The earliest occurrence of the ichnogenus Psilonichnus: a new record from the Mississippian of the West of Ireland

The ichnogenus Psilonichnus Fürsich, 1981 is recorded for the first time from the Mississippian (Brigantian regional substage) limestones of the Slievenaglasha Formation from the west coast of County Clare, Ireland. This record extends the known range of Psilonichnus which has previously been recorded from rocks of Jurassic or younger age. The trace maker of the burrows described herein has not been preserved and remains unknown; however, the ichnogenus Psilonichnus is generally ascribed to the burrowing activities of decapod crustaceans such as crabs or shrimp. Decapods from the Paleozoic are poorly known and assigning the traces to a maker must be treated with caution. However, this record indicates that burrowing organisms using a similar life strategy to modern decapods were established by Brigantian times. These examples are assigned to the Glossifungites ichnofacies, as they occur on an omission surface in a shallow marine carbonate firmground. The burrows were subsequently infilled with deposits of possible storm origin. Glacioeustatic sea-level fluctuation related to the onset of the Late Paleozoic Ice Age is a common feature of Carboniferous sedimentary successions at this level, and the Irish (Mississippian) Psilonichnus horizon occurs just below a significant regional sequence boundary at the top of the Slievenaglasha Formation.