Evaluating evidence from the Torridonian Supergroup (Scotland, UK) for eukaryotic life on land in the Proterozoic

Abstract

The Stoer, Sleat and Torridon Groups lie unconformably on Palaeoproterozoic Lewisian metamorphic rocks. They contain organic carbon microfossils claimed to be non-marine and to include eukaryotes. Evidence for terrestrial interpretations from each Formation of the Torridonian Supergroup is first considered. The range of sedimentary structures, and the boron content of illite lead to an overall conclusion that, on present evidence, the Torridonian Supergroup was likely entirely non-marine. Evidence for terrestrial life in these rocks comes from microbially induced sedimentary structures (MISS) including wrinkle structures with reticulate and elephant-skin fabrics. Organic remains and microscopic carbonaceous compressions mostly reported from phosphates in the grey shales of the Stoer, Aultbea and Applecross formations are dominated by sphaeromorph acritarchs. The Diabaig phosphatic lagerstätte includes three-dimensional preservation of eukaryotic and prokaryotic organisms, providing remarkable insights into non-marine life around 1 billion years ago. The Precambrian is no longer the fossil-barren 'Lost World' that intrigued Charles Darwin. Over the last several decades there have been significant advances in understanding of the evolving early marine biosphere (see for example Brasier et al., 2015). Questions remain, however, over the extent to which life had colonized terrestrial environments during the Proterozoic and early Palaeozoic. In

Detecting ancient life: Investigating the nature and origin of possible stromatolites and associated calcite from a one billion year old lake

“…An undulating palaeo-land surface of eroded Lewisian Gneiss crops out at Enard Bay ( Figs. 1 and 6; see also Stewart, 2009;Brasier et al, 2017). A mound of gneiss is draped in breccia that is carbonatecemented in places.…”

“…The Stoer Group is dominantly siliciclastic with few carbonate rock horizons (see Stewart, 2002;Parnell et al, 2014;Stueeken et al, 2017;Brasier et al, 2017). Notably there are layered carbonate units in the Stoer Group that have been interpreted as stromatolitic (Upfold, 1984;Krabbendam, 2011;Brasier et al, 2017), and these are the only potential macrofossils visible in the field, but their biogenicity has never been convincingly proven (Stewart, 2002).…”

“…The Stoer Group is dominantly siliciclastic with few carbonate rock horizons (see Stewart, 2002;Parnell et al, 2014;Stueeken et al, 2017;Brasier et al, 2017). Notably there are layered carbonate units in the Stoer Group that have been interpreted as stromatolitic (Upfold, 1984;Krabbendam, 2011;Brasier et al, 2017), and these are the only potential macrofossils visible in the field, but their biogenicity has never been convincingly proven (Stewart, 2002). More autochthonous evidence for Reduction spots with vanadium-rich micas at their centres are found in the Stoer Group rocks immediately below the Stac Fada Member and these have also been interpreted as a possible indicator of contemporaneous (Mesoproterozoic) intra-sediment microbial activity (Spinks et al 2010).…”

Physical and biological functioning in Proterozoic rivers: evidence from the archetypal pre-vegetation alluvium of the Torridon Group, NW Scotland

“…Numerous studies have discussed the holistic sedimentary history of the Torridon Group (e.g., Selley, 1965;Williams, 1966Williams, , 2001Stewart, 1969Stewart, , 1982Nicholson, 1993;McManus and Bajabaa, 1998), its provenance and geochemistry (e.g., Stewart, 1991;Stewart and Donnellan, 1992;Foden 2011, Krabbendam et al, 2017), and its palaeomagnetic (e.g., Stewart and Irving, 1974;Smith et al, 1983) and tectonostratigraphic (Kinnaird et al, 2007) characteristics. More recently, the succession has seen a resurgence of geobiological interest because microfossils extracted from Torridonian mudstones, first described by Teall (1907), have been recognized as Earth's oldest non-marine eukaryotes (Strother et al, 2011;Battison and Brasier, 2012) and sedimentological evidence for early microbial life on land has been described (Prave, 2002;Callow et al, 2011;Brasier et al, 2016; Strother and Wellman, 2016) (see Section 7).…”

“…Shale units, which contain randomly aligned, bifurcating symmetrical ripples and abundant desiccation cracks on certain bedding planes, contain the only potential evidence for microbial life in the Diabaig Formation. Previous reports include: 1) Wrinkle structures with sharp and smooth crests, attributed to microbial mats on ephemerally wetted surfaces (Prave, 2002); 2) Reticulate (and 'elongate' reticulate) fabrics comprising interconnected positive epirelief ridges (often associated with a negative hyporelief counterpart developed on the succeeding bed) (Callow et al, 2011);3) various discoidal fabrics, which may be the product of gas escape following biofilm decay (Callow et al, 2011;Brasier et al, 2016); and 4) positive epirelief sinuous ridges and grooves on bedding planes that have similarity to the microbially-related sedimentary surface texture 'Arumberia' (Callow et al, 2011;Brasier et al, 2016).…”