Three-dimensionally preserved stem-group hexactinellid sponge fossils from lower Cambrian (Stage 2) phosphorites of China

Abstract

Three-dimensionally, in situ preserved sponge fossils were collected from the phosphorites at the basal Niutitang Formation (Cambrian Stage 2) in Hunan Province, China. These fossils are preserved as nodular bodies in authigenic carbonaceous cherty Ca-phosphorites. Spicules are either completely embedded in cryptocrystalline phosphate or immediately overgrown by early isopachous phosphate cements, leaving the remaining interspace filled by other deposits. Two specimens are described in detail as examples. One is composed of hexactins of at least three size-hierarchies, the small spicules in which may be interpreted as microscleres. The other, investigated using grinding tomography, shows a skeletal frame composed of pentactins, hexactins, and diactins. These spicules exhibit a weak pattern of perpendicular orientation in 3D space and a possible differentiation of hypodermalia and parenchymalia. These skeletal architectures belong to new taxonomic groups and seem to represent very basic forms of hexactinellids. Disarticulated spicules scattered in other parts of the investigated phosphorites indicate a still unexplored diversity of the fossil community, and some show combined features, which were regarded characteristic and mutually exclusive in living hexactinellids and demosponges. This work suggests that the lower Cambrian phosphorite Lagerstätten have a great potential to preserve details of the early evolutionary history of sponges.