The origin of plastid in eukaryotes and the minimum age of this event based on microfossil recordare indirectly inferred by recognizing green algae among organically preserved, unicellularmicrofossils other than cyanobacteria. Single primary symbiosis of cyanobacterium establishedalgae and led to divergence of Archaeplastida, including Chlorophyta. The reproductive cystsof extant microalgae share characters derived from their early green algal ancestors. The geneticinheritance and eventually its phenotypic expression are universally shared. Similarly, the enzymesbinding specific compounds in photosynthesis evolved from cyanobacteria and are present in allphotosynthesising organisms since the Archean. Molecular clock analyses suggest that primaryplastid was established by c. 1.5 Ga, whereas by fossil record c. 1.8 Ga (Leiosphaeridia) or 2.1 Ga(Grypania). The fossil record of stem group eukaryotes without assignment to living groups isat 2.1 Ga by carbonaceous compressions (Grypania), however, the recognition as an alga is notexcluded, and it would provide the minimum age of the origin of plastid. Spheroidal microfossils(Leiosphaeridia) with multilayered cell-wall and trilaminar sheath structure that is the algal character ofChlorophyceae are documented throughout the Proterozoic into Cambrian. Because of the presenceof this character and interpreted as algal cysts, some leiosphaerids show record of chlorophyceanspersistently since 1.8 Ga. Microfossils with phycoma-like morphology suggesting prasinophyceanaffinity are known at minimum age of 1.4–1.2 Ga (Pterospermopsimorpha, Pterospermella, Simia,Tasmanites). Microfossils of the Proterozoic to Cambrian ages assessed by body plan, ornamentation,excystment structure, cell wall resistance and ultrastructure, and internal bodies defined by theirown walls, are recognized as algal zygotic cysts and phycomata by comparison with extant greenalgae. Internal bodies are a part of reproductive cysts, resembling those known in Chlorophyta.They are inferred to be endocysts containing zygote, if single, or offspring cells, if multiple, insexual and asexual generations of ancient taxa of the classes Chlorophyceae and Prasinophyceae.Based on the earliest occurrence of microfossils with morphologic characteristic of a zygoticcyst, multilayered cell-wall structure indicative of the primary and secondary wall, and with aninternal body in the Dictyosphaera-Shuiyousphaeridium plexus, the sexual reproduction is evident atc. 1.4-1.2 Ga. It became common in the Neoproterozoic (Cymatiosphaeroides, Trachyhystrichosphaera,Vandalosphaeridium, Tanarium, Asterocapsoides, Ancorosphaeridium, Densisphaera), and the Cambrian(Skiagia, Polygonium). The divergence of Chlorophyta from the lineage of Chloroplastida occurredat the minimum age of 1.8 Ga, and the origin of primary plastid prior to this time.
2014. Vol. 1, 256- p.
4th International Palaeontological Congress, 28 September - 3 October 2014, Mendoza, Argentina