Abstract
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 slo ...
Highlights
Vermeij (2017) calculated that this shell morphology has arisen independently more than 50 times in the evolutionary history of gastropods, and to this may be added a morass of monoplacophoran and poorly known bilaterally symmetrical shells in the early–middle Palaeozoic (Runnegar & Jell 1976; Peel 1991a, 1991b; Geyer 1994; Parkhaev 2008, 2017; Bouchet et al 2017)
Despite a host of modern phylogenetic and developmental studies (Koufopanou et al 1999; Wanninger et al 1999, 2000; Lindberg 2009; Kristof et al 2016), elucidation of their palaeontological record is frustrated by the morphological simplicity of their bilaterally symmetrical shell and muscle scar, the only characters that are preserved in most fossil scenarios
This paper examines a distinctive but poorly known Ordovician univalve which lies in limbo between gastropods and monoplacophoran molluscs
Summary
Univalved molluscs with capshaped or limpetformed shells have existed throughout the Phanerozoic. Vermeij (2017) calculated that this shell morphology has arisen independently more than 50 times in the evolutionary history of gastropods, and to this may be added a morass of monoplacophoran and poorly known bilaterally symmetrical shells in the early–middle Palaeozoic (Runnegar & Jell 1976; Peel 1991a, 1991b; Geyer 1994; Parkhaev 2008, 2017; Bouchet et al 2017). Eichwald’s (1860) species Cyrtolithes corniculum was formally fixed as type species of Pollicina by Bouchet et al (2017) under the name Cyrtolites corniculum Eichwald, 1860, with reference to Article 70.3 of the International Code of Zoological Nomenclature (ICZN 2012), in accordance with earlier usage by Koken (1897), Koken & Perner (1925), Knight (1941), Knight & Yochelson (1960), Kisselev (1994) and Evans & Cope (2003). Tentative identifications of Pollicina from the lower Cambrian of Denmark and southern Sweden by Poulsen (1967) and from the middle Cambrian of Australia by Runnegar & Jell (1976) are discounted This redescription of Pollicina is based on type and additional material of Pollicina corniculum from Russia and Pollicina crassitesta Koken, 1897 from Estonia. Muscle scar is a circumapertural band located at about half the distance from the apertural margin to the apex
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