Phosphatized animal embryos from the Neoproterozoic Doushantuo Formation at Weng'an, Guizhou, South China

Abstract

Phosphorites of the uppermost Neoproterozoic Doushantuo Formation (ca. 570 ± 20 Ma) exposed at Weng'an, South China, contain globular microfossils characterized by distinctively sculpted outer coverings and precise patterns of cell division. Forms assigned to the genera Megasphaera and Parapandorina were originally interpreted as problematica and volvocacean green algae, respectively, but newly discovered populations support their reinterpretation as the eggs and embryos of early animals. The fossils' large size (about half a millimeter in diameter), persistent enveloping membrane, precise cleavage pattern, and inferred physiology are fundamentally different from those of volvocaceans or other algae, but are expected features of animal embryos preserved in early stages of cleavage. In this light, Megasphaera is interpreted as an egg enveloped within its egg case, and Parapandorina represents equally and totally cleaving stereoblastulas. Despite their exquisite preservation, the phylogenetic placement of Megasphaera and Parapandorina cannot be resolved with confidence, due largely to the absence of recognizable adult morphologies to which they might be linked. Individual characters of Megasphaera and Parapandorina can be found in eggs and embryos of extant sponges, cnidarians, and bilaterians.Three other distinctive globular forms, Megaclonophycus onustus, Caveasphaera costata n. gen. and sp., and Spiralicellula? bulbifera? occur in the same deposit. Interpretations of these fossils are more problematic, although they may also be animal embryos.