Cartilage decays rather than mineralising, so no complete megalodon skeleton exists. What survives in abundance are teeth, which can exceed 17 centimetres in slant height, along with rare associated vertebrae. Body length is estimated by scaling relationships derived from living sharks, most often the great white, and different scaling choices give different answers.
Published estimates cluster around fifteen to eighteen metres for large individuals, with some analyses allowing up to about twenty metres. Figures of twenty-five or thirty metres circulate widely but come from extrapolations most researchers do not accept. The reliance on great white proportions is itself questionable, since megalodon is now placed in the otodontid family rather than close to the lamnids.
Bite marks on fossil whale bones confirm predation on large marine mammals. Extinction around 3.6 million years ago coincides with cooling seas, restructuring of marine mammal communities, and the appearance of the great white shark as a competitor. Claims that the animal survives in the deep ocean have no evidential basis: its teeth stop appearing in the fossil record entirely.