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Although Cyprideis torosa is one of the most studied ostracods, its ecophenotypic variability has always impeded a clear definition of its carapace morphology. As a consequence, it is often difficult to identify this species in fossil material and very little is known about its phylogenetic origin. In this paper, we attempt to answer two main questions: when and from what species did C. torosa originate? To reach this goal we first analysed living and Recent populations of C. torosa collected from different salinity environments to define its variability. We found that the valve outline, the size of rounded normal sieve pores and the percentage width of the anterior inner lamella seem to vary independently of salinity. Secondly, to look for possible ancestors we analysed several Neogene species and found that Cyprideis sp. from the mid-Serravallian of Spain could possibly be the common ancestor of the Late Miocene Mediterranean and Lake Pannon lineages and that C. torosa is strictly linked to the Mediterranean stock, particularly to the phyletic lineage C. ruggierii–C. crotonensis. In this framework Cyprideis gr. torosa originated in the late Tortonian and C. torosa s.s. is thought to have been differentiated from C. crotonensis at the beginning of the Calabrian.Supplementary material: biometric tables and the list of the examined material is available at https://dx.doi.org/10.6084/m9.figshare.c.3636170.v1

The living euryhaline species Cyprideis torosa (Jones) undergoes morphometric variations in size, noding and sieve-pore shape linked to environmental parameters. In particular it is known that salinity values around 8–9‰ represent the osmoregulation threshold and the turning point between smaller and greater valves and prevailingly noded as opposed to un-noded valves. Here, a character never studied before is analysed: the relationship between salinity and the C. torosa valve outlines, especially the morphology of the ventral border. Geometric–morphometric analysis was carried out on different populations of C. torosa collected in oligohaline (Massaciuccoli marsh, Tuscany, and Lake Trasimeno, Umbria) and hyperhaline waterbodies (Trapani–Paceco salt pans, Sicily). The results indicate that the species displays two morphotypes: a dominant one, characterized by a straight ventral border; and a morphotype with a sigmoidal profile ventral border. This latter morphotype seems to be related to higher salinity: the oligohaline waters are characterized only by C. torosa individuals with straight ventral outline, while the hyperhaline environments are marked by the occurrence of 17.6% of sigmoidal individuals. Furthermore, the sigmoidal ventral border is more emphasized in males than in females.

As a result of the use of geometric morphometrics, multivariate statistics and classical morphological observations the authors propose a taxonomic revision of the Neogene Italian Cyprideis species. On this basis, the Cyprideis valves collected in Tortonian, Messinian and Pliocene brackish deposits of Italy are referred to 12 species, six of which are new: Cyprideis alexandri nov. sp., Cyprideis arvedoi nov. sp., Cyprideis lepianensis nov. sp., Cyprideis rooki nov. sp., Cyprideis strollae nov. sp., and Cyprideis toscana nov. sp. Some species are rather widespread in the Italian Peninsula (Cyprideis ruggierii Decima and Cyprideis crotonensis Decima) or even in the whole Palaeomediterranean (Cyprideis agrigentina Decima and Cyprideis anlavauxensis Carbonnel). For C. ruggierii, C. agrigentina and C. crotonensis, a phyletic relation is suggested. The new species are confined both geographically and stratigraphically in several Tuscan athalassic brackish basins, suggesting endemic speciation similar to those occurring in ancient lakes.

The living euryhaline species Cyprideis torosa (Jones) undergoes morphometric variations in size, noding and sieve-pore shape linked to the environmental salinity. In particular it is known that salinity values around 8-9 psu represent the osmoregulation threshold and also the turning point between smaller and greater valve dimensions and prevailingly noded against un-noded valves. The variation of the percentage of round-, elongate- and irregular-shaped sieve-pores on the valves has shown an empiric logarithmic correlation with the water salinity from 0 to 100 psu. Due to this ecologically cued polymorphism, C. torosa represents 28 an invaluable palaeosalinometer for the Quaternary brackish basins. In this paper we attempt to verify whether the ecophenotypical behaviour of the post-evaporitic Messinian species Cyprideis agrigentina Decima was comparable with that of C. torosa. To reach this goal, three morphometric characters have been analysed: 1) size variability; 2) noding and ornamentation; 3) variability of the percentage of the sieve-pore shapes. The palaeoenvironmental interpretation was made using synecological and geochemical approaches [stable isotopes, trace elements, Sr-isotopes and natural radioactivity (NRD)]. For this study, the 250 m-thick Messinian Lago-Mare succession of Eraclea Minoa (Agrigento, Sicily) was chosen for the presence of monotypic assemblages made only by C. agrigentina for around 70 m of thickness. The results of the morphometric analyses showed that: 1) size variations are not related to the salinity changes recognized both from synecological and geochemical analyses; 2) no noded specimens have been recovered along the section; 3) the salinities calculated on the basis of the percentage of the sieve-pore shape are not correlated with the salinities inferred from the synecological and geochemical analyses. Thus in this paper we conclude that C. agrigentina cannot be considered a palaeosalinometer for the Messinian Salinity Crisis. There is a correlation of the 13 C and NRD data with the percentages of sieve-pore shapes, linking them to the oxygen availability at the bottom of the basin.