Palaeoecology of Small Round Holes in Mississippian Crinoids: Salthill Quarry, Clitheroe, UK

The ichnogenus Oichnus Bromley (= small round holes) is uncommon between the Carboniferous and Triassic. Fossil crinoids from Salthill Quarry, Lancashire, England (Mississippian, Chadian) were uncommonly infested by pits assigned to Oichnus paraboloides Bromley. These may be solitary in the disparid Synbathocrinus conicus Phillips or gregarious in the monobathrid Amphoracrinus gilbertsoni (Miller) and certain pluricolumnals. The infesting organisms were not predatory; the pits were certainly protective domiciles and harvesting of plankton may be considered parasitic on the host’s food source. The two contrasting habits of O. paraboloides suggests that these identical pits were produced by two different species with contrasting habits. The organisms that produced solitary O. paraboloides were likely reproducing by shedding their gametes into the water column. In contrast, gregariousness may have been necessary of an organism that reproduced by copulation, a habit similar to extant balanid barnacles.