Phosphatized Obruchevella and other microfossils from the Ediacaran-Cambrian transition (Terreneuvian, Maidiping Formation), southern Sichuan Province, China

Abstract

Phosphatization is known for its remarkably detailed preservation of cellular and subcellular microfossils through diagenetic phosphate replacement of the original organic materials. Thin-sections of the Ediacaran-Cambrian transition Maidiping Formation from borehole core Jinshi 103 from southern Sichuan Province, China, exhibit a newly discovered, diverse assemblage of helically coiled microfossils. The four genera recovered include three-dimensional preserved Obruchevella and fragments of cyanobacteria Siphonophycus and Oscillatoriopsis and coccoidal fossils Archaeophycus. The fossil assemblage was dominated by long-ranging hypobradytelic (exceptionally slow evolutionary rate) cyanobacteria. A large number of exquisitely preserved Obruchevella can be identified, including O. minor, O. parva, O. delicata, O. meishucunensis, and O. magna, which exhibit a variety of morphologies, ranging from tangled masses to well-developed helixes, from cocoons with thick mucilaginous sheaths to elongated spring-like forms. Preserved with exceptional fidelity, these Obruchevella species display distinct cross-partitioning and support affinities with modern helical cyanobacteria, such as Arthrospira or Spirulina. Based on traditional taxonomic parameters, such as filament diameter, helix diameter, and winding pattern, two possibilities for the morphological variety of the spiral rings of Obruchevella are proposed. Overall, the Maidiping fossil assemblage provides new paleontological data on the Ediacaran-Cambrian transition and opens an important window into the evolution of early ecosystems at the time of metazoan radiation.