The Dinosaur That Peacefully Grazed

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.