The Nation’s T. rex rolls into Washington from Montana to a dinosaur-loving crowd at the Smithsonian National Museum of Natural History. And the Wankel family, who discovered the fossil in 1988, says goodbye to their ‘baby.’ (Lee Powell/The Washington Post)

The FedEx truck arrived before daybreak Tuesday, backing up to the loading dock at the Smithsonian’s National Museum of Natural History.

The police escort — which rolled with the semi from Hagerstown, Md., more than an hour from the Mall — indicated a very special delivery. So, too, did the custom wrap on the trailer. “Delivering history: The Nation’s T. Rex,” it said next to a painted Tyrannosaurus rex.

The first of 16 wooden crates — each sealed with tape and marked “FRAGILE” multiple times — rolled off the 53-foot-long trailer at 6:26 a.m. A few minutes later, museum director Kirk Johnson signed for the delivery of a dinosaur. Within a half-hour, all of the bone-filled boxes were being inspected inside the museum, which had finally filled the T. rex-sized hole in its fossil collection.

“It’s like Christmas morning,” declared dinosaur curator Matt Carrano.

The Nation’s T. Rex, discovered by a Montana rancher near Fort Peck Reservoir in 1988 and owned by the Army Corps of Engineers, is on loan to the Natural History Museum for 50 years.

The National Museum of Natural History welcomes the Wankel T. rex.

It will replace a life-size T. rex replica that’s been displayed in the Natural History Museum’s dinosaur hall since 1999, shortly after the Smithsonian lost out on Sue, the world’s most famous dinosaur fossil, at a hotly contested auction.

“The absence of a T. rex was unforgivable,” Johnson said, standing among the crates. “It’s this amazing dream for me to get this iconic fossil.”

“It’s a wonderful day for the nation,” said Lt. Gen. Thomas P. Bostick, the Army Corps commander who signed off on the long-term loan.

The world’s second-most-visited museum has big plans for the borrowed king carnivore: It will stand as the centerpiece of the new dinosaur hall that’s scheduled to open in 2019, after a five-year, $48 million makeover. The hall — one of the most visited spaces at the Natural History Museum — closes April 28.

“It’s an amazing object,” Johnson said of the T. rex.

The 38-foot-long dinosaur died more than 66 million years ago in a riverbed and was frozen in time — and rock — for ages. It remained unseen and undisturbed from the late Cretaceous Period until around Labor Day in 1988, when rancher Kathy Wankel spotted a small part of an arm bone during a day hike in a wildlife refuge.

Wankel and her husband, Tom, eventually brought their find to the Museum of the Rockies in Bozeman, Mont., where the chief “preparator of paleontology,” Patrick Leiggi, began furiously puffing his Marlboro Light upon seeing the prehistoric find.

Eventually, an excavation team unearthed between 80 and 85 percent of the bones on the Army Corps site, making it one of the five most complete T. rex specimens ever recovered. It included the first T. rex arm ever discovered (“not much bigger than my arm,” Johnson noted), and it was dubbed “the Wankel T. rex” by the Museum of the Rockies.

“To see it come to the National Museum is pretty exciting,” Leiggi said Tuesday, standing between a polyurethane cast of the Wankel T. rex skull, and Kathy and Tom Wankel and 24 of their relatives — 25, if you count the T. rex.

“We’re very proud . . . parents,” Kathy Wankel said. “Big, ugly baby. [But] we think he’s beautiful.”

“Kind of old, too,” her husband said.

Tuesday, after the seal on the trailer was broken and the dinosaur was signed over to the Smithsonian, the museum began to reveal the old bones.

First came the T. rex’s right femur, which was enormous — roughly the size of an elementary school student and weighing at least 100 pounds.

Then, out of a box marked “WOW,” came components of the skull: the left maxilla and right dentary (roughly, the cheek and lower jaw). They contained massive banana-shaped teeth that, Johnson said, will help the museum win over children for years to come. “Dinosaurs are the gateway drug to science for kids,” he said.

Adults can’t resist meat-eaters, either. Shortly after the beast’s bones were introduced, #NationsTrex began trending on Twitter.

Once the fossil goes on display in 2019, hundreds of millions of people are expected to see it over the course of the 50-year loan.

For the time being, the bones are in an exhibition space dubbed the “Rex Room,” near the museum’s rotunda, where Smithsonian conservators, scientists, 3-D imaging technicians and others will clean, repair, reinforce and record each piece between now and National Fossil Day on Oct. 15, after which the specimen will be sent to Toronto for mounting work.

On Tuesday morning, crowds heavy on families with spring-breaking kids streamed past the Rex Room, peering through two metal gates for a glimpse of Washington’s new rock star. Most of the fossilized bones still were in their protective cradles inside the crates, so four skull casts — of a T. Rex and some related species — generated most of the oohs.

A sign outside the Rex Room announced, “Some assembly required.”

Good work if you can get it, said Carrano, the dinosaur curator. “This is a once-in-a-lifetime thing.”