The Dinosaur That Peacefully Grazed
Perfectly Adapted Creature Kept Its Head Down, Got New Teeth Once a Month

By David Brown
Washington Post Staff Writer
Friday, November 16, 2007

Could an elephant-size dinosaur with a skull so thin that a karate chop would have split it in two, teeth it shed once a month and a brain that, yes, was the size of a walnut, ever be considered one of evolution's success stories?

Paul C. Sereno thinks so.

The University of Chicago paleontologist yesterday unveiled Nigersaurus taqueti, a strange creature that is helping rewrite theories about how long-necked, plant-eating dinosaurs looked and behaved.

Nigersaurus appears to have spent a lifetime with its head in a hangdog position. Using a broad, tooth-filled mouth, it grazed on ferns and horsetails growing at most a couple of feet high. It couldn't even raise its head to horizontal. Getting at trees was out of the question.

Many other dinosaurs -- including the more famous and less bizarre Diplodocus -- probably behaved similarly, using their long necks as ground-mowing booms, not treetop cherry pickers, Sereno believes.

"It took an extreme dinosaur to open our eyes to this cow-like behavior," he said yesterday at the National Geographic Society's headquarters in the District, where a reconstruction of Nigersaurus was mounted. "It is sort of silly to think that something wasn't doing this. But we had missed the cows of the Mesozoic."

Other paleontologists agreed that the new dinosaur will further dispel the notion that long-necked dinosaurs were the prehistoric equivalent of giraffes, holding their heads high overhead.

"It would be hard to imagine a more compelling argument against" that view, said Kent A. Stevens, a computer scientist at the University of Oregon who has done extensive research on dinosaur posture.

Matthew T. Carrano, the curator of dinosaurs at the Smithsonian Institution, doubts there will be many arguments against a bovine dinosaur. There have been too many other strange-but-true discoveries in recent years.

"What we are seeing is something that we just didn't know about before. We are coming to accept the fact that this is going to be a regular thing for us," he said.

Sereno, who is also an explorer-in-residence at the National Geographic Society, discovered Nigersaurus in the Sahara in the North African country of Niger in 1997.

It was excavated over three years by a large team that included two paleontologists from the University of Niamey, in Niger, who attended yesterday's unveiling of a life-size model. Parts of five animals were found, with the skeleton 80 percent complete.

Sereno and his collaborators used CAT-scan technology to build models of what was inside Nigersaurus's skull. When they reconstructed the inner ear, which contains the semicircular canals that provide the sense of balance, they were forced to conclude that the dinosaur's normal head position was closer to straight down than horizontal.

"I think it could have gotten its muzzle 45 degrees forward. But that is not a very comfortable position for gnawing at bark," Sereno said.

Stevens agrees. But he doesn't think that rules out some long-necked dinosaurs browsing the tree-tops, thanks to their greater height and more favorable head and neck anatomy.

"We need to be careful to not extrapolate overly from Nigersaurus," he said.

A scientific paper describing the new dinosaur was published yesterday in the Public Library of Science's online journal PLoS ONE. The animal will also be featured in the December National Geographic Magazine.

Nigersaurus has extreme versions of several evolutionary adaptations seen in other grazing animals. For example, many grazers have broad faces and prominent teeth at the front of the mouth so they can take in lots of food with each bite. This is necessary, because grass and other ground plants tend to have low nutritional value.

Nigersaurus's mouth was wider than its skull -- it bears a remote resemblance to a platypus -- and is the only terrestrial animal with that feature. Its teeth, all incisors, were in a single line just behind the lips and far from the muscles at the back of the jaw. They exerted little force, Sereno believes, and were good only for clipping soft vegetation.

But this was not the only strange dental feature.

The teeth were in a single deep groove, not individual sockets, and were locked to each other. Microscopic examination of daily growth rings suggests they lasted about 35 days before they all fell out at once and were replaced by a row erupting from below and behind.

A few other types of dinosaurs had similar "tooth batteries," but Nigersaurus's was the largest and had the quickest turnover rate, Sereno said. X-ray images of one skull revealed nine rows of replacement teeth moving like a conveyor belt.

Nigersaurus also represents an extreme in the evolution of thin bones seen in some other large dinosaurs. The skull is translucent in places, and parts of the spine have window-like holes that lightened the overall weight of the skeleton.

Although the bones are impressively thin in places, "we can be assured that it certainly doesn't have less than is needed," Carrano said.

Also impressively lightweight was Nigersaurus's brain. The cerebrum, or thinking part, was the size of a walnut. But its life was simple, too: endless eating, with a little copulation and crocodile-avoidance on the side.

The Nigersaurus bones, on display at the society's headquarters, will eventually be repatriated to Niger. Sereno said he is raising money for the construction of a dinosaur museum there.