Dimyodon nilssoni

Cretaceous

Phylum: Mollusca
Class: Bivalvia
Order: Ostreoida

Cemented onto fish vertebra

Identification of these as bivalves took a very long time (historically) as few features are preserved. However, knowing that these are bivalves they can be easily spotted throughout much of the Jurassic and Cretaceous, cementing to hard substrates. The shape of the valve (of which only the inner layer remains) gives probably the most distinctive feature for this kind of animal.