Abstract
AbstractEarly-middle Ediacaran organic-walled microfossils from the Doushantuo Formation studied in several sections in the Yangtze Gorges area, South China, show ornamented cyst-like vesicles of very high diversity. These microfossils are diagenetically permineralized and observed in petrographic thin-sections of chert nodules. Exquisitely preserved specimens belonging to seven species of Appendisphaera, Mengeosphaera, Tanarium, Urasphaera and Tianzhushania contain either single or multiple spheroidal internal bodies inside the vesicles. These structures indicate reproductive stages, endocyst and dividing cells, respectively, and are preserved at early to late ontogenetic stages in the same taxa. This new evidence supports the algal affiliations for the studied taxa and refutes previous suggestions of Tianzhushania being animal embryo or holozoan. The first record of a late developmental stage of a completely preserved specimen of T. spinosa observed in thin-section demonstrates the interior of vesicles with clusters of identical cells but without any cavity that is diagnostic for recognizing algal cysts vs animal diapause cysts. Various lines of evidence to infer biological affinities of these microfossils – morphology, reproductive characters, spatial arrangement of cells, and biochemical properties of the vesicle wall – are collectively characteristic of algal clades. Recognizing the biological affinities of these microfossils is key to understanding whether animals capable of producing such morphologically complex diapause cysts had an early Ediacaran fossil record (633–610 Ma), or the microfossils were non-animal holozoans or algae as argued herein for Tianzhushania spinosa and other studied microfossils.
Highlights
Evolutionary innovations on an unprecedented scale are observed in Ediacaran biotas in regard to morphological disparity and ecological adaptations
We provide examples of extant algal taxa that are phenotypically analogous to these microfossils and have the same biochemical resistance properties to decay and we analyse various lines of evidence in the studied species to support an algal biological affinity and to question previous interpretations
The holozoan interpretation of microfossils attributed to the Tianzhushania life cycle was based on the elegant documentation of subcellular morphology and bodies potentially identifying nuclei in Megasphaera, abundant cells preserved inside peanutshaped specimens, and helical four cells in Spiralicellula by scanning electron microscope (SEM) and synchrotron X-ray microtomography (Huldtgren et al 2011; Donoghue et al 2015; Cunningham et al 2017; see Hagadorn et al 2006)
Summary
Evolutionary innovations on an unprecedented scale are observed in Ediacaran biotas in regard to morphological disparity and ecological adaptations. Ediacaran microfossils were considered to be largely phytoplanktonic and algal in affinity This interpretation of organically preserved microfossils as representing algal cysts was based on their morphological comparisons with extant taxa, vesicle wall biochemical resistance and was supported by case studies of the wall ultrastructure in certain species (Zang & Walter, 1989, 1992; Arouri et al 1999, 2000; Grey, 2005; Moczydłowska, 2005, 2016; Willman & Moczydłowska, 2007; Moczydłowska & Willman, 2009; Moczydłowska et al 2011). The illustrated specimens are designated by the prefix IGCAGS followed by the sample number and specimen position by England Finder graticules in thinsection orientated with its label to the left side
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