Exceptional preservation of two new early rossellid sponges: the dominant species in the Hirnantian (Late Ordovician) Anji Biota of China

Abstract

The Anji Biota of Zhejiang Province, South China, is an exceptionally preserved, sponge-dominated fauna from the latest Ordovician interval, representing a deep-water environment and containing more than 100 sponge species. Herein a complex of two common species that together dominate the deepest-water sponge assemblages within the sequence are described: Shouzhispongia coronata gen. et sp. nov. and Shouzhispongia prodigia gen. et sp. nov. The complex taphonomic pathway for sponge preservation is critical to interpretation of the fossils: prostalial hypodermal pentactins (a unique character of the hexactinellid family Rossellidae) were originally present and are locally visible, but many have been lost taphonomically because spicules are primarily preserved as moulds within soft tissues. The choanosomal skeleton is composed dominantly of very small hexactins and other triaxon spicules rather than diactins, suggesting an early branching position within the family stem group. Despite their abundance in the Anji Biota, and superficial similarity to certain extant rossellid genera such as Bathydorus, no closely similar sponges have yet been described. This highlights the extremely incomplete fossil record of truly deep-water ecosystems. Preservational alignment patterns of the sponges on bedding planes support previous ideas of nepheloid-layer collapse as the primary burial mechanism.