Ocruranus–Eohalobia Sclerites from the Cambrian Stage 2 Yanjiahe Formation in South China: Scleritome Reconstruction and Zoological Affinity

SIMPLE SUMMARY: New materials of Ocruranus and Eohalobia sclerites are found in Member 5 of the Yanjiahe Formation (Cambrian Stage 2). The extended and upfolded field on Eohalobia exhibiting dense wrinkles is most likely a weakly mineralized structure that joins the valves. The distribution of muscle attachment zones of the Ocruranus–Eohalobia group corresponds with that in modern chitons, which provides strong evidence for assigning them to the Polyplacophora. Furthermore, comparing the butterfly-shaped region on a unique specimen of Eohalobia with the sub-apical field on Ocruranus suggests that the original scleritome may consist of only Ocruranus and Eohalobia sclerites. The Ocruranus–Eohalobia group previously discovered from the Zhongyicun Member (Fortunian) in eastern Yunnan may have three types of plates, which is different from the reconstruction here from the Yanjiahe Formation (Stage 2), where only two plates are included. However, it is inferred that such differences might be due to the morphologic evolution (the reduction of the number of plates from three to two from the Fortunian to Stage 2) or due to other factors (geographical, biological, etc.). This paper provides a new insight into reconstructing the original configuration of the scleritome of the Ocruranus–Eohalobia group. ABSTRACT: 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.