A brachiopod assemblage from the Cambrian ‘Tsinghsutung Formation’ of Jianhe, Guizhou and its significance
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Abstract:
The ‘Tsinghsutung Formation’ (Cambrian Series 2 Stage 4) in Jianhe County, Guizhou is mainly composed of a lithological assemblage featuring carbonate rocks at the lower and top parts, with interbedded mudstones and argilla-ceous limestones with various thickness at the middle and upper sections. The carbonate layers of the Cambrian ‘Tsinghsutung Formation’ at the Songshan section were systematically sampled and processed using acid etching method, allowing for the study of the highly abundant and diverse micro-brachiopods. Five genera and six species of micro-brachiopods from the ‘Tsinghsutung Formation’ are identified, including Eohadrotreta zhenbaensis and Hadrotretaprimaea of the Acrotretoidea; Schizopholis napuru and Schizopholis yorkensis of the superfamily Acrotretacea; and Palaeobolus sp. and Eoobolus sp. of the superfamily Linguloidea. Eohadrotreta zhenbaensis is dominant, accounting for approximately 80% of the assemblage. Notably, E. zhenbaensis and S. yorkensis are discovered for the first time in the study area. A comparison of characters of the micro-brachiopod assemblage from the carbonate rocks of the ‘Tsinghsutung Formation’ with those of macro-brachiopods in clastic rocks, as reported by previous researchers from Jianhe, reveals a micro-brachiopod fauna dominated by Eohadrotreta in the carbonate rocks. In contrast, a large number of macro-brachiopod Nisusia, along with smaller amounts of Diandongia and Kutorgina, are uniquely preserved in the clastic rocks. This difference between these assemblages may be resulted from variations in burial conditions or changes in relative sea level and environmental substrates. This study not only enriches our understanding of the characteristics of the brachiopod assemblage from the ‘Tsinghsutung Formation’, but also provides more information for a comprehensive understanding of the brachiopod composition in Cambrian Stage 4 and their evolutionary adaptations to environmental changes.
