ABSTRACT
The histological study on the three dimensionally well-preserved multicellular fossils embedded in the phosphorites of the Neoproterozoic Doushantuo Formation in central Guizhou Province, south China, provides some details about the major evolutionary innovations that occurred prior to Ediacaran radiation and Cambrian “explosion”. A comparative histological study on the fossil forms in different grades of multicellular organization have offered the information that may be conductive to understanding the evolutionary transition and development of multicellularity. The study have also provided the evidence for sexual reproduction modes of the early multicellular organisms. It was possible that, the sessile thallophytes (e.g. Thallophyca), with polarized and directional growing thallus, possibly evolved from a prostrate colonial ancestral form, by changes of habit and growth pattern. The benthic photoautotrophic organisms adapted to a turbulent habitat by increasing stability, this promoted the evolutionary change of habits from prostrate to sessile, and spurred on tissue differentiation and structural complication. The evolutionary transition from prostrate colonial forms to sessile thallophytes had been possibly also promoted by competition for light. The prostrate colonies might expand their surface for light acceptance by upward growth and branching, thus resulted in polarization and differentiation. The study demonstrates that, some of the thallophytes possess the structures that are comparable to female and male reproductive organs. These thallophytes showed both sexuality and multicellularity, they have a certain level of tissue-differentiation and possibly a diploid-dominant sexual life cycle. Development of sexual reproduction in multicellular organisms led to an increase of genotype and phenotype variability, this preludes the remarkable diversification of the metaphyte and metazoans at the beginning of Phanerozoic.
