Abstract:Abundant, exquisitely preserved specimens of the enigmatic tubular microfossil Megathrix longus are reported from the early Cambrian Yanjiahe Formation in the Yangtze Gorges area. Studies of their morphology and taphonomy reveal that the inner cross-wall of Megathrix longus is topographically flat, the trichome terminus is blunt and closed, and incomplete cross-walls are regularly intercalated between complete ones. The deformation of trichomes, as well as the corrugation of their cross-walls, are postmortem f… Show more
“…Furthermore, the corrugated transverse wall of M. longus , the most distinctive character of the taxon, is a postmortem feature rather than the original biological feature. Our specimens show most of the key characters of the taxon presented by Shang et al (2015); thus we agree that M. longus was a planktic Oscillatoriopsis -like organism in the Cambrian ocean, which radiated along with the AHC assemblage.…”
Section: Descriptions Of the Acritarch And Other Microfossilssupporting
confidence: 86%
“…Megathrix longus Yin, 1987a, n. emend. Shang et al 2015, p. 5, figs 2A–2D, figs 3A–3L, figs 4A–4E, figs 5A–5N. figs.…”
Section: Descriptions Of the Acritarch And Other Microfossilsmentioning
confidence: 97%
“…Basal Cambrian successions in South China and Tarim. Occurrences in South China include the Yanjiahe Formation (Yin, 1987a; Yin et al 1992; Yin, Gao & Xing, 2003; Dong et al 2009; Shang et al 2015). Occurrences in Tarim include the Yurtus and Xishanblaq formations (Yao et al 2005; Dong et al 2009).…”
Section: Descriptions Of the Acritarch And Other Microfossilsmentioning
The Asteridium–Heliosphaeridium–Comasphaeridium (AHC) acritarch assemblage is composed of common organic-walled microfossils in the basal Cambrian chert–phosphorite units in South China, indicating that the AHC assemblage can be a useful biostratigraphic tool for the Ediacaran–Cambrian boundary successions in the Yangtze Platform. To test the validity of the AHC acritarch assemblage as a biostratigraphic tool, the stratigraphic range of the AHC acritarch assemblage was confirmed, and its spatial and temporal relationships to other bio- and chemostratigraphic tools were analysed in the Yanjiahe Formation, Yangtze Gorges area, South China. The result shows that the AHC assemblage temporally correlates to the Anabarites trisulcatus–Protohertzina anabarica Assemblage Zone, and spatially correlates to the large negative carbon isotope anomaly of the lowermost Cambrian (BACE) in the Yanjiahe Formation. This implies that the radiation of phytoplankton occurred slightly before the radiation of the small shelly fossils, and the AHC acritarch assemblage can be another important chronological reference to the lowermost Cambrian successions in South China, and potentially to global correlations.
“…Furthermore, the corrugated transverse wall of M. longus , the most distinctive character of the taxon, is a postmortem feature rather than the original biological feature. Our specimens show most of the key characters of the taxon presented by Shang et al (2015); thus we agree that M. longus was a planktic Oscillatoriopsis -like organism in the Cambrian ocean, which radiated along with the AHC assemblage.…”
Section: Descriptions Of the Acritarch And Other Microfossilssupporting
confidence: 86%
“…Megathrix longus Yin, 1987a, n. emend. Shang et al 2015, p. 5, figs 2A–2D, figs 3A–3L, figs 4A–4E, figs 5A–5N. figs.…”
Section: Descriptions Of the Acritarch And Other Microfossilsmentioning
confidence: 97%
“…Basal Cambrian successions in South China and Tarim. Occurrences in South China include the Yanjiahe Formation (Yin, 1987a; Yin et al 1992; Yin, Gao & Xing, 2003; Dong et al 2009; Shang et al 2015). Occurrences in Tarim include the Yurtus and Xishanblaq formations (Yao et al 2005; Dong et al 2009).…”
Section: Descriptions Of the Acritarch And Other Microfossilsmentioning
The Asteridium–Heliosphaeridium–Comasphaeridium (AHC) acritarch assemblage is composed of common organic-walled microfossils in the basal Cambrian chert–phosphorite units in South China, indicating that the AHC assemblage can be a useful biostratigraphic tool for the Ediacaran–Cambrian boundary successions in the Yangtze Platform. To test the validity of the AHC acritarch assemblage as a biostratigraphic tool, the stratigraphic range of the AHC acritarch assemblage was confirmed, and its spatial and temporal relationships to other bio- and chemostratigraphic tools were analysed in the Yanjiahe Formation, Yangtze Gorges area, South China. The result shows that the AHC assemblage temporally correlates to the Anabarites trisulcatus–Protohertzina anabarica Assemblage Zone, and spatially correlates to the large negative carbon isotope anomaly of the lowermost Cambrian (BACE) in the Yanjiahe Formation. This implies that the radiation of phytoplankton occurred slightly before the radiation of the small shelly fossils, and the AHC acritarch assemblage can be another important chronological reference to the lowermost Cambrian successions in South China, and potentially to global correlations.
