Supplementary material from "Developmental biology of the Ediacaran Megaclonophycus from the Weng’an Biota"

The Ediacaran Weng’an of 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 a 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 10201, with counts clustering around 2048 and 4096, consistent with binary division. However, the specimens have asynchronous binary cell division with cell sizes within a specimen varying by two-to threefold. Megaclonophycus increases in volume during division, showing no evidence of reductive cleavage or gastrulation, both being characteristics of metazoan development. Parapandorina, Megaclonophycus and Megasphaera all exhibit a similar sequence of development, size, and diagenetic impact on preservation indicating a developmental sequence of early to later stages of the same organism. These findings refute the crown group metazoan affinity suggested for this taxon, therefore molecular clock estimates for the origin of animals must rely on fossil calibration from sources other than Megaclonophycus but possibly still from the Weng’an Biota.