“Like a locomotive with a mouth full of butcher knives.”

That is how a shark expert, Matt Hooper, described Carcharodon megalodon to the police chief in Peter Benchley’s novel “Jaws.” He was referring to the 50-foot-long, 50-ton body and enormous six- to seven-inch-long teeth that made the extinct megalodon shark perhaps the most awesome predator that has ever roamed the seas.

Hooper had just gotten his first glimpse of the massive great white shark that was terrorizing the residents of Amity Island. Hooper explained that the Latin name for the great white was Carcharodon carcharias and that “the closest ancestor we can find for it” was megalodon. So maybe, he speculated, this creature wasn’t merely a great white, but a surviving sea monster from an earlier era.

Hooper was toying with a simple and long-established idea: that the most feared predator in the ocean today, the great white shark, evolved from megalodon, the most fearsome predator of a few million years ago.

That is how the two species had been viewed, until recently, when new ways of looking at shark teeth, and new shark fossils from a Peruvian desert, convinced most experts that great whites are not descended from a megatoothed megashark. Rather, they evolved from a more moderate-size, smooth-toothed relative of mako sharks.