conclude on the paleogeography of major Mongolian terrains using small skeletal fossil assemblages and a combined analysis of U‐Pb ages and typology of detrital zircons. We document a new Cambrian Stage 3 fauna from the Telmen Block located between the Dzhabkan and Tuva‐Mongolia cratons. Based on a hierarchical cluster analysis of 35 blocks, we observe the most robust biogeographic relationship between Telmen and Kusnetzk Alatau and between Tuva‐ Mongolia and Lake Zone. These results challenge the view that these cratons were assembled during the Ediacaran/Cambrian in a single ribbon‐shaped paleocontinent. Detrital zircon U‐Pb ages indicate a unique age population at ∼516 Ma at Telmen derived from I‐type granitoids, neither seen on Dzhabkan nor Tuva‐Mongolia cratons supporting their different tectonic evolution within the Palasian Ocean after the breakup of Rodinia.

Classical sections, such as the Maidiping and Daqiao Mine sections of South Sichuan (China), expose early Cambrian deposits that are crucial for understanding the biological and environmental evolution of Yangtze Craton. These sequences are rich in Terreneuvian small shelly fossils, which can be assigned to assemblages I and III from South China. The Anabarites trisulcatus – Protohertzina anabarica Assemblage Zone (Assemblage I) is recognized at the lower Maidiping Formation. The second assemblage ( Paragloborilus subglobosus – Purella squamulosa Assemblage Zone) cannot be verified in South Sichuan, although previous reports claimed its existence based on the occurrence of Paragloborilus subglobosus . The third assemblage ( Watsonella crosbyi Assemblage Zone) is confirmed in the upper Maidiping Formation. The abundant bioclasts in this interval indicate abrasions and bioerosions by winnowing or starved sedimentation. Carbon isotope values from the Maidiping section present no negative excursion at the presumed Ediacaran–Cambrian transition. A positive carbon isotope excursion is observed in the Upper Maidiping Formation (Assemblage III) which is correlated to the ZHUCE excursion in the Dahai Member of eastern Yunnan. The shallow-water deposits of South Sichuan can be correlated with the South China, western Mongolia and Siberia successions based on biozonations and carbon isotope trends. Supplementary material: Isotope data are available at https://doi.org/10.6084/m9.figshare.c.5326834 Thematic collection: This article is part of the Advances in the Cambrian Explosion collection available at: https://www.lyellcollection.org/cc/advances-cambrian-explosion