Developmental biology of Spiralicellula and the Ediacaran origin of crown metazoans

Weichen Sun, Zongjun Yin*, Liu Pengju, Maoyan Zhu, Philip C J Donoghue*

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

Abstract

The early Ediacaran Weng’an biota (Doushantuo Formation, South China) provides a rare window onto the period of Earth history in which molecular timescales have inferred the initial phase of crown-metazoan diversification. Interpretation of the embryo-like fossils that dominate the biota remains contentious because they are morphologically simple and so difficult to constrain phylogenetically. Spiralicellula from the Weng’an biota is distinguished by spiral internal bodies, allied through development to Megasphaera or Helicoforamina and interpreted variously as metazoans embryos, encysting protists, or chlorophyacean green algae. Here we show, using x-ray microtomography that Spiralicellula has a single-layered outer envelope and no more than 32 internal cells, often preserving a nucleus and yolk granules. There is no correlation between the extent of spiral development and the number of component cells; rather, the spiral developed with each palintomic stage, associated with the cell disaggregation and reorientation. Evidence for envelope thinning and cell loss was observed in all developmental stages, reflecting non-deterministic shedding of gametes or amoebae. The developmental biology of Spiralicellula is similar to Megasphaera and Helicoforamina which otherwise exhibit more rounds of palintomy. We reject a crown-metazoan affinity for Spiralicellula and all other components of the Weng’an biota, diminishing the probability of crown-metazoan diversification before the early Ediacaran.
Original languageEnglish
JournalProceedings of the Royal Society of London B: Biological Sciences
Publication statusAccepted/In press - 12 Apr 2024

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