A stromatoporoid-like sediment-agglutinating sponge from microbialites of Cambrian Stage 4

Many extant demosponges, especially the non-spicular taxa, incorporate foreign detritus in their spongin or chitinous skeletons. This supposed energy-efficient way of skeleton construction is, however, rarely known in the fossil record. In this study, based on careful petrographic analyses using standard optical microscopy, cathodoluminescence and fluorescence microscopy, and a three-dimensional reconstruction, a fossil from the Cambrian Stage 4 of Xuzhou City, Jiangsu Province, China, is established as a sediment-agglutinating sponge Psammolectospongia beiwangensis gen. et sp. nov. These fossils encrust stromatolitic microbialites and are often surrounded by Girvanella mats. This succession of microbialites and sponge fossils was developed in a turbulent hydrodynamic condition after the deposition of edgewise conglomerates. P. beiwangensis possesses labyrinthine skeletons with an architecture similar to that of some stromatoporoids, such as Stromatopora and Syringostromella, although dissepiments are absent in the Cambrian fossil. The skeleton of P. beiwangensis is mainly composed of packed silt-to sand-sized detritus grains and skeletal fragments of other animals, while in some places, it can contain various proportions of micrites. The morphology and skeletal composition of P. beiwangensis indicate an affinity to an agglutinating demosponge, which mainly constructed its skeleton using agglutinated and incorporated sediments, while the spongin skeleton was able to be biomineralized like that of Vaceletia. This study is the first report of a sponge fossil that mainly built its skeleton using foreign detritus, and it expands our knowledge about the physiology and ecological behaviors of early Paleozoic demosponges.