New occurrences of Sphenothallus in the lower Cambrian of South China: Implications for its affinities and taphonomic demineralization of shelly fossils

Abstract

Sphenothallus , a tubular fossil that occurs in Paleozoic strata around the world, may be related to cnidarians or ‘worms,’ but its affinities remain controversial due to preservational variations among localities. Here, to test hypotheses regarding Sphenothallus taphonomy and affinities, we investigate the preservation of new fossils from the lower Cambrian Shuijingtuo Formation at Heziao and Jijiapo (Hubei Province) and the equivalent Niutitang Formation at Siduping (Hunan Province) in South China. Morphology and allometry suggest that the fossils represent a single species, and their biostratinomy and authigenic mineralization indicate that they underwent similar pre-burial and early diagenetic processes, particularly transport and focused degradation via microbial sulfate reduction. However, the compositions and microstructures of the tests vary among localities. The Heziao tests consist of lamellar phosphatic material covered interiorly and exteriorly by layers of carbonaceous material. In contrast, the Siduping tests consist of non-lamellar phosphatic material associated with carbonaceous material, and the Jijiapo tests consist entirely of carbonaceous material. Based on these observations and experimental taphonomic data—which show that hydrochloric acid treatment of Heziao samples produces fossils that compositionally and microstructurally resemble the Jijiapo specimens—we propose that the new tests constitute a taphonomic continuum between shelly and carbonaceous preservation. This taphonomic model suggests that the preservation of shells as carbonaceous fossils results from kerogenization of the organic matrixes and subsequent or concomitant demineralization of the biomineralized microstructures of the shells. Hence, variations in Sphenothallus test composition and microstructure among localities may reflect varying extents of test demineralization, and the carbonaceous layers in Heziao specimens may have formed via demineralization of the tests' exterior and interior phosphatic lamellae. Our reconstruction based on this model indicates that pristine Sphenothallus tests consist of exteriorly sculptured and interiorly unsculptured organophosphatic lamellae. Thus, Sphenothallus microstructurally resembles and may be related to conulariids, and potentially, cnidarians. • Sphenothallus fossils in Shuijingtuo and Niutitang formations of Hubei and Hunan. • We provide data on their taphonomy, allometry, compositions, and microstructures. • Data illuminate Sphenothallus affinities and taphonomic diversity of Cambrian shells. • Sphenothallus microstructurally resembles conulariids and may be a cnidarian. • Data support taphonomic model for preservation of shells as carbonaceous fossils.