Four species of megathyrid brachiopods from the Lower Cretaceous (Berriasian) of Crimea, Ukraine, and one species of megathyrid from the lower Upper Cretaceous (Cenomanian) of England are reinvestigated using scanning electron microscopy. The species are variously assigned to one existing genus, Praeargyrotheca Smirnova (P. haxaplicata (Smirnova) and P. megatrema (J. de C. Sowerby)) and two new genera, Krimargyrotheca (K. concinna (Smirnova) and K. balkii n. gen. and n. sp.) and Evargyrotheca (E. alta (Smirnova)). Taxa are distinguished principally on differences in shell shape, plication, and, in particular, surface microornament. The shell microstructure of all five species is investigated with particular emphasis on endopunctation and microornament. Dental plates are present briefly early in ontogeny. Based on the possession of transient dental plates, a septalium, and conspicuous microornament, Praeargyrotheca is removed from the family Megathyrididae and assigned, along with the two new genera, to a new family Praeargyrothecidae. Scanning electron microscopy study of loop development in both fossil and Recent terebratelloids and megathyrids supports a recent immunological study of living taxa from both groups that suggests that the two groups are not as closely linked phylogenetically as previously maintained. The family Megathyrididae is thus removed from the superfamily Terebratelloidea and assigned, along with the new family Praeargyrothecidae, to a new superfamily Megathyridoidea.

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