ANIMAL EMBRYOS FROM THE TERMINAL NEOPROTEROZOIC DOUSHANTUO FORMATION: HOW DID THEY HATCH?

Globular microfossils interpreted as animal blastula embryos have been reported from phosphorites of the Neoproterozoic Doushantuo Formation (ca. 600 Ma) at Weng'an, South China. However, the later ontogenetic stages and adult morphology of the Doushantuo animal have been elusive. We report a series of new globular microfossils from the Doushantuo phosphorites at Weng'an. The fossils provide insights into the hatching process and adult morphology of the Doushantuo animal. It appears that the post-blastula and pre-hatching embryo remodeled itself into a spiral animal within the spherical embryo case; with no exceptions, the spiral embryo has three clockwise spires. In the same time, a spiral groove that matches the spiral embryo developed on the embryo case. The spiral groove then gradually developed into a spiral slit, along which the embryo case might have opened to release the spiral embryo. After hatching, the spiral organism began to uncoil slightly. The complete developmental sequence after hatching is uncertain, but it is possible that the hatched animal might have devolved into a tubular organism. If so, the new embryo fossils may represent a developmental transition that links the blastula embryos and tabulated tubular fossils that have been previously described from the Doushantuo phosphorites at Weng'an. This interpretation indicates that the Doushantuo animal was a direct developer, an interpretation that is consistent with the size of the Doushantuo embryos.