Perissocytheridea (Ostracoda, Crustacea) from Western Amazonia (Middle Miocene)

During the Miocene (c. 23–10 Ma), climatic and tectonic changes led to the development of a large wetland with shallow lakes and swamps in Western Amazonia (Hoorn et al. 2010). These new aquatic environments of the “Pebas” system were colonized by rapidly evolving endemic invertebrate faunas composed of mollusks and ostracods (Wesselingh, 2006; Purper, 1979; Sheppard & Bate, 1980, among others). The ‘Pebasian’ ostracods hold a highly endemic and diverse fauna, which led to the description of several new genera and species.

The goals of this study are to review the taxonomy of Amazonian Perissocytheridea species, analyze their intraspecific variability and ontogenetic development, and to identify evolutionary pathways.

We studied samples from eight localities in the Iquitos region (Peru) covering the Middle Miocene mollusk biozones MZ4–MZ9 (Wesselingh et al., 2006).

We identified two species: Perissocytheridea sp. 1 and Perissocytheridea sp. 2, both of which appear to be endemic to the Pebas system. Remarkably, the specimens belonging to Perissocytheridea sp. 2 display ‘inverse’ hinges. The two species co-occur in different stratigraphic sections, with Perissocytheridea sp. 2 being less abundant in all samples. Both species exhibit a kind of premature sexual dimorphism from the juvenile stages A-1 to A-3. Additionally, polymorphism was found in specimens of Perissocytheridea sp. 1, manifested in variations in surface ornamentation, in size and shape of the alar and caudal process.

So far, no chronological trend has been established and other features remain to be compared (e.g., inner lamella, pores).

Although there are many references to ‘Pebasian’ ostracods with ‘inverse’ hinges, all of them were documented in the genus Cyprideis (Purper & Pinto, 1983, 1985; Whatley et al. 1998; Gross et al. 2013, 2014). Here, ‘inverse’ Perissocytheridea is reported for the first time. The presence of these ‘inverse’ forms could explain reproductive isolation and therefore, sympatric speciation, as suggested for the ‘Cyprideis species flock’ (Gross et al. 2014). In any case, the trigger for the occurrence of these ‘inverse’ forms in ‘Pebasian’ ostracods is still unknown.

Finally, we prefer not to assign the specimens to already described Perissocytheridea species from the Pebas system, as potential synonyms are still under study.