Morphological and Chemical Analyses of Sphenothallus from the Early Cambrian Shuijingtuo Formation, China

Sphenothallus Hall, 1847, is an enigmatic tubular organism that has been interpreted as a plant, worm tube, and most recently, a cnidarian. The changing phylogenetic placement of this organism is due to its simple morphology and the lack of preserved soft parts. Its phosphatic tube is typically found as a pair of branches that have been interpreted as thickenings that supported the rest of the thin tube. Specimens of Sphenothallus have been found in the black shales of the early Cambrian Shuijingtuo Formation in Hubei and Hunan provinces of South China. The Hubei specimens were collected from a more heavily weathered section than the Hunan specimens. Chemical and morphological analyses were performed on these specimens to provide insights into their taphonomy and ontogeny. Environmental scanning electron microprobe analysis revealed shrinkage cracks on the specimens from the weathered locality as well as fine lamellae on specimens from both localities. Energy dispersive elemental analysis revealed that the tube wall is composed of predominantly organic carbon and calcium phosphate, although calcium phosphate in the weathered specimens has been severely leached. To investigate the overall morphology, landmarks that defined the outline of the specimens were identified. A Procrustes analysis was performed and no significant morphological difference between the two sites was found, indicating that they belong to the same taxon. Further, the width of the thickenings did not increase during ontogeny.