The prehistoric megalodon shark had fins as big as human adults and heads the length of cars, according to new research

• New estimates suggest the megalodon — a giant, ancient shark — had a head about 15 feet long and a dorsal fin more than 5 feet tall.
• Researchers used the shark's fossilized teeth and the proportions of its nearest living relatives, including the great white, to estimate the size of its body parts.
• Megalodons have been hard to study because they had soft cartilage skeletons that didn't fossilize.

The only fossils that remain of the megalodon — the largest known shark ever to exist — are its human-hand-sized teeth.

The prehistoric shark, which roamed Earth's oceans for over 20 million years, from 23 to about 2.6 million years ago, had a skeleton made of cartilage, as sharks still do today. But because the material is soft, it rarely survives fossilization, which makes ancient sharks difficult to study.

Previous research suggested that megalodons could reach lengths of almost 60 feet, roughly the size of a small yacht. But the proportions of their heads and fins — among other measurements — have remained undetermined.

Last week, researchers at the University of Bristol and Swansea University offered new estimates that give a sense of just how enormous the sea creature was.

According to their work, published in the journal Scientific Reports, the megalodon's dorsal fin was about 5 feet 3 inches tall (or 1.65 meters). (The dorsal is the triangle-shaped balancing fin that lurks above the water, a tell-tale sign of an impending shark attack in movies.) That's about the height of the average American woman.

The megalodon's head, meanwhile, stretched or about 15 feet long from front to back (4.65 meters) — roughly the length of a Toyota Camry.

The researchers arrived at these estimates by analyzing the body shapes of five living shark relatives of the megalodon, including the great white. All the sharks had similar body shapes, suggesting their giant ancestor probably did, too.

Plus, those sharks' body proportions don't change as they grow older, unlike humans, who start out with large heads and short, stubby legs as babies. The consistency of shark bodies over time makes them easier to research and compare, because biologists don't have to figure out how other species compare to the Megalodon at different ages or estimate changes to the shark's proportions as it grew.

"We were surprised, and relieved, to discover that in fact that the babies of all these modern predatory sharks start out as little adults, and they don't change in proportion as they get larger," Professor Mike Benton, a paleontologist at the University of Bristol and a study coauthor, said in a press release.

Studying the shark's modern relatives also helped the researchers estimate how the megalodon's fins were shaped, which in turn gives more insight into how it lived. For instance, the scientists think the shark's dorsal fin curved outwards, an adaptation that likely helped it swim for long periods of time. That also could have enabled the megalodon to swim in short, quick bursts to capture prey in its huge jaws.

The researchers hope that shedding new light on the megalodon's proportions can help others study the ancient creature, too — and perhaps by extention figure out what drove the shark to extinction.