Abstract

Argenthina emilyae is a new genus and species of extinct muricid gastropods from the early Middle Miocene of southern Argentina. Most of its shell features are consistent with those of the Acanthina-clade of Thais-like ocenebrines, including a broad, rounded body whorl, a large aperture lacking ornamentation, and a short, open siphonal canal, although other characters are consistent with the Ocinebrina-like ocenebrines, such as the differentiation of axials into varices and intervaricals and complex spiral sculpture. This combination of traits is unique in the Ocenebrinae. It is not yet clear whether this new taxon represents an episode of morphological experimentation by a stem group of Thais-like ocenebrines resulting in convergence on the Ocinebrina-like shell form or an evolutionarily intermediate step between Thais-like and Ocinebrina-like ocenebrines. Intermediates are expected from previously published phylogenetic hypotheses for the Ocenebrinae that suggest Ocinebrina-like ocenebrines have evolved multiple times from Thais-like ocenebrines. Until now, however, such intermediates have not been reported from the fossil record.

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