The discovery of widespread agrichnia traces in Devonian black shales of North America: another chapter in the evolving understanding of a “not so anoxic” ancient sea

For earth scientists, to examine how life forms interact with their environment is of key importance to understand evolutionary linkages and adaptations within the biosphere. The Devonian Period experienced many changes of marine and terrestrial ecosystems, most notably the proliferation of vascular plants that accelerated terrestrial weathering and nutrient flux to the oceans and resulted in globally extensive deposition of organic-matter-rich mudstones. In geologic history, such strata appear to be linked to global rise of sea level, greenhouse climate with elevated atmospheric pCO2 , active volcanism, and extinction events. A concerted effort in multiple fields is aimed at understanding the history of oxidation states, modes of organic-matter preservation, and how paleoredox conditions are influenced by local and global drivers (e.g., sea level, tectonics, volcanism). In this study, we document the discovery and unique preservational style of agrichnia (graphoglyptid) traces in Middle-Late Devonian organic-rich shales from three different basins across North America. On slabbed core surfaces the remains of these burrows appear as inconspicuous discontinuous, bedding parallel pyritic streaks, but X-ray computed tomography (CT) scans show bedding parallel pyritized polygonal structures that are interpreted as graphoglyptid burrows. Agrichnia are generally attributed to trace-makers capable of constructing interlaced branching structures to harvest microbial consortia in oxygen-stressed settings, and their discovery in black shales provides new perspectives for interpreting the benthic habitability of organic-rich depositional settings.