Early embryogenesis of potential bilaterian animals with polar lobe formation from the Ediacaran Weng’an Biota, South China

Abstract

Exquisite phosphatized eggs and embryos from the Ediacaran Doushantuo Formation (Weng’an biota, ca.580Ma) of Guizhou, southwestern China have greatly expanded our knowledge of the diversity and palaeobiology of early multicellular animals, and contributed to our understanding of the origins of sponge, cnidarian and other potential eumetazoans. However, the key question of whether triploblastic bilaterian animals are present in the Weng’an biota remains controversial. In the present study, scanning electronic microscopy (SEM) and propagation phase contrast synchrotron X-ray microtomography (PPC-SR-μCT) are used to prove the presence of bilaterian embryos in the Doushantuo phosphorite. Based on the well preserved connection neck between the polar lobe (PL) and the egg or blastomere as shown by the high-resolution internal sections conducted by PPC-SR-μCT, three types of PL-forming embryos (calabash-shaped (1-cell stage), trefoil and L-shaped (2-cell stage)) have been confirmed in the Doushantuo phosphorite. These PL-forming embryos are distinct from the PL-forming-like embryos, multicellular algal fossils, and even diagenetic artifacts in morphology and anatomy. In addition, an early developmental sequence for these PL-forming embryos can be inferred for the first time. The results not only provide more convincing evidence for animal affinities among the Doushantuo embryos, but also indicate that polar lobes, which break the symmetrical distribution of morphogenetic determinants in cytoplasm during the early developmental stages of living bilaterian embryos, are a conserved device that occurred already during animal embryogenesis even in the Precambrian.