Stenothecoida, a proposed new class of Cambrian Mollusca

Cambridium, Bagenovia, and Stenothecoides, composing the Family Cambridiidae, a monotypic superfamily and an order, were in 1960 assigned (although with a query) to the molluscan class Monoplacophora. The basic error of this assignment, according to the author, was the assumption that these specimens are univalves. One specimen from Siberia and a second from Alaska demonstrate that Stenothecoides is bivalved; Bagenovia was first described as a bivalve, but the implication of two valves was ignored.

Short internal ridges normal to the shell margin in Cambridium and Stenothecoides, described by Rasetti and Horný, show little resemblance to features of pelecypod shells. These markings are not homologous to paired muscle scars of monoplacophorans. The asymmetric bivalved shell and internal furrows are interpreted as features of class-rank significance; the extinct class Stenothecoida is proposed to accommodate these genera. These animals are most common in Lower Cambrian, but range into Middle Cambrian. They may have been functionally similar to brachiopods, but were unable to compete with those more efficient bivalves.