New and revised genera of Wenlock-Ludlow atrypids (Silurian Brachiopoda) from Gotland, Sweden, and the United Kingdom

Abstract

The late Llandovery through late Ludlow sequences of the stable carbonate platform of Gotland (Baltic Basin; western side of the Silurian paleocontinent Baltica) and the carbonate ramp of the Welsh Borderland area in England contain a richly fossiliferous fauna of atrypid brachiopods, including more than 60 species. This was a time of diversification for the atrypids in the tropical equatorial latitudes, with new lineages appearing for the first time that were to dominate the Devonian Period. Included in this burst of new groups was the ribbed atrypid group, the Plectatrypinae (new subfamily), which quickly led to the first spinose atrypids, the Spinatrypinae. For the Plectatrypinae, a new subgenus and species, Plectatrypa (Gutnia) capidula, and a new genus, Xanthea, are described. The evolution of expanded growth lamellae and frills on the shell at this time led not only to the growth of large shells assignable to Atrypa, but also to the rise of affiliated ribbed taxa, such as the perireefal and reefal genus Endrea echoica new genus and species (Wenlock-Ludlow). The nature, affinities and taxonomy of the smooth atrypid Lissatrypa obovata (Sowerby) from the U.K. are also clarified, indicating that the type species for the unrelated genus Glassia needs to be revised.