These fossils are a relatively rare find at Runswick. Go a few miles along the coast to Whitby and the east cliff is full of fat little bivalves. Here they're bigger and rarer. To the left, a bivalve found loose and smoothed after a while being tumbled in shingle (it may be Pleuromya sp.), to the right a bivalve graveyard mostly composed of Cardinia sp.
Below, hinge detail of the solitary (maybe) Pleuromya...
And here, the other side of the clump of Cardinia. They're much smashed up. Either they were damaged by geological activity after death and during fossilisation or they'd been eaten, churned up in the gut of a Jurassic marine reptile (icthyosaur or plesiosaur) and the indigestible shells excreted or vomited back in a marine midden. Robert Bakker makes a good case for land dinosaurs using gastric mills - eating stones to help the stomach grind up tough plant material as part of digestion - in The Dinosaur Heresies (a must for any dinosaur fan). Aragonite-covered shellfish would be even tougher dietary, er, nuts to crack. Any icthyosaur skeletons with stony stomach contents out there, I wonder? There are certainly icthyosaur fossils with ammonites where the digestive tract would have been in life.
More bivalves later. This time, I think, definitely a candidate for Jurassic CSI.