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The skeleton in Dippy's closet: He's more than one dinosaur

Sunday, July 04, 1999

By Bob Batz Jr., Post-Gazette Staff Writer

Dippy gets referred to as if it's an individual, but its fossil skeleton actually is composed of several different creatures.

 
The life-size, fiberglass model of Diplodocus carnegii that the Carnegie Museum is to unveil this week was made and erected under tarps by Royal Castings International of Toronto. (Darrell Sapp, Post-Gazette) 

Look closely at these 150-million-year-old bones and you get a good lesson in science, which always is evolving.

Same with Dippy's skeleton.

Most of it is the Diplodocus unearthed at Sheep Creek, Wyo., beginning on July 4, 1899. "CM 84," as Carnegie Museum's 84th specimen was cataloged, was the most complete ever found. In fact, it became the "type specimen" -- the one J.B. Hatcher later used to identify the new species, Diplodocus carnegii.

Nonetheless, it has missing pieces. Many were filled in from a similar-sized Diplodocus that a Carnegie team found at that same site the next spring: CM 94.

But the skeleton still wasn't perfect, and so scientists kept adding bits -- such as the more complete tail of a Diplodocus (CM 307) found elsewhere in Wyoming in 1903.

Nobody had found a complete skull, but that was no surprise. As Mary Dawson, Carnegie Museum's curator of vertebrate paleontology explains, skulls are relatively light and fragile, so they tend to roll away from the rest of the remains and disintegrate. Thus, it's often hard to match a dinosaur with its correct head.

The skull that was put on Dippy's skeleton was a model based on fragments from two other Diplodocus specimens: One found by the Smithsonian (USNM 2673) and one found elsewhere in Wyoming in 1902 by Carnegie Museum (CM 662).

Turns out, CM 662 isn't even the same species -- it's a Diplodocus hayi. But this was at the time the best guess of what the skull looked like. Scientists couldn't be sure until Carnegie Museum's Earl Douglass found, in Utah, an intact skull still connected to Diplodocus neck vertebrae.

You also can view that specimen today at Dinosaur Hall, where the basement "Big Bone Room" holds shelves and boxes of spare parts.

Wanting to use real fossils rather than casts for Dippy's mounted skeleton, the museum dipped into its collection for a missing left hind foot (from another partial Sheep Creek Diplodocus).

But because the only available right forelimb was from a smaller Diplodocus,, they had to make a larger-sized model limb for Dippy.

Then they made a mistake.

Thinking they had no Diplodocus front feet, scientists modeled some based on a Camarasaurus (from the American Museum of Natural History) that then was thought to be a close relative (both are long-necked, long-tailed "sauropods"). Likewise, they used a piece of Camarasaurus as a stand-in for a leg bone.

As Carnegie collections manager Elizabeth Hill quips, "It's got to look good, so we have to have some feet on it."

Eventually, the museum obtained molds of Diplodocus feet and recently made casts that are to be put on Dippy's skeleton by next weekend's big centennial bash.

"In fact, it's going to look a little strange," says Hill, explaining how the actual feet are shorter than the previous ones. They may have to raise the ground so the feet touch it, but, "It will be accurate," including the proper number of claws.

Such changes have occurred before. Dinosaur Hall's Tyrannosaurus rex was mounted with three claws on its forelimbs, a guess based on the knowledge that Allosaurus had three. When T. rex forelimbs were found with two claws, Hill recalls, those real dinosaur experts -- children -- started calling the museum to say its T. rex was wrong.

The museum removed the extra claws -- and its three-clawed mural. But knowledge keeps changing: Eventually, it plans to remount its T. rex skeleton into a more horizontal posture.

"It's all part of an evolution of a concept," Dawson explains.

Dippy's mounted skeleton will undergo more changes, too.

While its long neck was mounted correctly -- straight out in front of the body -- the tail was mounted along the ground.

Evidence doesn't support that. At sites where footprints have been found, there have been no marks of a dragged tail.

Based on that and other clues, such as how the tail vertebrae should fit together, scientists now believe Diplodocus walked with its tail raised off the ground, as a counter-weight to its neck and head. They eventually plan to remount it that way.

As Dawson puts it, "You're constantly interpreting and reinterpreting the way things were."

That's science.

Sometimes, scientists make changes for other reasons. In the case of the new Dippy model that's to be unveiled in front of the museum, it was the fear of hangers-on.

To keep college students and others from climbing aboard and being a pain in Dippy's neck, designers raised it a bit.

"That doesn't say when he was startled or looking around that he couldn't raise his neck," Dawson says with a laugh. "We're not wrong, we're just stretching it -- and stretching his neck."



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