Biostratigraphic and Paleobiogeographic Significance of Two Cap-Shaped Fossil Groups from the Cambrian Xinji Formation, North China: Evidence for North China–australia Faunal Links

The Xinji Formation represents the oldest Cambrian sedimentary unit on the North China Platform, widely distributed along its southwestern and southern margins. It yields abundant small shelly fossils, among which cap-shaped forms are particularly diverse. This study presents a systematic investigation of two types of cap-shaped fossils collected from four sections along the southern margin of the North China Platform (Zhoujiaqu, Longxian County; Dazhuang, Luonan County; Guanyintang, Baofeng County; Shuikunongdi, Yexian County). Based on surface ornamentation, they are classified into concentrically ornamented and tuberculate types. High-resolution scanning electron microscopy reveals fundamental differences in microstructures between the two groups, suggesting they likely belong to distinct animal phyla. The concentrically ornamented cap-shaped fossils exhibit bilateral symmetry, a posteriorly overhanging apex, and an anterior concavity, allowing their assignment to the helcionelloid mollusc Prosinuites Poulsen, 1967. Through comparative morphological analysis, energy dispersive spectrometer, and microstructural examination, a new species, Prosinuites dazhuangensis sp. nov., is established. The tuberculate cap-shaped fossils are covered with irregular papilliform tubercles and display a reticulate internal texture. They are assigned to Chalasiocranos theloides (Feng et al., 1994). Although the overall sclerite morphology, growth mechanism, and similarities to priapulid cuticular elements suggest a possible relationship with ecdysozoans, this interpretation remains tentative pending further evidence, and they are not considered typical molluscan shells. Although the two fossil groups share a superficial resemblance in overall shape, they likely belong to different metazoan phyla. The currently known distribution of C. theloides is restricted to North China and Australia, providing direct evidence for biostratigraphic correlation of Cambrian Stage 4 strata between these regions. Prosinuites, in contrast, ranged across South China, North China, Denmark, and France, indicating trans-plate dispersal of early Cambrian molluscs. This study contributes new insights into the taxonomic revision, phylogenetic affinities, paleobiogeography, and morphological convergence of early Cambrian cap-shaped fossils.