The reversal of valve overlap in Cyprideis (Ostracoda, Crustacea) as a mechanism for sympatric speciation in the Pebas System (Miocene) of Western Amazonia

During the Miocene, a huge (~1 million km²), long-lived (~10 myr) aquatic environment, the so-called Pebas System (Hoorn et al., 2010), existed in Western Amazonia, which is known for its highly diverse and endemic mollusk and ostracod faunas (Gross et al., 2014; Nuttall, 1990; Purper, 1979; Wesselingh, 2006). In the latter group, the asymmetrical-shelled genus Cyprideis is the most relevant, both in terms of abundance and number of endemic taxa, generating a flock of species in the Pebas System (Gross et al., 2014; Whatley et al., 1998). Although the understanding of certain aspects of this flock, such as taxonomy, has improved in recent years, the mechanisms responsible for this speciation remain elusive. In this study, we focus on a peculiar feature of the Cyprideis species flock, as about one-third of its species have a ‘reversed’ hinge and valve overlap compared to what is typical for the genus.

Several outcrops (Marañón Basin, Peru) and boreholes (Solimões Basin, Brazil) yielded abundant and well-preserved material of some little known species (Cyprideis caraionae Purper & Pinto, 1985, Cyprideis krsticae Purper & Pinto, 1985 and Cyprideis retrobispinosa Purper & Pinto, 1983) with complete ‘populations’ (males, females and juvenile stages) within the samples, allowing us to understand the variation of these species over a broad temporal span (~16-13 Ma). Our analyses demonstrate that the development of a shell reversal from previous ‘normal’ shelled Cyprideis species is an effective mechanism for reproductive isolation of the populations, and hence, of sympatric speciation in the Pebas biome.