Morphology and ethology of the Late Devonian trace fossilRhizocoralliumfrom the Xichuan section of Central China

Abstract

The trace fossil Rhizocorallium is abundant in the shallow-marine settings of the Upper Devonian Wangguangou Formation in Xichuan, Henan Province, northern margin of the South China block. Based on its morphological characteristics, the studied Rhizocorallium is attributed to Rhizocorallium commune var. auriforme. There is a significant compositional difference between the fillings of the marginal tube and the spreite. The fillings in the marginal tube are similar to the surrounding rock, with abundant bioclasts. The fillings in the spreite are relatively fine-grained and obviously affected by biological activities. Four types of pyrite framboids (pyritohedral, octahedron, cube and spherical) have been found within the Rhizocorallium. Moreover, the ratio of pyrite framboids diameter (D) to submicron crystal diameter (d) is <10, suggesting that they were formed by microbial activities. Relatively large pyrite framboids with smooth surface mainly occur in the marginal tube, while small pyrite framboids with sheath are abundant in the spreite. Energy spectrometer (EDS) analysis showed that the S/Fe ratio in the pyrite increased with decreasing crystal diameter, indicating different redox conditions in the marginal tube and the spreite. These features indicate that Rhizocorallium from the Xichuan section is not simply a foraging trace, but is probably a multi-functional, complex burrow system utilized for living, caching and farming microorganisms.