A bored cup of the Mississippian crinoid Synbathocrinus Phillips

Only the second bored cup of the disparid crinoid Synbathocrinus conicus Phillips is described, infested by a pit of Oichnus paraboloides Bromley. Both bored specimens are from the Mississippian of Salthill Quarry, Clitheroe, Lancashire, UK, although the search for others has extended from northern Europe to North America. The first, described 30 years ago, infested a plate triple-junction of the cup on the presumed up-current side of the crinoid; the new specimen, in contrast, is in the centre of a radial plate, which is inflated as a growth response to infestation. We informally name the producing organism the ‘Salthill bug’. Although unknown, this was a small, unmineralized invertebrate that commonly attached to elevated positions on living crinoids and was likely a filter feeder. It constructed a domicile by boring into the crinoid endoskeleton, and could invade both living and dead crinoid skeletons. On crinoid stems it was commonly gregarious, a habit perhaps favoured by secretions by the first ‘Salthill bug’ to settle, attracting conspecific larvae and similar to the settling patterns of some modern sessile invertebrates. Comparison with Oichnus from Trearne Quarry, Ayrshire, UK, reveals several differences between the pits in infested crinoids at the two sites.