Affinity, life cycle, and intracellular complexity of organic-walled microfossils from the Mesoproterozoic of Shanxi, China

Abstract

Abstract.-Light microscope and scanning electron microscope observations on new material of unicellular microfossils Dictyosphaera macroreticulata and Shuiyousphaeridium macroreticulatum, from the Mesoproterozoic Ruyang Group in China, provide insights into the microorganisms' biological affinity, life cycle and cellular complexity. Gigantosphaeridium fibratum n. 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 in Dictyosphaera and Shuiyousphaeridium are 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 in Gigantosphaeridium, 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 in Dictyosphaera and Shuiyouisphaeridium required 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.

A Mineralized Alga and Acritarch Dominated Microbiota from the Tully Formation (Givetian) of Pennsylvania, USA

“…However, in these genera reticulation is the result of small ridges or crenulations elevated above the vesicle surface; it is not due to cracks as in the specimen illustrated in Figure 7C. The tiles are also similar to polygonal platelets occurring in the wall structure of the Proterozoic microalga Dictyosphaera macroreticulatus described by Agić et al [61] (Figures 3 and 4). However, dictyosphaerid platelets have straight, linear edges and are therefore much more regularly geometric in their shape, than the tiles illustrated in Figure 7C.…”

“…Figure 8A shows a spheroidal specimen with a large crack in the lower right quadrant and with missing material near the left end of the scale bar. We interpret this rupture as a lateral excystment opening as described by Strother [61]. The spines are conical in shape, spaced about 20 µm apart, and arranged in a regular array.…”

Microfossils from the lower Mesoproterozoic Kaltasy Formation, East European Platform

“…Silicified coastal carbonate facies offer a view of benthic microbes, including abundant and diverse cyanobacteria (e.g., Zhang, 1981;Sergeev et al, 1995Sergeev et al, , 2007Kumar and Srivastava, 1995), while carbonaceous compressions in fine-grained siliciclastic lithologies record both benthic and planktonic microorganisms across a range of lagoonal to basinal environments (e.g., Prasad et al, 2005;Nagovitsin, 2009;Agi et al, 2015;Vorob'eva et al, 2015). In many basins of this age, microfossils thought to be eukaryotic are largely restricted to coastal waters (Javaux et al, 2001), and an explanation for this may lie in the physical nature of mid-Proterozoic oceans.…”

Silicified microbiota from the Paleoproterozoic Dahongyu Formation, Tianjin, China

“…According to microfossil records, Pterospermopsimorpha emerged as early as late Paleoproterozoic (Yan, 1985), but were more abundant in the late Proterozoic. For additional synonymy see Agić et al, 2015.…”

“…Their reticulate sculptures are both very small and simple. Recently, Agić et al (2015) emended Dictyosphaera as a monospecific genus, so we assign the new specimens to D. macroreticulata.…”