Insights on the history of the scorched mussel Brachidontes rodriguezii (Bivalvia: Mytilidae) in the Southwest Atlantic: a geometric morphometrics perspective

Abstract

Some authors support the idea that an important part of the Miocene marine mollusc fauna is still represented in the Argentinean Province. The fossil mollusc Brachidontes lepida (Philippi) is considered a subspecies of the extant B. rodriguezii (dˈOrbigny), a taxon currently present in the Argentinean Biogeographic Province. The aim of this study is to investigate the shell shape relationship between B. lepida and B. rodriguezii using geometric morphometrics. Samples of B. rodriguezii (n = 63) from four localities distributed in Uruguay and Argentina, of B. lepida from the Paraná Formation (n = 26) and of two other extant Brachidontinae present in the region were included in this study. Canonical variate analyses showed that extant species differed in shell shape, with the discriminant function properly allocating 93% of the individuals to their respective species. Using the extant discriminant function, approximate 92% of B. lepida individuals were allocated to B. rodriguezii. This result suggests that B. lepida is more similar in shell shape to B. rodriguezii than to the other extant species present in the region. Considering the material from the Pliocene of Cerro Laciar and from the Pleistocene deposits of Buenos Aires and Bustamante, the presence of B. rodriguezii from the Late Miocene in the warm temperate region is discussed.

Keywords:

• Mussels
• Brachidontes
• geometric morphometrics
• shell shape
• Late Miocene
• Southwestern Atlantic

Acknowledgements

We dedicate this paper to our co-author and dearest friend Jose Maria (Lobo) Orensanz who passed away in January 2015. We are grateful to Néstor Basso, Andrea Marin and Victor López for their photographic assistance. To Andreia Salvador and Harry Taylor (British Natural History Museum), and John Sime (Academy of Natural Sciences of Drexel University, Philadelphia) for excellent photographic and archival information of Brachidontes species. This contribution is part of the doctoral thesis of BT.