Large Fossil Plesiosaur Marine Dinosaur Vertebrae in a Matrix Cretaceous Period, 145-65 million years B.P. A group of four large complete or near-complete fossil vertebrae from a Plesiosaur sp. marine dinosaur, set into a sedimentary stone matrix with plaster backing jacket. See O’Keefe, F.R., The evolution of plesiosaur and pliosaur morphotypes in the Plesiosauria (Reptilia: Sauropterygia), 2002, Paleobiology, 28: 101-112. 11.5 kg, 67.5 cm long (26 5/8 in.). From a Cambridgeshire, UK, collection.