Article: The Middle Jurassic bivalve 'Cuspidaria' ibbetsoni: a corbulid not a septibranch

A distinctive shallow-burrowing bivalve with an elongate rostrum, common in the British Middle Jurassic, is frequently referred to as Cuspidaria ibbetsoni (Morris, 1853) and as such is recognised as one of the earliest members of the anomalodesmatan family Cuspidariidae. Since modern cuspidariids are highly specialised bivalves and are voracious carnivores in the deep sea, whereas C. ibbetsoni is prevalent in brackish-water facies, questions have been raised over its true affinities. We have investigated new and pre-existing material of C. ibbetsoni, and have concentrated on shell microstructure, morphology, and musculature. Our findings suggest that the inequivalve shell is composed of crossed-lamellar aragonite rather than either homogeneous or prismato-nacreous structures. It also lacks the complex musculature associated with the raptorial siphon of the predatory septibranchs. These characters suggest that C. ibbetsoni should more properly be placed within the myoid family Corbulidae and a new genus, Rostrocorbula, has been erected to accommodate it. It has also been necessary to establish a neotype. The significance of this familial redesignation to our understanding of the evolution of carnivory within the Bivalvia is discussed.