Kent Hups, a high school science teacher in Colorado, was nursing a bad knee when he accidentally stepped into paleontology history.

He was volunteering on a fossil dig in the North Dakota badlands last year, while recovering from knee replacement surgery, and was walking gingerly on easier, flatter routes, to avoid steep terrain.

That was when he spotted a lumpy triangle of hardened sandstone that looked like giant toes. He thought it was a dinosaur track, and he was right.

“I got out my tape measure, and I started measuring the angles and everything started to fit,” Mr. Hups said. “That’s when I realized I had something really good. That’s when I discovered that was only the third adult T. rex track ever found.”