KANAB, Utah — On a stormy day in southern Utah last summer, the paleontologist Alan Titus wandered from the roadside, hot, wet and annoyed. A team from California was supposed to assist him in a ground survey of the craggy, buggy badlands of Utah’s Grand Staircase-Escalante National Monument. But his colleagues had bailed because of the lousy weather.

His eyes scanned the flat ground near Wahweap Creek, about 200 yards from one of the few roads that wind through the Grand Staircase’s remote and rugged 1.9 million acres. Dr. Titus had walked this area before and found nothing.

This time, however, the skull of an adult tyrannosaur peered up at him. Nearby, Dr. Titus spotted something else: a tyrannosaur toe bone.

From their terrifying seven-inch teeth to their comically disproportionate arms, tyrannosaurs loom large in the public imagination. But these ancient predators are quite rare in the fossil record.