PHOENIX — Call it Triassic Park.

The National Park Service will sign a deal Thursday greatly expanding the boundaries of Petrified Forest National Park in northeast Arizona, where relatively small bipedal dinosaurs roamed hundreds of millions of years ago and their fossilized remains have been discovered in the stark, rainbow-hued landscape.

In acquiring 26,500 acres of private ranchland that has long been the envy of archeologists and paleontologists, the park will increase its boundaries by roughly a quarter as well as increase opportunities for both researchers and visitors to peer back into the Late Triassic period, which preceded the Jurassic period, during which giant dinosaurs thrived.

“The opportunities to find things new to science are pretty high,” said William G. Parker, the park’s paleontologist, noting that past expeditions by the Smithsonian, the American Museum of Natural History, the University of California, Berkeley, and the University of Texas at Austin have led to significant finds of plant and animal fossils on the acquired property.

Image A fossil found at the park. Credit... National Park Service

With more than 600,000 visitors a year, the park’s oddly shaped rock formations, in colors ranging from red and pink to green and purple, provide a firsthand glimpse of the ancient landscape of a very different earth.