Oichnus parabloides 'Salthill Bug'

Oichnus paraboloides Bromley, 1981, is an ichnospecies (trace fossil) consisting of small, shallow, parabolic pits bored into hard substrates. These pits are attributed to sessile or shelter-seeking invertebrates and are classified as domichnia, representing dwelling structures rather than feeding traces.

At Salthill Quarry in Clitheroe, Lancashire (Mississippian, lower Chadian, Lower Carboniferous), these pits are found in the fossil stems (pluricolumnals) of crinoids—marine echinoderms that formed dense carbonate sediments.

Donovan and Tenny (2022) hypothesise that these pits represent the domiciles of a small, soft-bodied invertebrate—possibly a worm sensu lato—with a free-swimming larval stage, which settled upon elevated crinoid stems and bored into the stereom (the crinoid's calcitic tissue) to form snug-fitting pits. It is called the 'Salthill Bug'. There is no soft-bodied preservation to aid identification, by it may have been an annelid within the Polychaeta class known as the bristle-worms. Many modern polychates are filter-feeding pit-boring marine invertebrates with gregarious settling behaviour much like the creators ot Oichnus parabloides here. However a number of theories have been posited from gastropods to crustaceans.

The clustering of pits along one (presumed up-current) side of the stem suggests that the organism oriented itself to intercept waterborne food particles, akin to modern filter feeders. The shallow nature of the pits and absence of soft tissue consumption, coupled with the host's growth response (swelling around the pit), support a commensal or shelter-seeking habit, rather than parasitism.

The close proximity of multiple pits indicates a gregarious settlement, possibly facilitated by chemical cues released by pioneer infesters that attracted conspecific larvae, similar to the settlement mechanisms seen in modern barnacles and other sessile invertebrates.

The “Salthill Bug” appears restricted to echinoderm substrates, particularly crinoid stems, and has not been observed on other fossil types (e.g., corals or brachiopods), hinting at a host or substrate selection preference.