Evolutionary implications of an exceptionally preserved Carboniferous microboring assemblage in the Buckhorn Asphalt lagerstätte (Oklahoma, USA)

In the Buckhorn Asphalt deposit, exceptional preservation of mollusc shells with original aragonitic mineralogy, owing to an early impregnation with migrating hydrocarbons, provides a ‘preservational window’ for studying a Late Palaeozoic microboring assemblage. The evaluation of thin-sections, bioclast surface features, and SEM analysis of epoxy resin casts reveals a total of 18 known ichnospecies and the new ichnotaxon Aurimorpha varia – reflecting the most diverse Palaeozoic microboring assemblage known to date. The ichno-inventory is dominated by the cyanobacterial traces Eurygonum nodosum, Scolecia filosa, and Fascichnus dactylus, complemented by the common Fascichnus frutex, Planobola macrogota and Cavernula coccidia, and the chlorophyte traces Cavernula pediculata and Rhopalia catenata. Presumable borings of heterotrophs are rare albeit diverse constituents of the microboring assemblage. The ichnocoenosis composition indicates a palaeoenvironment in the shallow-euphotic zone and is in many respects ‘modern’. The fact that 17 out of the 19 recorded ichnospecies are also known to neoichnology in closely similar ichnocoenoses of today’s shallow-euphotic seas underlines the pronounced longevity of microendolithic taxa and promotes their value as deep-time palaeoenvironmental indicator. Two thirds of the recorded ichnotaxa were reported for the first time from Carboniferous strata, thereby extending the known stratigraphic range of Cavernula pediculata, Cavernula coccidia, Rhopalia catenata, Rhopalia spinosa, Scolecia serrata, Polyactina fastigata, Saccomorpha terminalis, and Aurimorpha varia igen. n., isp. n. back into the Palaeozoic. This shows that at least 21 ichnospecies, including a high number of modern ichnotaxa, were established as early as the late Palaeozoic and reflects an increasing abundance and potential competition by chlorophyte algae and heterotroph microborers breaking the former strong dominance of endolithic cyanobacteria in microborer communities. The curve of Phanerozoic ichnospecies diversity suggests that the evolution of microbioerosion agents is to a certain degree parallel to that of the macroborers with main radiations in the Ordovician and Devonian. However, according to the current state of knowledge, the end-Permian mass extinction event seemingly had little or no impact on the diversity and evolution of microborers. Thus, diversity patterns of micorborings differ from those of marine invertebrates, most of which have a minimum in standing diversity at the Permian / Triassic boundary and a general turn-over from typical Palaeozoic to modern evolutionary faunas.