A multicellular organism with embedded cell clusters from the Ediacaran Weng'an biota (Doushantuo Formation, South China)

Developmental biology of Helicoforamina reveals holozoan affinity, cryptic diversity, and adaptation to heterogeneous environments in the early Ediacaran Weng'an biota (Doushantuo Formation, South China)

The exceptional fossil preservation of the early Ediacaran Weng'an biota provides a unique window on the interval of Earth history in which animal lineages emerged. It preserves a diversity of similarly ornamented encysted developmental stages previously interpreted as different developmental stages of one taxon. Although Helicoforamina wenganica is distinguished from other forms by a helical groove or canal, it has been interpreted as a developmental stage of cooccurring metazoan, nonmetazoan holozoan, or green algal taxa. Using x-ray microtomography, we show that Helicoforamina developed through one-, four-, and eight-cell stages, to hundreds and thousands of cells. Putative hatchlings are artifacts of incompletely preserved cyst walls. Our results preclude inclusion of Helicoforamina into life cycles assembled from other components of the Weng'an biota but support a holozoan affinity. The similarly ornamented encysted forms shared among the diverse Weng'an biota represent parallel adaptations to the temporally and spatially heterogeneous Ediacaran shallow marine environments.

Oxygen and the Energetic Requirements of the First Multicellular Animals

The appearance of multicellular animals during the Neoproterozoic Era is thought to have coincided with oxygenation of the oceans; however, we know little about the physiological needs of early animals or about the environment they lived in. Approaches using biomarkers, fossils, and phylogenomics have provided some hints of the types of animals that may have been present during the Neoproterozoic, but extant animals are our best modern links to the theoretical ancestors of animals. Neoproterozoic oceans were low energy habitats, with low oxygen concentrations and sparse food availability for the first animals. We examined tolerance of extant ctenophores and sponges-as representatives of extant lineages of the earliest known metazoan groups-to feeding and oxygen use. A review of respiration rates in species across several phyla suggests that suspension feeders in general have a wide range of metabolic rates, but sponges have some of the highest of invertebrates and ctenophores some of the lowest. Our own studies on the metabolism of two groups of deep water sponges show that sponges have different approaches to deal with the cost of filtration and low food availability. We also confirmed that deep water sponges tolerate periods of hypoxia, but at the cost of filtration, indicating that normal feeding is energetically expensive. Predictions of oxygen levels in the Neoproterozoic suggest the last common ancestor of multicellular animals was unlikely to have filtered like modern sponges. Getting enough food at low oxygen would have been a more important driver of the evolution of early body plans.