Non-metazoan affinity of embryo-like Megasphaera fossils from the Ediacaran Zhenba microfossil assemblage

The Ediacaran Doushantuo Formation of South China yields diverse phosphatised embryo-like fossils, among which the taxon Megasphaera stands out for its astonishing abundance and strong resemblance to extant invertebrate embryos. Our knowledge of Megasphaera has been increasing since their discovery in 1986. However, the controversy surrounding its biological affinity remains unresolved. Previous investigations have almost all been based on specimens from the Weng'an biota; here, however, we show new fossil material from the Zhenba microfossil assemblage, a possibly younger phosphatised fossil deposit from the base of the late Ediacaran Dengying Formation. The new material not only contains common specimens seen in the Weng'an biota but also provides single-celled specimens with a broader size range and Megaclonophycus-stage specimens with remarkably tiny daughter cells comparable to the smallest single-celled specimen in size as well as structure, and hence, allows a more complete reconstruction of the life cycle. Multiple methods analyses reveal a conspicuous volume increase during the vegetative stage, a ceaseless cell division without germ-layer or tissue-organ differentiation, and a probable phenomenon of encystment. These features are inconsistent with the embryogenesis of living animals, and therefore do not support the metazoan-embryo interpretation. In contrast, a more complete life cycle based on new specimens supports an encysting-protist affinity as the interpretation of the Ediacaran Megasphaera fossils.