巴基斯坦北部寒武纪小壳化石与地层延限图谱

埃迪卡拉纪与寒武纪早期是地球生命系统演化中的一段最为重要转折期之一，是地史上隐生宙向显生宙的过渡期，也是研究后生动物起源与演化最为热点的时期。骨骼化石在埃迪卡拉纪晚期和寒武纪早期的突然出现和快速辐射演化见证了地质历史上一项重要的演化事件，即寒武纪大爆发。这些骨骼化石因其通常都是矿化的且个体微小，又被笼统地称作“小壳化石”。小壳化石是最早的后生动物代表之一，因而对研究许多动物门类的起源和早期演化尤为关键。小壳化石在寒武纪早期有广泛的全球性分布，并且演化较为快速，因此也被当做寒武纪早期地层划分与对比和古地理重建的重要工具。巴基斯坦北部地区虽然发育有新元古代至寒武纪早期的地层，但一直以来都缺乏相关的基础研究工作，导致学界难以确定对这段地层的具体时代归属。虽然前人在Hazara盆地东部的寒武纪早期地层Hazira组中发现了一些小壳化石，但对研究的程度还显得尤为不足且存有问题，因此急需重新做开展相关的研究工作来理清这些化石的属种组成，为生物地层工作奠定基础。本次研究，对Hazara盆地东部的Salhad剖面与Kalu-di-bandi 剖面的Hazira组及其下伏的Abbottabad组进行了系统的化石采样。在实验室内岩石样品经5%冰醋酸浸泡、水洗、烘干后，在体视显微镜下挑选出小壳化石标本。选取保存较为完整的化石标本，粘到金属靶台上，再使用扫描电子显微镜拍摄化石照片。在两个剖面共计鉴定出小壳化石27属29种，且皆识别出三个相同的化石带，自下而上分别为：Anabarites trisulcatus–Protohertzina anabarica Assemblage Zone、Zhijinites longistriatus Assemblage Zone以及Sinosachites flabelliformis–Tannuolina zhangwentangi Assemblage Zone，时代上属于寒武纪纽芬兰世幸运期至第二期。

The Ediacaran and Early Cambrian periods represent one of the most critical transitional intervals in the evolution of Earth’s life system, marking the transition from the Cryptozoic to the Phanerozoic eons, and are the most intensively studied intervals for the origin and evolution of metazoans. The sudden appearance and rapid radiation of skeletal fossils in the late Ediacaran and Early Cambrian witnessed a pivotal evolutionary event in Earth’s history: the Cambrian Explosion. These skeletal fossils, typically mineralized and tiny in size, are collectively referred to as "small shelly fossils (SSFs)". As one of the earliest representatives of metazoans, small shelly fossils are particularly crucial for studying the origin and early evolution of numerous animal phyla. With a widespread global distribution in the Early Cambrian and rapid evolutionary rates, SSFs also serve as important tools for Early Cambrian stratigraphic subdivision, correlation and paleogeographic reconstruction. Although Neoproterozoic to Early Cambrian strata are well-developed in northern Pakistan, relevant basic research has long been scarce, making it difficult for the academic community to determine the precise chronological attribution of these strata. Although previous studies have discovered some small shelly fossils in the Early Cambrian Hazira Formation in the eastern Hazara Basin, the research remains insufficient and problematic. Therefore, there is an urgent need to conduct new relevant studies to clarify the genus and species composition of these fossils, laying a foundation for biostratigraphic work. In this study, systematic fossil sampling was carried out on the Hazira Formation and its underlying Abbottabad Formation in the Salhad Section and Kalu-di-bandi Section of the eastern Hazara Basin. In the laboratory, after soaking rock samples in 5% glacial acetic acid, washing and drying, small shelly fossil specimens were picked out under a stereomicroscope. Well-preserved fossil specimens were selected, mounted on metal stubs, and then photographed using a scanning electron microscope (SEM). A total of 27 genera and 29 species of small shelly fossils were identified from the two sections, and three identical fossil assemblage zones were recognized in both sections, in ascending order: the Anabarites trisulcatus–Protohertzina anabarica Assemblage Zone, the Zhijinites longistriatus Assemblage Zone, and the Sinosachites flabelliformis–Tannuolina zhangwentangi Assemblage Zone. Chronologically, these correspond to the Fortunian to Stage 2 of the Cambrian Terreneuvian Series.