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. Titus had walked this area before and found nothing. This time, however, the skull of an adult tyrannosaur peered up at him. Nearby, Titus spotted something else: a tyrannosaur toe bone.

From their terrifying 7-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.

By December, the bones of the excavated tyrannosaur lay encased in plaster in a lab in Kanab, Utah, awaiting closer analysis. “It was the find of my lifetime,” said Titus, a paleontologist with the Bureau of Land Management.

But it is just one of the many extraordinary discoveries made here. In the past 15 years, Titus and his colleagues at the bureau — along with the Natural History Museum of Utah, the Denver Museum of Nature & Science and hundreds of volunteers, interns and researchers — have excavated tens of thousands of fossils from an extraordinary part of the Grand Staircase monument called the Kaiparowits Plateau, a 50-mile-long, high-elevation ridge.

One of the richest troves of fossils from the Late Cretaceous Period, the Kaiparowits is providing a window into the hothouse world that was home to the dinosaurs in their twilight, about 10 million years before their sudden extinction. The place stands out for the sheer number of well-preserved, unique fossils, and finds from this ancient ecosystem are challenging long-held assumptions about dinosaur physiology, evolution and environment.

Most fossils have been excavated from a part of the plateau called the Kaiparowits formation, a multilayered band of sandstone and mudstone dating from 76.6 million to 74.5 million years ago. Quickly deposited layers of sand and mud buried the fossils in a pristine state.

Their preservation is spectacular: articulated skeletons, fossilized skin, plants so shockingly fresh that their delicate leaves can be peeled right off the rocks. When they are illuminated beneath an epifluorescence microscope, their cuticles, or waxy leaf coverings, fluoresce bright green, revealing their cellular structures.

Among the animals discovered here are 21 never-before-seen dinosaurs. Many are ceratopsids, or horned-face dinosaurs, including the ornately frilled Kosmoceratops richardsoni (named after Scott Richardson, a paleontologist with the Bureau of Land Management) and Nasutoceratops titusi (named after Titus), a herbivore with a skull 7 feet long, an oversize nose and forward-facing horns. Two new species of tyrannosaurs have been found on the plateau: the 12-foot-tall Teratophoneus currei (“monstrous murderer”), which died 75 million years ago; and Lythronax argestes (“king of gore”), at 81 million years old the oldest true tyrannosaurid known to science.

Today the Kaiparowits is a craggy expanse of shrub-covered rock and sheer cliffs with little moisture. But 75 million years ago, the region was a steamy, swampy, coastal forest. Giant pine trees draped with moonseed vines towered over an Everglades-wet forest floor blanketed with gingers, ferns, duckweed, water lettuce and floating, flowering plants.

In the Late Cretaceous, southern Utah was home to nine species that weighed well over 2,000 pounds as adults. Compare that with present-day Africa, which is five times as large but supports only a handful of animals that big: elephants, giraffes, hippopotamuses, buffaloes and rhinoceroses.

“It’s a tiny land mass,” Scott Sampson, a paleontologist at the Denver Museum, said of the plateau. “How did you get so many giants in such a small piece of real estate?”

The answer may lie in the leaves, specifically those from the moonseed family. Found in abundance in the Kaiparowits, the heart-shaped moonseed leaves indicate the presence of a dense vine system in the Late Cretaceous. The presence of so much moonseed suggests that this ecosystem was fantastically dense, a salad bar for giants.

Paleontologists working the Kaiparowits hope their finds also may shed light on the greatest dinosaur mystery: their sudden disappearance.

“What was going on before that major extinction, and what happened in its aftermath?” Titus said. “How did it lead to our modern world?”