Muscle scars, mode of life and systematics of Pollicina (Mollusca) from the Ordovician of Baltica

Pollicina is a distinctive, but uncommon, univalved mollusc originally described from the Middle Ordovician (Darriwilian Stage) of the Baltic. The slender, bilaterally symmetrical shell expands slowly and is curved through up to about 90 degrees, but straightens in the latest growth stages. Pollicina corniculum, the type species from the St Petersburg region of Russia is redescribed, as is Pollicina crassitesta, the most common representative from the Tallinn area of Estonia. Muscle scars in P. crassitesta form a continuous circum­apertural band on internal moulds about half way between the apex and the apertural margin. The bilaterally symmetrical shell, orthocline aperture, circum­apertural muscle scar and frequent displacement of the apertural margin, as evidenced by dislocations in growth ornamentation, indicate that Pollicina lived as a limpet, clamped against the substrate. Suggestions that it was an open coiled gastropod lying on the sediment surface are rejected. As with most Ordovician limpet­formed shells, assignment of Pollicina to Tergomya or Gastropoda is equivocal, even controversial, not least on account of the tall shell and muscle band. Despite similarities with the tergomyan Cyrtolites, Pollicina is placed tentatively together with the archinacelloidean gastropods.