Conodont biostratigraphy in the Middle–Upper Ordovician boundary beds of Estonia

V Viira

doi:10.3176/earth.2008.1.03

Abstract

More than 150 Ordovician and Early Silurian brachiopod species have been assigned to the genus Platystrophia King, 1850 mainly on the basis of their Spirifer-like shell exteriors. King's concept of the genus was based on Platystrophia biforata King, which is not conspecific with Terebratulites biforatus Schlotheim, traditionally regarded as the type species of Platystrophia. Porambonites costatus Pander, 1830 is formally proposed as the type species of the genus to replace P. biforata; the latter is considered to be a nomen dubium. In our revised diagnosis, Platystrophia is restricted to a group of Arenig to upper Caradoc species from Baltica and Avalonia, whereas the Ashgill and lower Silurian taxa of these regions, hitherto assigned to Platystrophia, are placed in the new genus Neoplatystrophia. Platystrophia ponderosa Foerste, 1909 from the Upper Ordovician of North America is proposed as the type species of a new genus Vinlandostrophia. Two new species, Platystrophia baltica and Platystrophia pogrebovi from the Llanvirn-Caradoc of the East Baltic are also described.

Highlights

“The name Platystrophia proposed by Dr King, has come into very general use for a group of orthids having a strikingly spiriferoid exterior” (Hall & Clarke 1892, p. 201)
We provide an emended diagnosis of the genus Platystrophia and a description of two new genera, Neoplatystrophia and Vinlandostrophia, from the Upper Ordovician and lower Silurian of Baltica and from the Upper Ordovician of Laurentia, respectively
This assumption was supported by our examination of brachiopod collections which have been collected by various authors from the Siljan district

Summary

INTRODUCTION

“The name Platystrophia proposed by Dr King, has come into very general use for a group of orthids having a strikingly spiriferoid exterior” (Hall & Clarke 1892, p. 201). Despite over a century of research on this group of orthide brachiopods, our current knowledge of the morphology and systematics of the many taxa assigned to “Platystrophia” remains poor This is unfortunate since the Platystrophia plexus is one of the most distinctive brachiopod groups within the benthic faunas of the palaeocontinents of Baltica, Avalonia, and Laurentia during the Ordovician and Early Silurian. We provide an emended diagnosis of the genus Platystrophia and a description of two new genera, Neoplatystrophia and Vinlandostrophia, from the Upper Ordovician and lower Silurian of Baltica and from the Upper Ordovician of Laurentia, respectively. Both are externally similar to Platystrophia, but have distinctive, contrasting dorsal interiors. The morphology of the Platystrophia-like genera Gnamptorhynchos Jin, 1989 and Siljanostrophia Zuykov & Egerquist, 2005 from the Ashgill of Anticosti Island, Canada, and the Siljan district, Sweden, respectively, are briefly discussed below

MATERIAL AND METHODS

Discussion

Findings