Developmental biology of Helicoforamina reveals holozoan affinity, cryptic diversity and adaptation to heterogeneous environments

The early Ediacaran Weng’an Biota provides a unique window of exceptional fossil preservation on the interval of Earth History in which molecular clocks estimate the fundamental animal lineages to have arisen. It is composed in large part by a diversity of embryo-like developmental stages that share a similarly ornamented envelope, leading many authors to conclude that they represent different developmental stages of the same organism. Helicoforamina wenganica is distinguished by its helical groove or canal, but it has nevertheless been integrated into competing life cycles comprised of co-occurring embryo-like fossils from the Weng’an Biota, supporting their interpretation as metazoans, non-metazoan holozoans, or green algae. In large part, this diversity of interpretation is possible only because the biology of Helicoforamina is so poorly known. We characterised the structure of more than 300 exceptionally preserved specimens of Helicoforamina using nanofocus and synchrotron radiation X-Ray Tomographic Microscopy, revealing a range of developmental stages, from one cell, through four cell, eight cell, and then hundreds, to hundreds of thousands of component cells. We also elucidate the taphonomy of Helicoforamina, demonstrating that putative hatchlings are perservational artefacts, endocasts of incompletely preserved cyst walls. Our results preclude inclusion of Helicoforamina into lifecycles assembled from other components of the Weng’an Biota because it exhibits equivalent developmental stages. These results highlight the challenge of reconstructing the developmental biology of fossil organisms but also betrays the rich diversity of the Weng’an Biota otherwise obscured by similarly ornamented embryo-like fossils.