The excellent preservation of Lower Pleistocene Glycymeris

Species belonging to the genus Glycymeris Da Costa, 1778, commonly named ‘dog-cockles’ or ‘bittersweets’, appear in the Lower Cretaceous and have a rich fossil record; they are free-living seafloor inhabitants (up to 100 m of depth), widespread and very common in fossil faunas and are also very abundant in present seas and oceans, being absent from polar, deep-sea regions and brackish water. Species of the genus Glycymeris have an excellent potential as palaeoenvironmental archives and are thus important tools for palaeoclimatic reconstructions and sclerochronological analyses. The study of the shell ultrastructure under scanning electron microscope (SEM) is a fundamental step before doing geochemical and isotopic analyses; it is very important for fossil specimens, but also for recent ones, to test the preservation of the shell ultrastructure in order to exclude diagenetic alteration which could affect the results of the geochemical and isotopic analyses. In the present analysis new data about the shell ultrastructure of species of the genus Glycymeris are obtained through a comparison between the fabric of recent specimens from Brittany (France) and fossil specimens collected from the Lower Pleistocene Castell’Arquato Formation cropping out along the Arda River in Western Emilia (Italy). This comparison, made using Scanning Electron Microscope (SEM), results in a strong similarity between the two fabrics, highlighting the great preservation of fossil one. Both fossil and recent specimens show a well preserved outer simple crossed lamellar layer and an inner irregular and cone complex crossed lamellar layer; the inner and outer layers are separated by an irregular simple prismatic pallial myostracum; these mineralized layers are penetrated by parallel, not ramificated and not bifurcated cylindrical tubules, which represent a peculiar character of the Arcoida shells. This analysis, besides revealing new data on the pattern and origin of the tubules, shows that the fossil species of Glycymeris are very well preserved and can be used for subsequent geochemical analysis. It also shows that Glycymeris shell fabric has not changed for at least the last 2 million years, underlining the conservative character of the genus not only in its morphology but also in its ultrastructure.