Late Ediacaran life on land: desiccated microbial mats and large biofilm streamers.

S Mcmahon,J J Matthews,J Still,A Brasier

doi:10.1098/rspb.2021.1875

Abstract

The Ediacaran period witnessed transformational change across the Earth–life system, but life on land during this interval is poorly understood. Non-marine/transitional Ediacaran sediments preserve a variety of probable microbially induced sedimentary structures and fossil matgrounds, and the ecology, biogeochemistry and sedimentological impacts of the organisms responsible are now ripe for investigation. Here, we report well-preserved fossils from emergent siliciclastic depositional environments in the Ediacaran of Newfoundland, Canada. These include exquisite, mouldically preserved microbial mats with desiccation cracks and flip-overs, abundant Arumberia-type fossils and, most notably, assemblages of centimetre-to-metre-scale, subparallel, branching, overlapping, gently curving ribbon-like features preserved by aluminosilicate and phosphate minerals, with associated filamentous microfossils. We present morphological, petrographic and taphonomic evidence that the ribbons are best interpreted as fossilized current-induced biofilm streamers, the earliest record of an important mode of life (macroscopic streamer formation) for terrestrial microbial ecosystems today. Their presence shows that late Ediacaran terrestrial environments could produce substantial biomass, and supports recent interpretations of Arumberia as a current-influenced microbial mat fossil, which we here suggest existed on a ‘streamer–arumberiamorph spectrum’. Finally, the absence of classic Ediacaran macrobiota from these rocks despite evidently favourable conditions for soft tissue preservation upholds the consensus that those organisms were exclusively marine.

Highlights

As participants in primary production, biological and oxidative weathering, nutrient cycling, river channel stabilization, clay mineralization and pedogenesis, late Precambrian terrestrial ecosystems may have been important agents of global biogeochemical change (e.g. [1,2,3,4])
We report fossil microbial matgrounds, exceptionally well preserved in epirelief on sandstone bedding planes capped by mudstone veneers
The exceptionally preserved flipped-over desiccated microbial mats preserved at Bear Cove Point, as well as the ribbon-like current-induced streamers at Ferryland Head, provide new windows onto late Ediacaran life on land

Summary

Introduction

As participants in primary production, biological and oxidative weathering, nutrient cycling, river channel stabilization, clay mineralization and pedogenesis, late Precambrian terrestrial (tidal, fluvial, lacustrine and soil) ecosystems may have been important agents of global biogeochemical change (e.g. [1,2,3,4]). The studied approximately 10 m thick succession is dominated by the red-brown siltstone-rich facies, which on reddish mudstone horizons preserves diverse microbial mat textures, in some instances cross-cut by sand-filled desiccation cracks (e.g. figure 2b) Many of these textures can be identified with known varieties of Arumberia, a problematic sedimentary fabric associated with Ediacaran matgrounds and characterized by a distinctive, lineated surface texture (see electronic supplementary material, note S2 for discussion of Arumberia nomenclature). The S1 surface could not be sampled, similar material sampled 5 cm higher in the stratigraphy was thin-sectioned at a low angle to bedding, revealing that clay- and phosphate-rich laminae contained filamentous structures that we cautiously interpret as probable fossil microorganisms (figure 4c) These filaments, like the ribbons themselves, appear to have been preserved three-dimensionally by authigenic clay replacement (aluminosilicification), in common with Arumberia at Ferryland Head and at other localities globally Our reinterpretation of Arumberia, extend the fossil record of terrestrial streamer formation into the Precambrian

Discussion

Methods

57. Anderson RP et al 2020 Aluminosilicate haloes