New insights on the paleobiology, biostratigraphy and paleogeography of the pre-Sturtian microfossil index taxon Cerebrosphaera

Abstract

Important biological and geological events occurred during the early to middle Neoproterozoic. Among diversifying eukaryotic assemblages, populations of Cerebrosphaera, a distinctive and robust organic-walled vesicular microfossil (acritarch), show restricted stratigraphic distribution in several late Tonian to early Cryogenian worldwide successions. Here, we report the first occurrence of this taxon in Africa, in the Bouenza Subgroup (Republic of the Congo), enlarging its paleogeographic distribution and biostratigraphic significance. We also attempt to determine its biological affinity, using a combined analytical approach on specimens from the Kanpa and Hussar formations, Australia, and from the Svanbergfjellet Formation, Spitsbergen. Morphological and quantitative analyses were performed using light microscopy and scanning electron microscopy. The analyses show fine-scale morphological details and a morphological continuum between the former species Cerebrosphaera ananguae and Cerebrosphaera buickii, confirming their synonymy as proposed by a recently revised taxonomy. These observations also highlighted the presence of a thin external envelope, previously reported but formerly described and illustrated here for the first time. The characteristics of this envelope, the large diameter range of the vesicles, and the absence of excystment structure, suggest that Cerebrosphaera was a metabolically active growing cell. Ultrastructural analyses performed with TEM revealed a complex multilayered wall ultrastructure. The molecular composition and thermal maturity of the organic walls were estimated using Infrared and Raman microspectroscopies. The wall of Cerebrosphaera has a highly aromatic composition with short/highly branched aliphatic chains. The complex morphology and wall ultrastructure, combined with the large size (not a criterion by itself) of Cerebrosphaera, confirm its eukaryotic nature. Comparison with strikingly similar modern analogues permits to suggest a possible affinity to stem metazoan eggs, based on morphology and ultrastructure, but the chemical composition is unlike known biopolymers. This hypothesis is also consistent with estimates from molecular clocks. If confirmed, our results would provide an older direct evidence for stem metazoans than the Cryogenian biomarker and Ediacaran body fossil records. Our study reveals that Cerebrosphaera populations are important for Neoproterozoic biostratigraphy, but also participated to the diversification of eukaryotes in worldwide connected oceans.