After watching the movie Jaws, I had a deep sense of dread about going into the water—even if the water was in a swimming pool. A similar hydrophobia came over me while reading the riveting new book Resurrecting the Shark, by Bozeman, Mont.–based writer Susan Ewing. And this anxiety was even less rational than any arising from a Jaws viewing, as the book's subtitle makes clear: A Scientific Obsession and the Mavericks Who Solved the Mystery of a 270-Million-Year-Old Fossil. That's right—the cartilaginous creature in question, now called Helicoprion, was extinct before dinosaurs ever existed. But in its time, Helicoprion was quite the sea monster.

You know the Jaws scene in which the shark chomps away at Quint? It starts at the hunter's toes, bites away at his calves, rips into his knees, and so on. If Helicoprion had been working his way up Quint, it wouldn't have hit pay dirt until it got to the top of his inseam. Because its teeth weren't spread out along the long axis of its mouth, the way teeth are in great white sharks, humans and pretty much anything you can think of that has teeth. From Quint's point of view, the dental death coming at him would have looked like the cutting edge of a vertical buzz saw, like you'd see in an old lumber mill in a silent movie. Yikes.

Before continuing, two things: the teeth of Helicoprion (from the Greek for “spiral” and “saw”) were actually one humongous tooth, with dozens of visible crowns erupting from a single, continuous root. And to be taxonomically truthful, Helicoprion was not a shark. More on that bothersome bite ... I mean, bit ... of accuracy in a moment.

The first fossil finds of Helicoprion and related species were made in the 19th century. They were shaped like the remains of ammonoids, spiral-shelled marine mollusks. But with studs along the spiral. Trained eyes recognized the fossils as, well, lots of different things. Definitely fishy. Some kind of weapon. But where did it go on the fish?

“Scientists threw themselves into contortions keeping that tooth spiral out of the animal's mouth,” Ewing told me. “They put it on its head, they put it curling up over its nose, they put it on its tail, they put it on its back. They wanted to put it everywhere except in the mouth.” Because not even evolution could be that crazy, they thought. But, as Kramer said to Seinfeld, “Mother Nature's a mad scientist!” (Season Eight, Episode 19: “The Yada Yada.”)

Only in the past decade did researchers doing painstaking analysis of CT scans of fossils find evidence for the connective cartilage that nailed down the way the whorl was actually situated. “The thing that was so confusing about Helicoprion,” Ewing says, “was that that tooth whorl was a midline structure. So it was like a pizza cutter stuck in a quart of ice cream in the middle of the shark's lower jaw.”

But what could Helicoprion possibly chomp on with a vertical row of tooth-teeth? Well, its whorl's similarity in appearance to ammonoids was actually a clue: the shape made it good for snagging ammonoid flesh and then ripping them out of their shells. As Ewing writes, “The eminent French paleontologist Philippe Janvier likened the tooth whorl to a fourche d'escargot, a snail fork.” But the size of a large dinner platter or even a bicycle wheel and embedded in a sharky beast at least 20 and perhaps 30 feet long.

As to Helicoprion's sharkitude: “Because of the way Helicoprion's jaws were attached to its skull,” Ewing explains, “it technically is not a true shark. And it's not in the lineage that became the true sharks ... the purists really don't like us to call Helicoprion a shark, but it's really hard not to ... it surely looked like a shark.” And it led what would look to us like the epitome of a shark's life: an apex predator ruling the waters 270 million years before the first orthodontist adjusted an improper bite.