Ghost from the dawn of the dinosaurs.

Wading on all fours through streams and swampy lowlands, a 200-pound dinosaur the size of an ostrich met its death 225 million years ago. Its skeleton, recently discovered in Arizona's Petrified Forest National Park, was described last week by paleontologists.

Of the convincingly dated specimens, "this is the oldest...dinosaur skeleton in the world," says Robert Long, the leader of the group of University of California at Berkeley paleontologists working on the Arizona site. He and his colleagues believe the find represents a type of dinosaur never before described, although they think it lived too recently to represent a missing link between the two major branches of dinosaurs.

The ancient dinosaur bones were found late last summer near a popular Painted Desert vantage point named Chinde Point. Because "Chinde" is a native American word for ghost, Long says he may name the newly discovered dinosaur Chindesaurus.

The site of the 225-million-year-old skeleton, a layer of the desert's silty clay, is embedded with dozens of bones. "We might have a whole skeleton," Long says. "Right now it looks really good."

One complete hind leg has already been examined. All its joints and bones, including delicate toe bones, are in place. The number of thigh bones found indicate that the site actually contains more than one skeleton of the species.

The bones examined so far are "very distinctive," Long says, "with a combination of functions I've never seen in a dinosaur before. It seems to be a heavy-boned animal resembling a plateosaur, but there are...characteristics that are confusing."

The date of the dinosaur was established by the known dates of ancient leaves, spores and pollen found mingled with the bones. The age of the ancient plant material was determined from European sites associated with volcanic ash, which is dated by the decay of radioactive potassium to argon. The dinosaur of the Petrified Forest is the first to be convincingly dated back to the time known as the Carnian stage of the Triassic period. The peak of the dinosaur age was much later, about 65 million years ago.

This specimen is one of more than two dozen ancient reptile skeletons that have been found in the national park. The site provides "by far, the best picture anywhere in the world" of the dawn of the dinosaur age, Long says. The dinosaur fossils there are better preserved, more complete and more extensive than others that might be from that era.

The scientists now plan to cut around the siltstone that encases the dinosaur skeleton and pour a plaster casing around the block. Then it will be lifted by helicopter, loaded onto a truck and taken to the U.C.-Berkeley Museum of Paleontology. Long estimates it will take three to six months to clear the rock away from the skeleton once it reaches Berkeley. In about a year, after the specimen has been officially described, it will be returned to the museum at the Petrified Forest.

The additional study should clarify the evolutionary position of the newly discovered skeleton. On the question of whether the animal could be a common ancestor of the two major lines of dinosaur, Long says, "It's possible, but until we get the skeleton cleaned up it is difficult to say. My feeling is it is too late in time and the skeleton is too specialized." Although this is a very early dinosaur, he would expect a common ancestor to be at least 10 million years older.

"This isn't the ancestral stock," agrees Long's colleague Michael Greenwald of Berkeley, "just another page in the book pushing back the ancestry of dinosaurs another 5 million years."