Evolution and ecology of Baltic late Ordovician univalved molluscs

Abstract: Univalved molluscs (Gastropoda and Tergomya) are common fossils in the Palaeozoic succession across Baltoscandia, but most of the more embracive descriptive works on this group appeared before or near the turn of the 19th century. The study presented here aims at revision and description of some Baltoscandian late Ordovician/early Silurian univalved molluscs with emphasis on their shell morphology. Variation of shell characters and palaeoecology are investigated from the standpoint of function. A total of 51 species are documented, of which 32 are new. While considerable diversity is evident individual species are usually represented by few specimens. The main focus is on the isostrophically coiled forms, the Tergomya and the bellerophontoid Gastropoda. Isostrophic shells coil in one plane normal to the axis of coiling, and are bilaterallysymmetrical about the plane in which they coil. Isostrophism is characteristic of Tergomya, but uncommon in the Gastropoda where the shell is usually helically coiled (anisostrophic).Case studies of shell repair from failed predation in the well-preserved gastropod fauna of the Ordovician carbonate mounds of Central Sweden are described. Shell repair is displayed in 6 genera of various sizes and morphologies that appear structurally weak by modern standards. The frequency of shell repair was about 7%. The identity of the predator(s) is unknown, but cephalopods are likely culprits. Shell repair was also found in the trochiform Semizona bella gen. et sp. nov. from the Boda Limestone, a large species with a thick shell and sub-sutural knobs on the upper surface. The aperture is tangential and rounded which made it possible for the animal to clamp the shell against the substrate. Semizona is compared with the early Silurian genus Kiaeromphalus, the aperture in the latter is radial which suggests that the large shell was lying flat on the surface with the aperture perpendicular to the substrate.Bucaniids may be considered to be a well-defined group of bellerophontoidean gastropods characterized by their high number of barely contiguous whorls, moderately narrow to wide umbilici, apertural margins tending to flare, a relatively short slit open at the margin, and frequent revolving ornamentation. Baltoscandian members of the Subfamily Bucaniinae show unusual features, such as the development of a carina in species of Paramegalomphala gen. nov., Bucania and Phragmolites and the expression and combination of crenulated growth lines and spiral ornamentation in the two latter genera. Development of a single median exhalant opening in the last whorl (trema) in species attributed to Bucania, further demonstrates wide development of a character once considered diagnostic of the genus Salpingostoma.A high number of whorls, thick shell and accompanying relatively narrow umbilici characterize species of Tropidodiscus. Lateral lips cover the umbilici by infolding in Cymbularia. The presence of umbilico-lateral sinuses in Sinuitina? planilata sp. nov. allows interpretation of this species as an exogastric tergomyan.