Sphenothallus from the Balang Formation (Stage 4, Cambrian) of South China and its ecological implications
Sphenothallus Hall, 1847, a genus of benthic Cnidaria characterized by elongate cones and tubes, first occurred in the Cambrian and extended to the Permian. This genus has been reported from the Niutitang Formation (Stage 3, Series 2), Tsinghsutung Formation (Stage 4, Series 2), and the Kaili Formation (Wuliuan Stage, Miaolingian Series) in Guizhou Province. It has also been recently discovered in the Balang Formation (Stage 4, Series 2). Based on the characteristics of these tubular fossil specimens, two species, S. kozaki and S. kozaki?, are described from the Balang Formation.Previously, Sphenothallus kozaki was only found in the Jince Formation of the Czech Republic and the Shipai Formation of Hubei Province of South China. Compared with other species of this genus, S. kozaki possesses a long, narrow, straight, conical tube with a circular cross-section and insignificant thickening marks on the inner wall, indicating that the wall of S. kozaki is thin and weak. No transverse striae on the longitudinal thickening are observed. These tubes are straight throughout their length and have a very small angle of expansion. The longitudinal thickenings are not prominent in these specimens. Sphenothallus kozaki? has smaller expansion angles compared with S. kozaki, and the extension from the base to the upper carapace is approximately parallel. The specimens of S. kozaki? have multiple inclined, irregular, longitudinal ridges inside, presumably resulted from taphonomic bias. We describe the holdfast morphology of Sphenothallus kozaki and S. kozaki? and analyze their attachment strategies, providing useful information on the dynamics of of substrate conditions in the Cambrian. The apices of S. kozaki and S. kozaki? specimens from the Balang Formation are conical, and the basal attachment disks are not preserved. These characteristics suggest that the attachment strategies of these specimens are different from other genera. The result shows that S. kozaki and S. kozaki? rely on self-weight to insert their tips into the soft seafloor, anchoring themselves to the soft substrates. The majority of S. kozaki and S. kozaki? specimens are found in mudstone, confirming this observation. Some species of the Cambrian Sphenothallus evolved discoidal holdfast, whereas most post-Cambrian species of the genus are found with discoidal holdfast, possibly indicating that different species of the genus had different attachment strategies during the Cambrian. This change might be an adaptive response to varying substrate conditions during the Cambrian. Comparative analysis of the depositional environments indicates that both the Jince Formation and Shipai Formation are shallow-water platform facies, while the Balang Formation is a deep-water slope belt facies. The new discovery of S. kozaki in the Balang Formation suggests that this species is well adapted to different water depths and is, therefore, more widely distributed. The discovery also provides new information on the cnidarian, evolution, geographic distribution and species diversity of Sphenothallus during the Stage 4 Age (Series 2, Cambrian), and in understanding the benthic community composition of the Balang Biota.