Tube growth patterns and microbial mat-related lifestyles in the Ediacaran fossil Cloudina, Gaojiashan Lagerstätte, South China

Abstract

Cloudina are important Ediacaran index fossils, as they have a consistent terminal Neoproterozoic age and a worldwide distribution. The morphology of Cloudina has been reconstructed primarily on the basis of three-dimensionally preserved Chinese and Spanish specimens, but questions remain surrounding their phylogenetic placement. Understanding Cloudina paleoecology is an important aspect for the interpretation of these enigmatic organisms. Although their paleoecological dynamics remain unresolved, a possible “mat sticker” life mode has found support from recent taphonomic investigation of Paraguayan Cloudina specimens. Here, from analysis of fossil material from the upper part of the Ediacaran Dengying Formation (Gaojiashan and Lijiagou sections, Ningqiang area, southern Shaanxi Province, South China), we document complex tube growth patterns of Cloudina in addition to a close association of these fossils with preserved microbially induced sedimentary structures (specifically matgrounds). Sedimentological and taphonomic data of Cloudina-bearing carbonate sequences at the Gaojiashan section show a relationship between Cloudina and microbial mats, and further document erectly preserved Cloudina tubes. In conjunction with observations from the Paraguay material, the Dengying fossils described here provide supportive evidence for the paleoecological interpretation of Cloudina as inhabitants of a microbially bound carbonate substrate. The apical-most part of the tube (or cone) may have served as an attachment apparatus, situated within or beneath a microbial mat. Three-dimensionally phosphatized tubes of Cloudina from the Lijiagou section reveal that single tubes often have multiple growth orientations, which is demonstrated to have resulted from the curved growth of the tube walls (both cones and funnels). As these organisms exhibited a sessile, epibenthic life mode, the observed plasticity in growth direction and orientation may have been a competition-driven adaptation to ensure wider access to available food sources and potentially greater feeding efficiency. Although Cloudina have been found to co-occur in some deposits with biostromes and bioherms (e.g., stromatolites and thrombolites), sedimentological and taphonomic features of the Ningqiang Cloudina suggest that they inhabited a stratified carbonate substrate exhibiting matground features, but where biostromes and bioherms were absent.