Revision of Amazonian Perissocytheridea Species (Ostracoda, Crustacea) from the Pebas Formation (Middle Miocene)

During the Miocene (c. 23–10 Ma), a large wetland with shallow lakes and swamps developed in Western Amazonia (Hoorn et al., 2010). This predominantly aquatic environment – the ‘Pebas system’ – was colonized by rapidly evolving endemic invertebrate faunas, with mollusks and ostracods being the best documented (e.g., Wesselingh, 2006; Purper, 1979).

Over the last four decades, ‘Pebasian’ ostracods have been thoroughly studied. However, most research has focused on the genus Cyprideis Jones, which typically constitutes the bulk of the ostracod fauna (Purper, 1979; Muñoz-Torres et al., 1998; Gross et al., 2014).

In this study, we focus on the genus Perissocytheridea Stephenson (Cytheridae). The material examined consists of approximately 1,400 mostly well-preserved valves and carapaces. These specimens come from 14 fertile samples, collected from eight outcrops in the Iquitos region (Peru), which cover the Middle Miocene mollusk biozones MZ4–MZ8 (Wesselingh et al., 2006).

Eight taxa were identified. The most abundant are Perissocytheridea ornellasae and Perissocytheridea? elongata, followed by Perissocytheridea sp. 1, Perissocytheridea sp. 2 and fewer specimens of P. acuminata and Perissocytheridea sp. 3. All taxa appear to be endemic to the Pebas system. Notably, the specimens assigned to Perissocytheridea sp. 2 and Perissocytheridea sp. 3 display ‘inverse’ hinges. Perissocytheridea sp. 2 is recorded only in the stratigraphically oldest sections (mollusk zone MZ4; Boca Napo and Santa Teresa localities), together with Perissocytheridea sp. 1. In contrast, P. ornellasae occurs in samples from several localities and biozones (MZ5, MZ7 and MZ8), and is associated with P.? elongata in MZ7 (Puerto Almendras) and with P. acuminata in MZ8 (Palo Seco). The later was only recorded from one sample in the youngest biozone (MZ8). Unfortunately, in the samples analysed from MZ6 we not recorded Perissocytheridea.

Several intraspecific variations were observed. For example, specimens assigned to P.? elongata from MZ7 (Puerto Almendras and Tamshiyacu) exhibit smooth, reticulated, or strongly ornate surfaces, as well as the presence of nodes, which appear to have an ecophenotypic origin. Additionally, analyses of the ontogeny of P. ornellasae, Perissocytheridea sp. 1 and Perissocytheridea sp. 2, suggests that it is possible to distinguish a form of incipient sexual dimorphism in the final instars.

Although numerous references exist to ‘Pebasian’ ostracods with ‘inverse’ hinges, all previously documented cases correspond to the genus Cyprideis (Purper & Pinto, 1983, 1985; Whatley et al., 1998; Gross et al., 2013, 2014). The presence of such ‘inverse’ forms may indicate reproductive isolation and, consequently, sympatric speciation, as suggested for the ‘Cyprideis species flock’ (Gross et al., 2014). Nevertheless, the trigger for the occurrence of these ‘inverse’ forms in ‘Pebasian’ ostracods remains unknown.