“…Bed 1 contains the Asteridium-Heliosphaeridium-Comasphaeridium acritarch assemblage zone (Yao et al, 2005;Dong et al, 2009;Ahn and Zhu, 2017) and probable sponge spicules (Chang et al, 2017). Bed 2 is 2.6 m thick and consists of sili-ceous-phosphatic intraclastic dolostones bearing shelly fossils from the Anabarites trisulcatus-Protohertzina anabarica Assemblage Zone (Guo et al, 2014), in addition to the tubular taxon Megathrix longus (Shang et al, 2016). Bed 3 is about 18.5 m thick and consists of grey-black limestone interbedded with silty shale.…”
The Ediacaran to Cambrian transition is a critical interval of time during which major evolutionary changes occurred. Recently, abundant Protoconites minor have been recovered from the silty shales of the lower Cambrian Yanjiahe Formation (Terreneuvian, Fortunian-Stage 2) in the Three Gorges area of South China. These fossils represent an important ecological diversification of macroscopic organisms at the onset of the Cambrian. Protoconites minor is a probable cnidarian-grade organism preserved by carbon compression. Herein, geometric morphometric analyses are applied to crack out specimens of P. minor to reveal any cryptic morphological details that may have implications for their morphological diversity, ontogenetic processes, and taxonomic identification. These statistical analyses reveal a strong relationship between size and shape, which indicates that the overall shape of P. minor was mainly controlled by allometric growth. The smaller specimens are generally wider at the anterior and more commonly have straight-sides. Larger individuals tend to be narrower at the anterior, with bending more common. Our analyses demonstrate that there are transitional forms between larger, strongly bent specimens and smaller, straight specimens, suggesting that the assemblage likely consists of a single species.
“…The Shibantan limestone is overlain by up to 60 m thick, light grey, thick-bedded dolostone of the Baimatuo Member, which features abundant dissolution structures and is unconformably overlain by the lower Cambrian Yanjiahe Formation. The Baimatuo Member in the Yangtze Gorges area and its equivalent strata in southern Shaanxi yield the biomineralized tubular microfossils Sinotubulites and Cloudina (Zhao et al 1988; Hua et al 2000; Cai et al 2010, 2011, 2015), while the Yanjiahe Formation bears basal Cambrian small shelly fossils, the tubular microfossil Megathrix longus Yin, 1987 and acritarchs ( Asteridium – Comasphaeridium – Heliosphaeridium assemblage) indicative of an early Cambrian Terreneuvian age (Chen, 1984; Yao et al 2005; Dong et al 2009; Guo, Li & Li, 2014; Shang et al 2016). Figure 1. (a) Map showing the geographic location of the Yangtze Gorges area (star) in the Yangtze Block.…”
Non-biomineralizing Ediacaran macrofossils are rare in carbonate facies, but they offer valuable information about their three-dimensional internal anatomy and can broaden our view about their taphonomy and palaeoecology. In this study, we report a new Ediacaran fossil, Curviacus ediacaranus new genus and species, from bituminous limestone of the Shibantan Member of the Dengying Formation in the Yangtze Gorges area of South China. Curviacus is reconstructed as a benthic modular organism consisting of serially arranged and crescent-shaped chambers. The chambers are confined by chamber walls that are replicated by calcispars, and are filled by micritic sediments. Such modular body construction is broadly similar to the co-occurring Yangtziramulus zhangii and other Ediacaran modular fossils, such as Palaeopascichnus. The preservation style of Curviacus is similar to Yangtziramulus, although the phylogenetic affinities of both genera remain unresolved. The new fossil adds to the diversity of Ediacaran modular organisms.
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