Is the genus Retrotapes del Río 1997 (Bivalvia: Veneridae) a monophyletic group?

MAXIMILIANO ALVAREZ; DAMIÁN PÉREZ

Mendoza

Congreso; 4º International Palaeontological Congress; 2014

International Palaeontological Association

Retrotapes is a genus of the Subfamily Tapetinae created to group some extant and Cenozoic species from Patagonia and Chile that have three cardinal teeth inclining backwards or with the anterior tooth nearly vertical. Several authors have questioned the validity of this genus, proposing the synonymy of Retrotapes with Frigichione Fletcher, and that the extant species of Retrotapes, R. exalbidus and R. lenticularis, are members of the genus Eurhomalea Cossmann. Recently, others authors recognized the validity of Retrotapes suggesting the inclusion of the extant species mentioned above and others from the Eocene of Antarctica previously identified as members of genus Eurhomalea and Atamarcia Marwick, within Retrotapes. In the present contribution we analyzed the monophyly of genus Retrotapes considering as ingroup the extant species from Argentina and Chile, fossil species from the Miocene of Patagonia and Chile and Eocene of Antarctica, Frigichione permagna (type species of Frigichione, Miocene of Kerguelen Island), the extant species Eurhomalea rufa (type species of Eurhomalea) and Atamarcia sulcifera (type species of Atamarcia, Miocene of New Zeland). Additionally, we considered 16 species of the Subfamily Tapetinae as outgroup for this analysis. To test the affinities of Retrotapes we performed a cladistic analysis based on 67 shell characters of 31 species with TNT 1.1 through a heuristic search of 50 replicates of Wagner trees followed by TBR branch swapping algorithm holding 10 trees per replicate, under implied weighting with K values between 8 and 19. We obtained a tree with Retrotapes as a monophyletic clade, that includes R. newtoni, R. fuegoensis and R. robustus as successive sister groups; and two other clades, one grouping R. striatolamellatus and R. ninfasiensis both from the Miocene of Patagonia and the other grouping the extant species with R. andrillorum (Miocene, McMurdo Sound, Antarctica), R. antarcticus (Eocene, Marambio Island, Antarctica) and R. navidadis (Miocene, Chile). Retrotapes is supported by five unambiguous synapomorphies: wide hinge plate, right anterior tooth subvertical, lunule delimited by a deep groove, valves ornamented with comarginal lines more separate in juveniles but closer in adults and have some growth lines more pronounced than others. Moreover, Retrotapes is more closely related to the clade formed by Paleomarcia tatei (Miocene of Kerguelen Island) and Atamarcia sulcifera (Miocene of New Zeland). Frigichione was recovered as the most basal of the Tapetinae and Eurhomalea rufa was resolved in a different clade more closely related to species from the Northern Hemisphere.