In 2015 Philip Mullaly was strolling along a beach in Victoria, Australia, when he spotted what looked like a shining serrated blade stuck in a boulder. Using his car keys, Mr. Mullaly carefully pried from the rock a shark tooth about the size of his palm. He didn’t know it at the time, but the tooth he uncovered once belonged in the mouth of a 25-million-year-old giant shark that was twice the size of a great white.

“It was an awesome creature, it would have been terrifying to come across,” Mr. Mullaly said.

Though Mr. Mullaly, who is a schoolteacher and amateur fossil hunter, has collected more than a hundred fossils, he never before found a prehistoric shark tooth. He returned to the boulder a few weeks later and to his surprise dug up several more three-inch teeth.

“It dawned on me when I found the second, third and fourth tooth that this was a really big deal,” said Mr. Mullaly.

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He contacted Erich Fitzgerald, a paleontologist at the Museums Victoria in Melbourne, which announced the find on Thursday. Dr. Fitzgerald identified the teeth as belonging to a type of mega-toothed shark called the great jagged narrow toothed-shark, or Carcharocles angustidens.