The degradation-resistant organic-walled cell envelopes of acritarchs are the most abundant microfossils in Proterozoic and Cambrian rocks. These microfossils reveal diversity fluctuations that illuminate the nature of the record of primary producers near the Proterozoic/Phanerozoic boundary. Neoproterozoic radiations, some 1000–542 m.y. ago, reached levels comparable to those observed in the Cambrian Period. The microbiotas from rock successions from 13 Cambrian biochrons display significant fluctuations in the total number of microfossil taxa belonging to discrete microfossil assemblages. The assemblages reveal that Cambrian protist assemblages evolved over relatively short time spans, apparently out of low-diversity remnant populations after gradual declines in diversity. The characteristic microbiotas of the terminal Neoproterozoic and the Early, Middle, and Late Cambrian blossomed over relatively narrow time ranges, subsequently collapsing to nearly the initial levels. By virtue of the decreasing time spans involved in the late Vendian, Early, Middle, and Late Cambrian respectively, the tempo of specific turnover appears to have varied considerably. Speciation levels gradually decreased during Early and Middle Cambrian times and during Early Cambrian times were accompanied by rising levels of extinction. This latter feature seems to have reversed during Middle Cambrian times, lasting well into Late Cambrian times. Acritarchs were at the base of the marine trophic chain together with bacteria and other protists that are largely unrepresented in the fossil record. For this reason, the rise of diverse Cambrian protistan plankton must have been essential for early marine metazoan differentiation. Indeed, patterns of total diversity, speciation, and extinction of Cambrian acritarchs clearly mirror those of contemporaneous marine invertebrate faunas at the generic level.

Light microscope and scanning electron microscope observations on new material of unicellular microfossilsDictyosphaera macroreticulataandShuiyousphaeridium macroreticulatum,from the Mesoproterozoic Ruyang Group in China, provide insights into the microorganisms’ biological affinity, life cycle and cellular complexity.Gigantosphaeridium fibratumn. gen. et sp., is described and is one of the largest Mesoproterozoic microfossils recorded. Phenotypic characters of vesicle ornamentation and excystment structures, properties of resistance and cell wall structure inDictyosphaeraandShuiyousphaeridiumare all diagnostic of microalgal cysts. The wide size ranges of the various morphotypes indicate growth phases compatible with the development of reproductive cysts. Conspecific biologically, each morphotype represents an asexual (resting cyst) or sexual (zygotic cyst) stage in the life cycle, respectively. We reconstruct this hypothetical life cycle and infer that the organism demonstrates a reproductive strategy of alternation of heteromorphic generations. Similarly inGigantosphaeridium,a metabolically expensive vesicle with processes suggests its protective role as a zygotic cyst. In combination with all these characters and from the resemblance to extant green algae, we propose the placement of these ancient microorganisms in the stem group of Chloroplastida (Viridiplantae). A cell wall composed of primary and secondary layers inDictyosphaeraandShuiyouisphaeridiumrequired a high cellular complexity for their synthesis and the presence of an endomembrane system and the Golgi apparatus. The plastid was also present, accepting the organism was photosynthetic. The biota reveals a high degree of morphological and cell structural complexity, and provides an insight into ongoing eukaryotic evolution and the development of complex life cycles with sexual reproduction by 1200 Ma.

Biosynthesis of C ₂₉ sterols, enhancing eukaryotic temperature resistance, evolved during the global Neoproterozoic glaciations.

Early Cambrian acritarchs from the Lükati Formation of Estonia are described, based on new material, and their biological affinities reviewed. The phenotypical variants and the inferred function of their vesicles are indicative for recognising zygotic/resting cysts and vegetative cells. They represent the Chlorophytes in the class Prasinophyceae, which reproduce asexually (forming resting phycoma cysts), and Chlorophyceae, which reproduce sexually (zygotic cysts). Some species had a complex life cycle, with alternating vegetative and reproductive generations that differed morphologically. Based on acritarch records from the Baltica palaeocontinent, of which the Lükati Formation assemblage is a part, and the global PhytoPal Taxonomic Database (PhytoPal Project, Leverhulme Trust, Leicester University, 2003–2006), the early Cambrian diversification of phytoplankton is reviewed. The initial Cambrian diversification, and a recovery after the end-Ediacaran extinction, was rapid and exhibited a stepwise pattern of radiation episodes covering short intervals equivalent to biochrons lasting a few million years, estimated from the numerical ages of the strata containing them and the current time scale of the early Cambrian. However, the first appearance datum (FAD) of new species within the biozones/biochrons appears in a succession rather than at one stratigraphical level, indicating true biological speciations. The only genera surviving from the Proterozoic Eon are Leiosphaeridia, Tasmanites, Pterospermella and Pterospermopsimorpha; these are Lazarus genera radiating new species in the early Cambrian. All other Cambrian genera and species are new, morphologically innovative, and much smaller.

Metazoans (multicellular animals) evolved during the Ediacaran Period as shown by the record of their imprints, carbonaceous compressions, trace fossils, and organic bodies and skeletal fossils. Initial evolutionary experiments produced unusual bodies that are poorly understood or conceived of as non-metazoan. It is accepted that sponges, ctenophorans, cnidarians, placozoans, and bilaterians were members of the Ediacaran fauna, many of which have uncertain affinities. The fossilSabellidites cambriensisYanishevsky, 1926, derived from the terminal Ediacaran strata, is the earliest known organically preserved animal that belonged to a newly evolving fauna, which replaced the Ediacara-type metazoans. Morphologically simple soft-bodied tubular fossils, such asS. cambriensis, and biomineralized, as contemporaneousSinotubulitessp., are not easy to recognize phylogenetically because many unrelated organisms developed encasing tubes independently. Therefore, in addition to morphologic information, evidence derived from the microstructure of the organic wall and its biochemistry may be vital to resolving fossil origins and phylogenetic relationships. Here we present morphological, microstructural and biogeochemical studies onS. cambriensisusing various microscopic and spectroscopic techniques, which provide new evidence that supports its siboglinid, annelidan affinity. The late Ediacaran age ofSabelliditesfossil constrains the minimum age of siboglinids and the timing of the divergence of including them annelids by fossil record and this could be tested using molecular clock estimates. The fine microstructure of the organic tube inSabelliditesis multi-layered and has discrete layers composed of differently orientated and perfectly shaped fibers embedded in an amorphous matrix. The highly ordered and specific pattern of fiber alignment (i.e., the texture of organic matter) is similar to that of representatives of the family Siboglinidae. The biogeochemistry of the organic matter that comprised the tube, which was inferred from its properties, composition, and microstructure, is consistent with chitin and proteins as in siboglinids.