Developmental biology of the Ediacaran <i>Megaclonophycus</i> from the Weng’an Biota

The Ediacaran Weng'an Biota of South China yields embryo-like microfossils preserved with sub-cellular fidelity, previously interpreted as the oldest evidence of animals. Megasphaera dominates the assemblage and preserves the earliest stages of binary reductive division. It is assumed to develop into Megaclonophycus, which is composed of hundreds to thousands of cells; however, this developmental link has not been tested. We used synchrotron and computed tomography to characterize hundreds of specimens of Megaclonophycus and determine cell division patterns. Specimen cell counts range from 14 to 10 201, with counts clustering around 2048 and 4096, consistent with binary reductive cell division. However, the specimens have asynchronous binary cell division with cell sizes within a specimen varying by two- to threefold. The volume of Megaclonophycus is variable during development, showing no evidence of gastrulation, characteristic of metazoan development. Megaclonophycus and Megasphaera exhibit a similar sequence of development, size and taphonomy indicating a developmental sequence of early to later stages of the same organism. These findings are incompatible with the crown group metazoan affinity suggested for this taxon, and so molecular clock estimates for the origin of animals must rely on fossil calibration from sources other than Megaclonophycus.