Abstract:
Oxygenation of the ocean is presumed to be an important factor stimulating the evolution of multicellular animals. The appearance of the Ediacaran-type biota (ca 575 Ma) was assigned to the aftermath of the Gaskiers glaciation (ca 580 Ma), when substantial oceanic oxygenation is believed to have started. However, several lines of evidence reveal that at least sponges evolved before this oxygenation. For understanding the first stage of animal evolution, we propose the hypothesis that Dissolved Organic Carbon (DOC) Stimulated the evolution for Animal Multicellularity (DOXAM). Recent geochemical studies of the Ediacaran sedimentary sequences have indicated that a substantial DOC mass was developed in the stratified ocean after the Marinoan glaciation (655-635 Ma), and this was supported by the inorganic and organic carbon isotope profiles of the Doushantuo Formation in South China. The DOC mass was an oxygen consumer in the water column; however, it could have provided a food source for filter-feeding animals such as sponges and cnidarians, and established a primitive food-web. Such an ecological structure is recognized in modern deep-sea coral mounds. Results from the integrated ocean drilling program (IODP) Expedition 307 for a mound in northeastern Atlantic suggested that organic carbon suspended around the density boundary in the water column is the key feature to feed the heterotrophic deep-sea coral community. Our hypothesis is consistent with the fact that the two most primitive animal phyla (Porifera and Cnidaria) are filter feeders. The evolution of filter feeding ecosystems removed the DOC mass and may have contributed to ocean oxygenation in the terminal Neoproterozoic when animal evolution passed into the second stage, with the appearance of bilaterians. Abstract Copyright (2011), Blackwell Publishing Asia Pty Ltd.