SCANNING ELECTRON IMAGING OF SHELL CONSTRUCTION IN THE PROBLEMATIC TUBULAR FOSSIL SPHENOTHALLUS FROM THE EARLY CAMBRIAN (SERIES 2) OF SOUTH CHINA

The origin of biomineralized skeletons in the late Neoproterozoic may have been an ecological response to an escalating arms race between predator and prey, which likely contributed to the radiation of modern animals. However, many early Cambrian small shelly fossils remain problematic, and their significance in understanding the selective pressures exerted on epibenthic organisms during the Cambrian explosion is not fully appreciated. The tubular Sphenothallus (early Cambrian–Devonian)—previously compared to algae, worms, and cnidarians—represents one such taxon. Sphenothallus consists of a lamellar, phosphatic tube with a pair of oppositely-situated longitudinal thickenings separated by weakly mineralized (if not entirely organic) membranes that are rarely preserved. To better understand the shell structure, ecology, and affinities of Sphenothallus, we collected specimens from the shales of the early Cambrian (Series 2) Shuijingtou Formation of south China for imaging using light and scanning electron microscopy (SEM). The material preserved both skeletal and organic features.

In some outcrops, the apatite comprising Sphenothallus shells has been dissolved away, leaving fossils preserved as frameworks of carbonaceous material and authigenic minerals. SEM imaging reveals that, when mineralized skeletons are preserved, their longitudinal thickenings consist of phosphatic lamellae surrounded by an external organic layer. In rare specimens, the membranes separating the longitudinal thickenings also possess evidence of weak biomineralization, following a growth pattern consistent with more strongly mineralized tissue. This study highlights the importance of analyzing taphonomic modification of skeletal fossils in order to understand the extent of original biomineralization and its ecological implications.