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The skull of a T-rex.

A close-up of the skull of the Tyrannosaurus rex skeleton known as the Wankel T.rex which was installed in front of the Museum of the Rockies at Montana State University in Bozeman, Montana in 2001.

Photograph courtesy Museum of the Rockies via SI

Brian Switek

National Geographic

Published October 16, 2013

National Fossil Day, an annual celebration of all things fossil, has come around again. But not everyone is jubilant. As the government shutdown ticks on—with debate fossilized, you might say—a mighty Cretaceous carnivore has been left in limbo on the day it was supposed to be acclaimed. There is no joy in Washington, D.C., for mighty T. rex has struck out.

The dinosaur in question, fondly known as the Wankel rex, was due to arrive today, shipped off from the Museum of the Rockies in Bozeman, Montana, for a ceremonial greeting at the Smithsonian's National Museum of Natural History. That warm welcome is delayed until the spring.

Even amid the other victims of the federal shutdown, the Wankel rex truly deserved fanfare. The skeleton is one of the most important Tyrannosaurus rex fossils ever found.

To date, fossil hunters have excavated roughly 50 T. rex skeletons, ones anywhere from 5 to 80 percent complete. (That's not counting all the isolated bones and teeth that have turned up.) That's actually quite impressive, making T. rex remarkably well represented by fossil standards.

But not all of these specimens are equally important. Some are literal rock stars. The Wankel rex is one of them.

A Rock Star Is Born

Discovered in 1988 by rancher Kathy Wankel, the fossil of this imposing predator was not only large, but the skeleton also included the first complete T. rex forelimb discovered by paleontologists. Based on past finds, they had expected that T. rex had short, stocky, two-fingered arms, but the Wankel rex finally gave researchers a complete look.

Mocking the dinosaur's puny arms would have been a mistake, though. A recent study found that the Wankel rex would have weighed in the neighborhood of nine tons, making it one hefty carnivorous customer.

Some Dinosaurs Are Bigger Than Others

But which was the most impressive and important T. rex ever found? There's no shortage of candidates, and each has its own charms.

One of the first distinctive T. rex skeletons ever found is now across the Atlantic at London's Natural History Museum (NHM). Discovered in 1900 in Wyoming, and later sold to the U.K. museum during the 1960s, the partial skeleton was originally given a different name.

American Museum of Natural History (AMNH) paleontologist Henry Fairfield Osborn first called the NHM skeleton Dynamosaurus imperiosus in the same paper that he named a different skeleton Tyrannosaurus rex. When he saw his mistake and realized both skeletons were the same species, he selected T. rex as the preferred name for the animal. Where old Dynamosaurus stands in relation to other T. rex isn't totally clear. "I don't think we have an accurate length estimate, as it's pretty fragmentary," says NHM paleontologist Paul Barrett. But one of the lower jawbones of this dinosaur is on display at the museum to give visitors some idea of the animal's size.

The One and Original

Much better known is the skeleton that Osborn originally dubbed Tyrannosaurus rex. That skeleton was sold by the AMNH to the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania, in 1941 for the equivalent of $110,000 in today's dollars. As the representative fossil, it reigns as what Carnegie paleontologist Matthew Lamanna calls "the world's first specimen of the world's most famous dinosaur."

And the Carnegie T. rex is a big one. The complete femur of the dinosaur is about 4.2 feet long, and the latest analysis of the whole animal estimates that it was just over 39 feet long and weighed a little more than eight tons. "The holotype is estimated as a big, but not the biggest, known T. rex individual," Lamanna says. Still, the dinosaur will always hold the pride of place as the name-holder for the species.

Only One Can Claim the Crown

Of course, the NHM and Carnegie T. rex fossils were both early finds of important historical significance. They helped outline the image of what T. rex was like. But multiple specimens have been found since then, and they continue to be uncovered in the 68- to 66-million-year-old rock strata of western North America.

"While it is true that we are learning something from all the specimens," says University of Maryland tyrannosaur expert Thomas Holtz, Jr., "the most informative specimens have been Sue and Stan." Found in 1990 and 1987, respectively, these T. rex skeletons are the most complete found so far and nicely complement each other.

Sue, on display at Chicago's Field Museum, has become a fossilized atlas of T. rex anatomy by dint of being the most complete. And the virtue of Stan, kept at the commercial Black Hills Institute of Geological Research in South Dakota, is that the dinosaur "had a nicely disarticulated but complete skull, thus allowing us access to all sides of the various bones inside," Holtz says.

Who is the king of all T. rex is trickier to answer. "Sue does seem to be the largest one, or at least the largest one we can clearly determine the size for," Holtz says. This famous T. rex stretched approximately 40 feet long and is estimated to have weighed about nine and a half tons. But the largest T. rex may have been even bigger still.

Based on clues inside the microstructure of the dinosaur's bones, Holtz says, Sue was fully grown at the time of death. But individuals vary in how large they can get, and chances are that Sue represents the average full-grown T. rex rather than an extreme example. Given the way that animals vary in terms of size and growth, Holtz suggests that "it is very reasonable to suspect that there were individuals that were 10, 15, or even 20 percent larger than Sue in any T. rex population."

So, the biggest and baddest of the tyrant dinosaurs may yet be awaiting discovery by some lucky bone sharp.

Follow Brian Switek on Twitter.

9 comments
Charlaine Boa
Charlaine Boa

I have heard that way back then, girls were bigger than boys.  True??

Julie Neltner Reizner
Julie Neltner Reizner

@Charlaine Boa Girls are STILL bigger than boys in most species of animals - we mammals are kind of unusual.  Fish, reptiles, birds... anything that lays eggs, pretty much, the females tend to be larger.  So we can assume the same about dinosaurs, just like we'll assume the same about prehistoric fish.  Girls larger than boys seems to be the "default" for vertebrates - it's only flip-flopped in mammals due to male/male competition (physical fighting) for mates.

Large size must be clearly beneficial in order to evolve, and since females are the ones that produce the babies and feed them, they generally need more calories and so must be able to hunt larger prey.  That's why female hawks and owls can be up to 30% larger than the males!

Mohammad Abdullah
Mohammad Abdullah

@Charlaine Boa Even though that theory is popular, I've to say no, obviously. 

Julie Neltner Reizner
Julie Neltner Reizner

@Charlaine Boa ...but we haven't conclusively determined sexual dimorphism in any species of dinosaur.  The few specimens we have that we KNOW are female, due to the presence of a special bone tissue we only see in pregnant, egg-laying females today, like birds (check out Mary Schweitzer's work on a T. rex called "B-rex," in North Carolina), are quite large though.


Julie Neltner Reizner
Julie Neltner Reizner

@Mohammad Abdullah @Peo Huang @Charlaine Boa  Since it happens in all animals that the dinosaurs are closely related to, like reptiles and birds, we're kind of assuming that but exceptions could certainly occur.  We don't know, because we can't yet determine males from females except in special circumstances, like Mary Schweitzer's research I mentioned above.  Larger females is the default in vertebrates, so it's better to assume that for dinosaurs, rather than that they are like us unusual mammals.

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