The dinosaur hall first opened in 1911, when standards for handling fossils were very different. Many specimens, especially those acquired in the early years, weren’t fully excavated. Others were drilled, touched-up, or embedded in plaster or concrete. So for the museum’s preparators, the act of renovating the hall and getting its precious inhabitants up to modern standards is much like organizing a new dig. They are effectively performing re-paleontology, excavating their own dinosaurs from the museum itself.

The process began in April 2014, and it took just over a year to dismantle the old exhibits. Even when fossils were free-standing, they still had to be removed from the supporting metal brackets that acted as skeletons for their skeletons. Every piece was photographed and documented, “just like when you’re researching in the field,” says Starrs. Each fragment was cleaned, tested for cracks or weaknesses, retouched, and housed in bespoke plaster jackets or padded crates.

This isn’t just an aesthetic exercise. Some of the half-prepared dinosaurs are type specimens—the ones that were first used to name and describe their species. These are incredibly important individuals, avatars for their entire kind. They’re the ones to which scientists repeatedly return. But that’s hard to do when a specimen is half-immersed in plaster and stuck on a high wall, as was the case for the Smithsonian’s shovel-snouted Edmontosaurus. “Not only could visitors not get to it, but researchers couldn’t either,” Johnson says. “It does need to be liberated.”

In some cases, the renovation team has made new discoveries. The type specimen of Thescelosaurus, a human-sized plant-eater, had only been prepared on one side; when Starrs’ team exposed the other, they found beautifully preserved tendons and cartilage on its ribs. “Even the curator didn’t know they were there,” she says.

Similarly, the 80-foot-long Diplodocus, which was collected in 1887 and has long stood in the middle of the hall, still had bits of rock around its skull. When the team chipped that away, “suddenly all these amazing pencil-like teeth emerged in its face that weren’t there before,” Johnson says. “It’s a fantastic fossil. They just never finished excavating it.” In the process of exposing the teeth, the preparators also found more fossils—bits of plants, pollens, and other tiny fragments. All of those had to be photographed and described, too.

Once fully prepared, the fossils will be mounted onto new brackets. These are being designed so that individual bones can be snapped in and out, like “when you click a gemstone into the setting on a ring,” says Johnson. That will make them far easier to disassemble in the future, or to use in research. Some of these mounts are being built in the museum’s basement; others, for the larger specimens, are being crafted by the Toronto-based company Research Casting International. They’re one of just three big companies in the dinosaur-building game; a fourth recently went extinct.