Palaeoecological insights from a large Oichnus Bromley in a Carboniferous crinoid stem

Small round holes or pits, Oichnus Bromley, are rare trace fossils in Mississippian shells, but are a locally common feature of crinoid pluricolumnals in some sites of the British Isles. Numerous examples have been found from mudrocks in the Brigantian (Mississippian) Blackhall Limestone, Lower Limestone Formation, at Trearne Quarry, near Beith, north Ayrshire. Until now, all have been assigned to Oichnus simplex Bromley. These trace fossils are typically associated with growth deformities of the pluricolumnals, which are commonly swollen and more rarely grow a lip around the pit. A new specimen, Oichnus isp. cf. O. paraboloides Bromley, adds to the ichnological diversity of Trearne Quarry. It is likely the spoor of a different producer to O. simplex . Oichnus isp. cf. O. paraboloides infests the stem of Rhabdocrinus scotocarbonarius (Wright) and is an elongate, rounded, slot-like depression with sloping sides. It infested the live crinoid as established by the swollen growth reaction of the stem.