Cretoxyrhina mantelli prehistoric sharks, artwork. This species of prehistoric shark existed during the late Cretaceous period (roughly 100MB-82 million years ago). Measuring over six metres in length, it was the largest shark of its time and it preyed on large marine reptiles, turtles and giant armoured fish. Shark skeletons are made of cartilage and do not fossilise well, although several well preserved Cretoxyrhina skeletons have been discovered.