Doushantuo embryos preserved inside diapause egg cysts

Microfossils: mixed message The controversy generated by the identification of phosphatized microfossils from the Doushantuo Formation in southern China as the embryos of early animals continues. A recent suggestion, that the roughly 600-million-year-old microfossils may be giant sulphur-oxidizing bacteria similar to the extant Thiomargarita , has been put to the test and found wanting — at least for some of the specimens. New observations reveal that some of the embryo-like fossils are found within elaborate organic vesicles called acritarchs, consistent with a reproductive strategy known as embryonic diapause, in which early embryos enter a dormant 'egg cyst' state. This suggests that these organisms were eukaryotes. Further arguments against the 'giant bacteria' explanation are presented online in a Brief Communications Arising contribution, which includes the suggestion that bacteria-like microfossils may have been stripped of their outer layers by the vagaries of fossil formation and preservation. Phosphatized microfossils in the Ediacaran (635–542 Myr ago) Doushantuo Formation, south China, have been interpreted as the embryos of early animals 1 , 2 , 3 , 4 . Despite experimental demonstration that embryos can be preserved 5 , microstructural evidence that the Doushantuo remains are embryonic 6 and an unambiguous record of fossil embryos in Lower Cambrian rocks 7 , questions about the phylogenetic relationships of these fossils remain. Most recently, some researchers have proposed 8 that Doushantuo microfossils may be giant sulphur-oxidizing bacteria comparable to extant Thiomargarita sp. Here we report new observations that provide a test of the bacterial hypothesis. The discovery of embryo-like Doushantuo fossils inside large, highly ornamented organic vesicles (acritarchs) indicates that these organisms were eukaryotic, and most probably early cleavage stage embryos preserved within diapause egg cysts. Large acanthomorphic microfossils of the type observed to contain fossil embryos first appear in rocks just above a 632.5 ± 0.5-Myr-old ash bed 9 , suggesting that at least stem-group animals 6 inhabited shallow seas in the immediate aftermath of global Neoproterozoic glaciation.