Not dasycladalean alga, but an Odyssey of the earliest Phanerozoic animal reef-builders

Abstract

The compacted macrofossil Protomelission? sp. from the early Cambrian Xiaoshiba Lagerstaette was recently ascribed to early dasycladalean green algae and used to disprove the bryozoan affinity of coeval phosphatized microfossils, which made the puzzling question whether the bryozoans originated in early Cambrian pending again. Our new analyses of multiple specimens which are conspecific with Protomelission? from the Chengjiang Lagerstaette indicate that they are not dasycladaleans but one of the three groups of archaeocyath-like sponges that atypically inhabited siliciclastic substrates. All the archaeocyath-like fossils share the same preservation mode and exhibit archaeocyath-type external skeletal features. Particularly, the Protomellision? -like fossils preserve structures indicative of archaeocyath aquiferous system and ontogeny. They represent the first recognized one-walled archaeocyath sponges in South China and evidence the niche expansion of archaeocyaths on their way of global radiation from Siberia, 518 million years ago. The origin of the bryozoans remains a mystery.