The rise of complexity : Macroscopic Branching Organic Siliceous Structures from the c. 2.4 Ga Turee Creek Group, Western Australia

Abstract

The Great Oxidation Event (GOE: c. 2.45-2.3 Ga) was accompanied by a series of global glaciations that placed significant environmental pressure on extant microbial life and, perhaps, led to the evolution of eukaryotes. However, the direct impact of the GOE on life remains unclear due to a lack of well-preserved, fossiliferous strata from this time.

Here, a novel fossil form is described from the well-preserved shallow water part of a microbialite reef complex that lies between the 1st and 2nd of three global glaciations of the GOE, within the c. 2.4 Ga Turee Creek Group, Western Australia. Referred to as branching organic siliceous structures (BOSS), these novel fossils are organic-rich structures, up to 3 cm high, that are attached to the sides and tops of microbialites in up to 4 separate beds that have intercolumn sediment composed of phosphatic peloidal grainstone to pebble conglomerate. The unique occurrence of BOSS with phosphorite shows that they thrived in an oxygenated, nutrient-rich environment.

BOSS display a range of morphology, from simple buds, through cylindrical or simple branching structures, to taller multi-level branching structures. The best-preserved BOSS are composed of kerogen-rich micrite with a faint texture of fine fibres that show a weakly radiating texture from the base to the outer, upper margins of BOSS. The interior parts of BOSS are commonly partially to wholly replaced by fine-grained silica (chalcedony to microquartz), which formed early in diagenesis, or possibly even during growth. Silica replacement of BOSS commenced as point-source growth of individual scattered spherulites and progressed to whole-scale replacement of the interior. BOSS interiors also contain two forms of pyrite: as necklaces of fine aggregates distributed in clusters; and as euhedral, sometimes zoned, grains in the exterior rinds of BOSS with silicified core zones. Sulfur isotopic data from the fine pyrite necklaces provides evidence of a sulfate reducing microbial community within BOSS.

Petrographic analysis and morphological comparison show that BOSS are biological and dissimilar to any known type of abiological structure or microbialite. Rather, BOSS are most similar to eukaryotic algal-grade organisms based on their size, morphology and carbon isotope data. BOSS potentially illustrate that eukaryotes evolved up to 750 Ma earlier than previously recognised, coinciding with significant environmental change during the GOE.