Zygospira and some related Ordovician and Silurian Atrypoid brachiopods

The Zygospiridae are redefined to include 'primitive' atrypoid brachiopods with dorsally to dorsomedially directed spiralia, normally with fine ribs and a one-piece jugum dorsal to the spiralia. The Ordovician-Silurian family is divided into three: Zygospirinae, Catazyginae (new sub-fam.), and Tuvaellinae. Internal structures of Zygospira, Anazyga, Catazyga, Pentlandella, and Tuvaella are evaluated in terms of their evolutionary significance. A new genus Zygatrypa is erected. Eospirigerina, which was probably derived from zygospirid stock in the late Ordovician, differs in having ventrally located, separated jugal processes and trends towards 'frilly' shells: this represented a major jump towards a postulated Zygospiraella-Protatrypa-Gotatrypa lineage. In late Caradoc-Ashgill times Catazyga inhabited deeper water, whereas Zygospira lived in shallow-water communities. The Clintonellinae, including Alispira, are tentatively assigned to the Atrypidae, with Silurian taxa such as Nalivkinia and Anabaria.