Morphology and ontogeny of some Middle Ordovician gonambonitid brachiopods from Baltoscandia

The Volkhovian-Kunda boundary deposits of the Leningrad Region contain two closely related genera, Antigonambonites and Anchigonites. The latter genus was previously known only by valve moulds; here its shell exterior and interior, and ontogeny are described in detail based on the collection of well-preserved disarticulated valves of Anchigonites conulus. Antigonambonites and Anchigonites share similar developmental and morphological features, and ecological strategies including the attachment by cementation by a "pedicle tube" and are thus possibly phylogenetically related. A new diagnosis for the genus Anchigonites and description of A. conulus are provided; other species of Anchigonites are briefly revised. The moulds of outer epithelial cells are described for the first time for the class Strophomenata.