Sponge spicule fossils from the Cambrian Shuijingtuo Formation at Qiaojiaping Village, Yichang, and their palaeontological significance

In the study of early sponge evolution, disarticulated sponge spicules have received far less attention than articulated macrofossils because they rarely preserve the skeletal architecture of individuals and are often of limited use in taxonomic determinations. However, quantitative analysis shows that spicule assemblages can still reflect the variation of sponge diversity within specific spatiotemporal ranges. In this study, over 2,300 sponge spicules were recovered via acid maceration from limestone nodules in Member 3 and black shales in Member 4 of the Shuijingtuo Formation at Qiaojiaping Village in Yichang. These spicules are classified into seven major categories and 15 subtypes based on their morphology. Comparative analysis shows that the spicule assemblages from the two horizons are similar in composition, both dominated by siliceous hexactins, pentactins, and tetractins. Notably, inflated hexactins and drop-like pentactins exhibit diverse morphologies and are present in both horizons, distinguishing the Qiaojiaping spicule assemblage from other early Cambrian assemblages previously reported from South China. Additionally, polyactins and tylodiscs were recovered exclusively from the limestone nodules, whereas inflated pentactins were found only in the black shales. The inflated triaxons in the Qiaojiaping Village assemblage are comparable to the components of certain Early Paleozoic spicule assemblages, such as Thoracospongia, and can thus be reconstructed as elements of a similarly layered hexactinellid dermal skeleton. This indicates that the Thoracospongia-type body plan already existed in Cambrian Stage 3 of South China. Remarkably, none of these distinctive spicule types—including the drop-like pentactins, inflated hexactins, and the tylodiscs—have been reported in previously described sponge macrofossils from the same locality. This discrepancy highlights the necessity of microfossil studies to complement the analysis of articulated sponge fossils in elucidating early sponge evolution.