Morphometric divergence in <i>Cyprideis</i> (Ostracoda) during the Middle and Late Miocene of the Central Paratethys realm

Abstract. A morphometric outline analysis of the brackish-water ostracod Cyprideis examines a 2.0-million-year period during which the Central Paratethys region underwent significant geographic and environmental transformations. Based on hingement anatomy, the polymorphic Cyprideis pannonica, characterized by considerable variation in valve outline and size, inhabited the marginal marine environments of the Central Paratethys during the late Middle Miocene. With the transition to lacustrine conditions in the Late Miocene, Cyprideis colonized the brackish Lake Pannon and adapted to different water depths. However, compared to its Middle Miocene ancestors, it lost its total variability in outline due to a fragmented lacustrine environment. Less than 0.5 million years after the lake's formation, a regression event eliminated littoral C. pannonica morphotypes. When a new transgressive cycle began, these morphotypes were replaced by neoendemic littoral taxa, which exhibited only limited morphometric similarity to their ancestors and contemporaneous sublittoral relatives. While littoral taxa evolved rapidly in terms of outline, sublittoral species increased in size, consistent with Bergmann's rule, and were more conservative in the outline, maintaining a higher degree of morphometric similarity to their ancestors. Despite a physiological adaptation to brackish waters, Cyprideis were outnumbered in Lake Pannon by primarily freshwater candonins and marine/brackish leptocytherids. It is concluded that lacustrine habitat heterogeneity and tectonic activity in the Central Paratethys had impacted the adaptive radiation and the polycyclic evolution of Cyprideis.