扬子地台西北缘镇巴地区寒武系底部发育一套厚约8.5 m的含磷生物碎屑碳酸盐岩, 与上下地层均呈平行不整合接触。该套地层, 仅含少量海绵骨针与腕足类化石, 其归属长期存在争议, 存在三种观点: 或归入灯影组顶部、或独立为西蒿坪段, 或划属水井沱组。本研究关注到水井沱组底部及下伏西蒿坪段富集的小壳化石层, 系统采集古盘虫类三叶虫头盖化石, 结合前人报道的西蒿坪段存疑标本Parabadiella cf. huoi, 通过主成分分析(PCA)、典型变量分析(CVA)、非度量多元标度变换(NMDS)及聚类分析(CA)开展几何形态测量学研究。结果表明: (1)新采集的30件标本可归入Tsunyidiscus yanjiazhaiensis, 其头盖轮廓形态与模式种高度一致; (2) Parabadiella与Abadiella在形态空间内连续分布, 支持属级合并; (3)基于平行不整合界面及Abadiella和Tsunyidiscus的首现层位, 建议将西蒿坪段作为独立岩石地层单元, 时代厘定为寒武系第三阶下部。本研究为华南寒武纪早期生物地层对比及三叶虫分类修订提供了定量形态学证据。

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.

科技部重大专项(2023YFF0803601)和国家自然科学基金项目(42172014, 41621003)联合资助