Ichnotaxonomy of microbial trace fossils in volcanic glass

Ancient microbial activity in volcanic glass creates micron-sized cavities that can be regarded as trace fossils. These are common in glassy rims of oceanic pillow lavas and volcanic breccias. Morphologically comparable mineralized traces are also found in (meta)-volcanic glasses from ophiolites and Precambrian greenstone belts. Multiple lines of evidence indicate microbial formation of these borings, although the affinity of the trace maker(s) is poorly constrained. Two broad morphological types have been previously recognized and termed ’granular' and ’tubular' bioalteration textures. Here optical microscopy and SEM observations are used to erect two new ichnogenera: Granulohyalichnus igen. nov. and Tubulohyalichnus igen. nov. Five ichnospecies are also defined: Granulohyalichnus vulgaris isp. nov., a granular species; Tubulohyalichnus simplus isp. nov., an unornamented tubular species; Tubulohyalichnus annularis isp. nov., an annulated tubular species; Tubulohyalichnus spiralis isp. nov., a helicoidal tubular species; Tubulohyalichnus stipes isp. nov., a branched tubular species. This systematic taxonomy is advanced to allow reliable comparisons to be made between new and existing reports of these microbial borings. Moreover, the adoption of a taxonomic framework will aid the development of these ichnofossils as palaeoenvironmental indicators and tracers of microbial evolution.