Some small shelly fossils are important index fossils for global stratigraphic subdivisions and correlations of the Cambrian Terreneuvian. The first appearance datum (FAD) of the cosmopolitan mollusk Aldanella attleborensis has been suggested as one of the potential markers for defining the base of Cambrian Stage 2. Aldanella fossils were uncommon in South China, and if occurring, were often described as Aldanella yanjiaheensis, A. attleborensis, or indeterminate species in the literature, while A. yanjiaheensis was often taken as a junior synonym of A. attleborensis. Nevertheless, a detailed taxonomic revision of A. yanjiaheensis based on material from its type locality awaits to be made. In this study, we systematically re-evaluated A. yanjiaheensis based on the numerous specimens collected from the base of Member 5 of the Yanjiahe Formation in the Three Gorges area, western Hubei Province of South China. Detailed taxonomic comparison further substantiates that A. yanjiaheensis is a junior synonym of A. attleborensis, signifying its strong potential for a global correlation across paleocontinents. Morphological parameter analyses indicate that the length and width of shell tube of A. attleborensis shows allometric growth. The nearly cosmopolitan distribution and characteristic morphology of A. attleborensis indicate that it can play a significant role in the subdivision and correlation of Cambrian Stage 2. The co-occurrence of A. attleborensis and Watsonella crosbyi from the base of Member 5 of the Yanjiahe Formation corroborates that Member 5 belongs to Cambrian Stage 2.

Extant ecdysozoans (moulting animals) are represented by a great variety of soft-bodied or articulated organisms that may or may not have appendages. However, controversies remain about the vermiform nature (i.e. elongated and tubular) of their ancestral body plan. We describe here Beretella spinosa gen. et sp. nov. a tiny (maximal length 3 mm) ecdysozoan from the lowermost Cambrian, Yanjiahe Formation, South China, characterized by an unusual sack-like appearance, single opening, and spiny ornament. Beretella spinosa gen. et sp. nov has no equivalent among animals, except Saccorhytus coronarius, also from the basal Cambrian. Phylogenetic analyses resolve both fossil species as a sister group (Saccorhytida) to all known Ecdysozoa, thus suggesting that ancestral ecdysozoans may have been non-vermiform animals. Saccorhytids are likely to represent an early off-shot along the stem-line Ecdysozoa. Although it became extinct during the Cambrian, this animal lineage provides precious insight into the early evolution of Ecdysozoa and the nature of the earliest representatives of the group.

The isolated sclerites of the Ocruranus and Eohalobia group are abundant among the early Cambrian small shelly fossil assemblages, which were recently assigned to the same scleritome as an early member of the polyplacophoran (chiton) stem lineage. However, the scleritome reconstruction and zoological affinities of these sclerites are still controversial due to the lack of exceptionally preserved articulated specimens with in-situ sclerites. Herein, we report new specimens of Ocruranus and Eohalobia sclerites from Member 5 of the Yanjiahe Formation, which provide new insights into the reconstruction of the original scleritome. The Eohalobia sclerites from the Yanjiahe Formation have an extended and upfolded proximal field with dense wrinkles, which seems to be a weakly mineralized structure and acted as a joint with another sclerite, Ocruranus. Comparing the butterfly-shaped proximal field on a unique sclerite of Eohalobia with the sub-apical field on Ocruranus sclerites suggests that the original scleritome of this group may consist of only two types of sclerites: the Ocruranus-type and the Eohalobia-type. The polygonal structure on the internal mold of Eohalobia sclerites is interpreted herein as the muscle attachment zone; their distribution corresponds well with that of the modern chitons, which provides strong evidence to support the close relationship between the Ocruranus–Eohalobia group and the Polyplacophora.