Cell differentiation and germ-soma separation in Ediacaran animal embryo-like fossils

Abstract

Phosphorites of the EdiacaranDoushantuo Formation (600 million yearsold) yield spheroidalmicrofossilswith a palintomic cell cleavage pattern1,2. These fossils have been variously interpreted as sulphuroxidizing bacteria3, unicellular protists4, mesomycetozoean-like holozoans5, green algae akin to Volvox6,7, and blastula embryos of early metazoans1,2,8-10 or bilaterian animals11,12. However, their complete life cycle is unknownand it is uncertain whether they had a cellularly differentiated ontogenetic stage, making it difficult to test their various phylogenetic interpretations. Here we describe new spheroidal fossils from black phosphorites of the Doushantuo Formation that have been overlooked in previous studies.These fossils represent later developmental stages of previouslypublishedblastula-like fossils, and they show evidence for cell differentiation, germ-soma separation, and programmed cell death. Their complexmulticellularity is inconsistent with a phylogenetic affinity with bacteria,unicellular protists, or mesomycetozoean-like holozoans. Available evidence also indicates that the Doushantuo fossils are unlikely crown-group animals or volvocine green algae.Weconclude that an affinity with cellularly differentiated multicellular eukaryotes, including stem-group animals or algae, is likely butmore data are needed to constrain further the exact phylogenetic affinity of the Doushantuo fossils.