Sizing a Shark - The Cosmopolitan Predator 

I came across THIS ARTICLE in The Guardian, it is based on this ACADEMIC PAPER, and concerns a study of the extinct shark Otodus megalodon. 

Sharks do not have bones, so fossils are rare. Most shark remains are teeth So to get a picture of megalodon the researchers had to create a 3D model. And the results were that megalodon was 16 metres long and weighed 70 tons. We can be pleased that it went extinct 2.6 million years ago.

The beasts teeth are found all over the (marine) world, hence its description as a cosmopolitan predator.

The authors go into speculation about the affect such creatures would have on their surrounding ecology and come to the conclusion that it would have been the apex predator, able to eat anything in the ocean, including the largest whales.

Megalodon seems to compete with Tyrannosaurus rex for the title of scariest dead beast. I do not think I would like to meet either of them.

At around 50 feet (16 metres) from nose to tail, the megalodon was longer than a bus. Photograph: JJ Giraldo/AP