During the 2014 expedition to Marambio Island, more than 640 specimens of the studied species were collected at 33 localities (Fig. 1) distributed along all the allomembers defined by Marenssi (1998a). Precise information about the mentioned localities is available in Appendix 1. We also used the material donated by W. Zinsmeister to the MACN-Pi collection, which was collected during the field seasons 1990–1994. Most of the materials collected by W. Zinsmeister and J. Stilwell are housed at the Paleontological Research Institute (PRI), and were recently studied by CJR, who noted that said materials are currently being reclassified and renumbered. For this reason, we decided to not include these specimens in our analysis, considering that the materials housed at the MACN-Pi collection and the new materials collected in the Field Season 2014 deposited in the IAA are sufficient to perform the present study. Geographic and stratigraphic distributions of each studied species are summarized in Supplementary Data Set 1, and the materials corresponding to taxa used for comparison and phylogenetic analysis are summarized in Supplementary Data Set 2.

Systematic analysis shows that the Southern Hemisphere bivalve genus Retrotapes includes the Antarctic species R. antarcticus, R. newtoni, and R. robustus and recognizes for the first time the presence of Katelysia represented by K. florentinoi. Two new genera were erected in this study: Marciachlys new genus to include M. inflata new combination, and Adelfia new genus, which includes A. austrolissa new combination and A. omega new species from the Eocene of Antarctica, and the late Eocene Chilean A. arenosa new combination. Eurhomalea carlosi was synonymized with K. florentinoi; Cyclorismina marwicki with R. antarcticus; Gomphina iheringi was considered an indeterminate species; and Cockburnia lunulifera was excluded from the Tapetinae. These systematic assignments are supported by a phylogenetic analysis, which recognizes an Austral clade of Tapetinae, comprising all the genera mentioned above, and additionally Marcia, Paleomarcia, Atamarcia, and Protapes.