Morphometric studies on three ostracod species of the genus Digmocythere Mandelstam from the middle Eocene of Egypt

Ostracoda are a sub-class of small crustaceans (living forms range from 0.3 to 30 mm long), which occur in practically every aquatic environment. In today’s oceans they are found living from the abyssal depths to the shoreline. They also inhabit estuaries, lagoons, freshwater lakes, ponds and streams, salt lakes, hot springs, damp vegetation and even the water which collects in bromeliad leaf bases. Ostracods may be free-swimming for all or part of their life cycle, or, more commonly, are benthonic, living among aquatic plants or crawling on or through the sediment. A number of interstitial forms are known. Ostracods display a variety of feeding habits; some are filter-feeders, others are scavengers, detritivores, herbivores or predaceous carnivores. A few species are parasitic or commensal on other crustaceans, polychaete worms and echinoderms.

The most distinctive feature of the Ostracoda is the calcareous bivalved carapace which can totally envelop the body and limbs, but from which various appendages are protruded for locomotion, feeding and reproduction (after Athersuch et al., 1989). The word ostracod or ostracode is derived from the Greek word "ostrakon" (a shell); this shell or carapace has numerous morphological characters that allow taxonomic and phylogenetic studies to be made on living and fossil specimens.

In this paper, canonical variate analysis as well as relative warps and thin plate spline analyses have been applied to specimens questionably assigned by Elewa (1994) to the ostracod Cyamocytheridea? sp. and two other related species of the ostracod genus Digmocythere: D. omarai Cronin and Khalifa and D. ismaili (Bassiouni) to differentiate between the shapes of the three related forms studied by means of multivariate statistics and geometric morphometrics to establish whether there is an adequate shape variation between the forms separating them into three different species, and thus attributing one to a new species.

The reported results show that C.? sp. is a new species which is more similar to D. ismaili than it is to D. omarai. Additionally, the morphometric analyses support the suggestion of Boukhary et al. (1982) in placing D. omarai Cronin and Khalifa as the ancestor of D. ismaili (Bassiouni). These results also indicate that the geometric measurements (TPS) were helpful in differentiating the three studied species, whereas the distance measures were not. However, the distance measurements could help in differentiating males from females.