Characterization of the Multicellular Membrane‐Bearing Algae From the Kuanchuanpu Biota (Cambrian: Terreneuvian)

The diversity of small shelly fossils (SSFs) demonstrates that multicellular organisms underwent large-scale radiation at the beginning of the Cambrian, which is highlighted by the coexistence of various metazoans and the occurrence of their embryo fossils. However, little is known about early Cambrian eukaryotic multicellular algae, the primary producers that replaced oxygenic cyanobacteria and played a crucial ecological role in matter cycling and energy dynamics in marine ecosystems. In this study, hundreds of microscopic three-dimensionally preserved multicellular aggregate fossils were obtained from the Kuanchuanpu Formation (Cambrian: Terreneuvian, ca. 535 Ma) in southern Shaanxi, South China, which consisted of several tightly packed multicellular clumps encapsulated in a thin organic membrane. Synchrotron tomography analysis further revealed that the cells of the whole aggregate, although partitioned into different clumps by a gelatinous membrane, were distinctly differentiated into an outer conical cell layer and an inner spherical-cell layer, thus suggesting of a cortex-medulla-like differentiation. These characteristics resemble those of multicellular algae (e.g., Wengania, Gremiphyca, and Thallophyca) from the Weng'an biota (Ediacaran, South China) in morphology, size, and internal cell structure. Furthermore, a potential asexual life cycle for these membranous algae was proposed based on their morphological and structural characteristics.