True Dalmanella and taxonomic implications for some Late Ordovician dalmanellid brachiopods from North America

Abstract

Examination of topotype material of the type species of Dalmanella, D. testudinaria, from the Hirnantian strata of Borenshult, southern Sweden, revealed several diagnostic characters that distinguish the type species from some common North American species assigned previously to the genus. The typical Dalmanella testudinaria has a consistently developed, primary medial interspace in the dorsal valve, a cardinal process with an invariably bilobed myophore, punctae of two distinctly different sizes, and a lack of aditicules. In contrast, many Late Ordovician species from North America, such as ‘Dalmanella’ meeki and ‘Dalmanella’ multisecta from the Cincinnati type area, consistently have a dorsal medial costa, with a strong tendency to develop aditicules and a trilobate cardinal process. This study confirms the previous notion of many authors that true Dalmanella testudinaria, or the genus as a whole, is either rare or entirely absent in North America. Typical Dalmanella was predominant in cool-water depositional environments (such as the Hirnantia Fauna), whereas the North American dalmanellids (e.g. Cincinnati-type ‘Dalmanella’, Paucicrura, and Diceromyonia) were most abundant and diverse in tropical, epicontinental sea settings. A clear definition of the true Dalmanella, therefore, has important implications for the study of Late Ordovician–Early Silurian brachiopod evolution, paleoecology, and paleobiogeography.

Keywords:

• Dalmanella
• Orthida
• Brachiopoda
• Late Ordovician
• Sweden
• North America

Acknowledgements

The Swedish Museum of Natural History, Stockholm, kindly loaned topotypes of Dalmanella testudinaria for this study and provided working facilities for one of us (JB). We also thank Brenda Hanke of the Cincinnati Museum of Natural History for providing collections of ‘Dalmanella’ from the Cincinnati type area for comparison. Yutaro Suzuki kindly allowed us to use unpublished information on stratigraphy and faunas in Dalarna. David Harper and anonymous reviewers provided many valuable comments in their reviews of the manuscript. Funding for this study was provided by the Natural Sciences and Engineering Research Council of Canada.