New evidence on geometric morphometrics and stratigraphic attribution of trilobite fossils from the Xihaoping Member and basal Shuijingtuo Formation (Cambrian Series 2, Stage 3) in Zhenba, southern Shaanxi, China
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Abstract:
The Zhenba area, situated on the northwestern margin of the Yangtze Platform, hosts an approximately 8.5-meter-thick phosphatic bioclastic carbonate unit at the base of the Cambrian. This unit, characterized by parallel unconformities with overlying and underlying strata and sparse sponge spicules and brachiopod fossils, has been the subject of stratigraphic debate: (1) assignment to the uppermost Dengying Formation; (2) designation as an independent Xihaoping Member; or (3) inclusion in the basal Shuijingtuo Formation. This study focuses on a layer rich in small shelly fossils at the base of the Shuijingtuo Formation and underlying Xihaoping Member, systematically collecting eodiscoid trilobite cranidia, and integrating previously reported specimens of Parabadiella cf. huoi from the disputed Xihaoping Member. Geometric morphometric analyses (PCA, CVA, and NMDS) of 30 well-preserved eodiscoid trilobites from the Shuijingtuo Formation, along with previously documented Parabadiella (Abadiella) specimens, were conducted. Our key findings include: (1) The newly collected specimens are assignable to Tsunyidiscus yanjiazhaiensis, with cranidial contour morphology closely matching that of the holotype; (2) The continuous morphospace distribution between Parabadiella and Abadiella supports their synonymization at the genus level; and (3) Based on parallel unconformities and the first appearance datum (FAD) of Abadiella and Tsunyidiscus, we propose that the Xihaoping Member represents an independent lithostratigraphic unit, constrained to the lower Cambrian Stage 3. This study provides quantitative morphological evidence for refining trilobite taxonomy and enhancing biostratigraphic correlations of the lower Cambrian strata in South China.
