New Miocene fossil taxa illuminate the evolution and paleobiogeography of the Ponto-Caspian gammaroid amphipod radiation

The Ponto-Caspian radiation of gammaroid amphipods is one of the world’s most diverse lacustrine crustacean radiations as well as the only lacustrine amphipod radiation with a fossil record (dating back 9 Ma). However, the phyletic relationship between fossils and extant species was never formally investigated, hampering our evolutionary understanding of this diverse clade. Here, we present the first morphology-based phylogeny (114 characters) of Ponto-Caspian gammaroids that includes 99% of extant taxa, all previously known fossils, as well as newly discovered fossil specimens from the Late Miocene (10.5 Ma) of eastern Romania. Our phylogenetic and phenetic results strongly support the monophyly of all fossil taxa. However, despite its widespread Late Miocene distribution throughout the Paratethys Sea, this fossil clade left no modern descendants. Its exact phylogenetic position is ambiguous, although it is nested in the radiation’s crown group. Morphometric analyses further indicate that fossil species occupy a narrower and peripheral morphospace relative to extant taxa. Our results support the assignment of the newly discovered Romanian fossils to a new genus, Eogmelina gen. nov. with two new species Eogmelina moldavica gen. et sp. nov. and Eogmelina prisca gen. et sp. nov. Finally, our findings call for a fundamental systematic restructuring of Ponto-Caspian gammaroids at genus and family level.