The arm was found first, by a hiker in a rugged section of a Montana wildlife refuge. The body had been frozen in time — and rock — for ages, stuck in a death pose for posterity.

When paleontologists finished excavating the old bones, they had recovered one of the most complete Tyrannosaurus rex skeletons, a major specimen that is coming to the Smithsonian’s National Museum of Natural History on a long-term loan.

The museum announced Thursday that it will borrow the T. rex for 50 years from the Army Corps of Engineers, which owns it, and the state of Montana, which has had it since the late Cretaceous Period.

The big beast — named the Wankel T. rex after Kathy Wankel, the rancher who made the prehistoric find — will be trucked to the Mall for National Fossil Day on Oct. 16, then put on temporary display until the museum’s dinosaur exhibit closes for a $48 million renovation next spring. Eventually, the 35-foot-long skeleton will be mounted in a lifelike pose in the new dinosaur hall when it opens in 2019.

The trip will end the Smithsonian’s long, frustrating search for the king carnivore. It will also add considerable heft to the Natural History Museum’s collection: The Wankel T. rex will surpass just about every one of the roughly 127 million specimens and artifacts held by the world’s second-most-visited museum.

The current exhibition of dinosaurs is seen at the Smithsonian National Museum of Natural History, which is acquiring the bones of a Tyrannosaurus rex on a 50-year loan from the U.S. Army Corps of Engineers. (Sarah L. Voisin/The Washington Post)

“It will be one of our most important and iconic objects,” said Kirk Johnson, the museum’s director. The Hope Diamond remains the crown jewel of the collection. But, Johnson said, a natural-history museum is nothing without dinosaurs, and no dinosaur captivates people quite like Tyrannosaurus rex.

“If you stand next to a real T. rex, it is just an awesome experience,” he said. “Their teeth are the size of bananas. Their skulls are huge. They’re one of the great predators of history. They’re impressive in size, scale — everything. Just imagine an animal that big, that awesome, alive.”

The Wankel T. rex — estimated to have weighed six to seven tons — died in a riverbed near the eventual site of Fort Peck Reservoir. By the time Wankel stumbled upon the first lower-arm bones of a T. rex ever found, the land was controlled by the Corps. Thus, the Corps owns the skeleton, although the fossils have been conserved, studied and displayed at Montana State University’s Museum of the Rockies in Bozeman.

That the Corps had a T. rex to lend was news to many of its senior leaders. “They didn’t know we had a dinosaur,” said Sonny Trimble, who oversees curation and management of archaeological collections for the Corps. People transfer, he said. Many retired. So “the chief engineer doesn’t wake up in the morning saying, ‘How’s our dinosaur doing?’ ”

In fact, the Corps has two: A T. rex — known as Peck’s Rex — was found near Fort Peck in 1997. It, too, is at the Museum of the Rockies, where it will go on display.

When Corps leaders learned that the Natural History Museum was interested in borrowing the Wankel T. rex, Trimble said, they were happy to oblige.

The Wankel T. rex is currently crated and stored in a warehouse in Montana. (Secrecy abounds, given the sky-high prices the bones would fetch in the shadowy commercial fossil market.)

At the Museum of the Rockies, the staff has been planning to say farewell to a very old friend.

“Some people are sad about it leaving — it’s kind of like seeing your beloved kid go off to college and into the real world,” said Shelley McKamey, the museum’s executive director, who helped excavate the Wankel T. rex, which is also known as MOR 555. “You’re going to miss him, but you’re also really proud of him. . . . We’re happy to share the best we’ve got with the world.”

The Smithsonian already has a T. rex, sort of. One recent morning, a young boy entered the dinosaur hall and walked straight to the life-size replica.

Its pose suggested that it was walking and just beginning to crouch — “and presumably about to inflict mayhem on somebody,” said Hans-Dieter Sues, the Natural History Museum’s curator of vertebrate paleontology.

“Look at the size of it!” the boy said. “Whoa.”

His father nodded, then read the sign near the dinosaur: “They found this in South Dakota.” He did not mention that it was a cast made from “Stan,” a T. rex found on private land in the Hell Creek Formation in the 1980s.

Many of the museum’s more than 7 million annual visitors don’t realize it’s a replica (even though it says so on the sign), or they don’t care, officials said.

So, why the years-long obsession over getting a real specimen?

“Think about what the museum is,” said Johnson, the museum’s director. “It’s a place where real treasures of the natural world are on display. If I said I have a glass replica of the Hope Diamond, you’d be less impressed. . . . It’s really important for us to have a real object for people to see and experience and be amazed by.”

Fewer than 50 T. rexes have been found — and only about a quarter are considered “nearly complete,” meaning that more than half of the bones were retrieved, Johnson said. Between 80 and 85 percent of the Wankel T. rex bones were collected, making it the fifth- or sixth-most complete T. rex skeleton in existence.

It was found just before Labor Day in 1988. The Western states were dried out by a historic drought, and Yellowstone National Park had been burning for most of the summer.

Kathy and Tom Wankel were camping with their family near the reservoir ,and their three kids were with an uncle. They walked across what was once a bay to what was once an island, and Kathy Wenkel was looking down — she was a rockhound and knew that there could be dinosaur bones around. She wasn’t from the area, but she’d seen bones before at a local bar.

She saw something — “like just a corner of an envelope sticking out,” she recalled. “Tom said, ‘Hey, I think I’m finding some bone stuff down here.’ And I said, ‘No, you’d better come up here.’ We started chiseling away around the bone and didn’t get it all dug, so we covered it up and came home.”

When the Wankels returned, weeks later, they uncovered the bones of . . . something. They took the fossilized bones to their camper. That night, a violent thunderstorm pounded them. “It was almost like the gods were saying, ‘Don’t take them out,’­­ ” she said.

But they did, and that Thanksgiving, the family drove them to the Museum of the Rockies.

The museum’s chief “preparator of paleontology,” Patrick Leiggi, was smoking a Marlboro, Kathy Wankel said. When he saw what they had in their station wagon, “he really got puffing. He said, ‘You guys better follow me.’ We went downstairs to their lab, and pretty soon people were coming out of their offices, and they were all puffing on Marlboros and talking.”

The paleontologists asked the Wankels whether they’d take them to the site the next year, and when they did, they located the golden prize — the skull — along with some of the vertebrae. Because the bones were penetrating the hillside, they had to return in 1990 to finish the dig with a larger crew and some heavy machinery to move the over-dirt.

The Wankels knew that they’d found the bones on federal land and that the dinosaur belonged to the Corps. “We laugh, because our ranch is about 50, 60 miles away,” she said, “and if he would have just taken some giant T. rex steps as he was dying and staggered down here, we could have paid off our ranch mortgage.”

She laughed and then remembered something about the aftermath of her discovery.

“We got a hand-slap letter from the Army Corps of Engineers,” she said. “They said, ‘You did the right thing by bringing it to the museum and not taking it in the middle of the night and selling it to the Japanese or something. But you weren’t supposed to be digging it out.’ We didn’t know.”