The weight of internal and external constraints on Pupilla muscorum L. (Gastropoda:Stylommatophora) during the Quaternary in Europe

Abstract

Pupilla muscorum L. is a terrestrial gastropod which is very common in European Quaternary deposits. It shows great morphological variability, mainly in shell size. To understand how this variability is expressed, a survey of modern and Quaternary European specimens of Pupilla muscorum was undertaken using multivariate analyses to characterize the morphology. This was followed by an ontogenic study of shell sections to determine the changes in the growth timing. The first step relates to intra-population variability which is mainly expressed in shape variation. The shape of the shell lies between two extremes:a fat shell with a slightly protruding aperture and a slim shell with a strongly protruding aperture. The second step describes inter-population variability, both temporal (for fossil populations) and spatial (for modern populations), which is mainly in size. During the Quaternary, the size variation parallels the climatic cycles:large shells during pleniglacial phases, small shells during interglacial ones. At present, N—S and E—W size gradients are present with the adult size varying between 2.5 and 4.7 mm. The final step addresses the occurrence of this growth variability by studying morphological indices, which characterize size and spire of the shell, against the number of whorls taken as age reference. The variations in these indices show that heterochronic processes (sensu McNamara 1986), acceleration and hypermorphonis, include a supermumerary part-whorls of the spire in the juvenile phases of the growth. Because P. muscorum is in evolutionary stasis during the Quaternary, such morphological variations, channeled by external and internal constraints, are good example of ecophenotypic iterative changes at the intraspecific level.