Peculiar bundles and a knot of thin filaments in microbial mats from the Lower Devonian Rhynie and Windyfield cherts of Scotland

Microbial mats are common as fossils in the aquatic facies of the Lower Devonian Rhynie and Windyfield cherts. The framework-builder of these structures was most often a cyanobacterium described as Croftalania venusta; however, little is known about the other mat components. This study presents a new feature of the microbial mats that occurs in the form of variously sized bundles of entwined thin filaments; some of the bundles are sheathed, others are surrounded by a wall. A knot (or tangle) of filaments from which extend several bundles is also described. Filament size (2-4 mu m wide) and morphology strongly suggest that both the bundles and the knot are manifestations of C. venusta. Simple, linear bundles correspond in basic morphology to bundles seen in the extant cyanobacterium Microcoleus (Oscillatoriales). Larger bundles that are walled and the knot likely represent parts of other organisms, such as charophyte algae or fungi, that had been secondarily invaded by C. venusta, which continued to function and proliferated inside the host. The triggers for bundle formation in C. venusta remain unknown. This discovery is nevertheless important because it supports the perception that microbial mats in the Rhynie paleoecosystem were complex communities. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

Microbial mats from the aquatic facies of the Lower Devonian Rhynie and Windyfield cherts contain new features such as entwined filament bundles and knot structures, suggesting potential internal invasion by different organisms or continued activity of C. venusta. This discovery supports the idea that microbial mats were complex communities in the Rhynie paleoecosystem.