How these prehistoric animals compare
Otodus, the giant shark group that includes the megalodon, and Mosasaurus, a giant marine reptile, are often compared, but they lived tens of millions of years apart and neither was a dinosaur.
Fish versus reptile
Otodus is a cartilaginous fish, a shark relative that breathes with gills. Mosasaurus was an air-breathing marine lizard relative, a squamate, that lived in warm Cretaceous seas. Their similar streamlined bodies are a result of adapting to life in open water, not close kinship.
Separated by an extinction and a geological era
Mosasaurus belongs to the Late Cretaceous and disappeared at the end of the Cretaceous. Otodus megalodon belongs to a much later part of the Cenozoic era. The two never shared the same ocean, so a meeting between them is a modern invention.
What the record is based on
Shark skeletons are made largely of cartilage, so the fossil record of Otodus relies heavily on teeth and occasional vertebrae. Mosasaurus is known from skulls and more complete skeletons. Because of this, size comparisons are based on different kinds of evidence and carry different uncertainties.
Sources and specimen context: Otodus references · Mosasaurus references · Geological timeline
