Millions of years ago the oceans were ruled by a bus-sized, whale-eating shark with teeth as big as your hand.

Scientists have long believed that the prehistoric megalodon shark evolved into today's scariest sea predator, the great white. However, a new study comparing teeth suggests that the great white is more closely related to the smaller, but equally vicious mako shark.

"Most scientists would probably say the great whites evolved from the megalodon line, which existed from two to twenty million years ago," said Chuck Ciampaglio, a geologist and paleontologist at Wright State University. "However, our research, which is based on analyzing fossils of several hundred shark teeth, shows that the great white shares more similarities with the mako shark."

A great white shark compared to the much larger megalodon, and a hapless hypothetical human. LiveScience Illustration

Reconstructing prehistoric sharks is difficult. A shark's skeleton is made of cartilage, which decomposes quicker than bone. Researchers have only recovered a few fossilized megalodon vertebrae.

"Teeth are the thing to go on," Ciampaglio told LiveScience.

Sharks replace their teeth regular, so they can be found on the seafloor. Fossils of megalodon teeth are collected on ancient seabeds now exposed. Ciampaglio digitized hundreds of teeth -- upper, lower, front, and back teeth from the three species, and analyzed their sizes and shapes.

The analysis showed great whites and makos have very similar tooth and root structure. "The great whites and makos lay right on top of each other," Ciampaglio said. They also have very similar growth trajectories - how a tooth changes in size and shape as the shark grows to its adult size.

The great white and megalodon, however, shared none of these characteristics. Serration is their only common trait, said Ciampaglio, but the other characteristics are more important.

This evidence "strongly supports the theory that the great white is descended from the prehistoric mako group," Ciampaglio said. The megalodon was probably the end of a run of giant sharks that died out 2-3 million years ago, he said.

Recently, a few fossil species was found off South America that look like an intermediate between the great white and mako, Ciampaglio said. This further strengthens his theory on great white evolution.

While great whites typically grow up to 20 or 25 feet in length, the megalodon was twice as long and had a gaping maw that a human could climb into, should someone so desire.

"They were huge sharks, approximately the length of a Greyhound bus and possessing teeth that were up to six inches long," said Ciampaglio.

In Greek, megalodon means "big tooth." Thanks to those teeth, scientists have been able to estimate the size of the megalodon to have been at least 40 feet in length. Many researchers suspect they were 50 feet or larger, and some have speculated they were much bigger.

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