John H. Ostrom, a paleontologist influential in the revival of scientific research about dinosaurs, notably previously unsuspected clues to their speed and agility and their probable ancestral link to modern birds, died on Saturday in Litchfield, Conn. He was 77.
His death, from complications of Alzheimer's disease, was announced by Yale, where he was an emeritus professor of geology and geophysics and emeritus curator of vertebrate paleontology at the Peabody Museum of Natural History.
Although Dr. Ostrom had long since withdrawn from the field work of fossil hunting, two of his early discoveries had a profound effect on dinosaur research in the last half of the 20th century, shattering stereotypes and inspiring sweeping changes in thinking about their lives and times. His work attracted many young researchers to dinosaur studies, a field that had been moribund for several decades.
Dr. Ostrom's first important discovery was made late one afternoon in August 1964. While tramping along a slope in central Montana, Dr. Ostrom and an assistant, Grant E. Meyer, came upon a macabre sight: large and sharp claws reaching out of an eroded mound. ''We both nearly rolled down the slope in our rush to the spot,'' Dr. Ostrom recalled later.
They uncovered the rest of a powerful, three-fingered grasping hand and then a foot. The inner toe stuck out like a sharply curved sickle. After further research, Dr. Ostrom determined that the claws and feet belonged to a fleet, predatory dinosaur that lived 125 million years ago. He gave it the name Deinonychus, meaning ''terrible claw.''
In a research report, Dr. Ostrom described this dinosaur as a raptor, an active predator that killed its prey by leaping and slashing with its fierce claw. Such behavior, he suggested, meant that the animal had a high metabolism rate and was warmblooded.
This interpretation stimulated a polarizing debate among scientists over the revolutionary idea that at least some dinosaurs, like Deinonychus and related Tyrannosaurus rex and Velociraptor, had more in common with mammals and birds than with ordinary coldblooded reptiles. This in turn led to the revival of a 19th-century hypothesis that dinosaurs were direct ancestors of today's birds.
Dr. Ostrom became a leading proponent of the dinosaur-bird link after he made his second significant discovery, in a museum at Haarlem in the Netherlands.
On a visit there in 1970, he saw a fossil specimen identified as a pterosaur, a gliding reptile, that to him did not look like a pterosaur. It came from the same Bavarian quarries that had yielded other fossils of Archaeopteryx, a curious mix of dinosaur and bird characteristics and generally accepted as the earliest known bird, from about 150 million years ago. The rediscovery prompted Dr. Ostrom to study the evolution of birds and bird flight.
“Ever since I've had to give up field work,'' Dr. Ostrom told a reporter years later, ''I've said the best discoveries are made in museum storerooms.''
Dr. Ostrom, a soft-spoken, scholarly professor, found himself in the middle of stormy controversy. ''Warmblooded'' dinosaurs became popular subjects in magazines and books. Over time, he put some distance between himself and some of his younger, more outspoken allies. The issue has yet to be resolved, though many experts agree that some dinosaur behaviors are most unreptilian.
But discoveries in China and Mongolia in the last decade seem to support the hypothesis of a close dinosaur-bird relationship. Dr. Ostrom led a delegation of scientists to examine a feathered dinosaur excavated in China in the early 1990 's.
''I never expected to see anything like this in my lifetime,'' he said after the trip. ''I literally got weak in the knees when I first saw photos. The apparent covering on this dinosaur is unlike anything we have seen anywhere in the world before — quite different from modern feathers or hair, but also different from the skin of other dinosaurs.''
John Ostrom was born in New York City and grew up in Schenectady, N.Y. As an undergraduate at Union College there, he prepared himself for medical school, but an elective course in geology changed his life. When the lectures turned to paleontology and he read a book on evolution, he decided to be a paleontologist.
Karen Ostrom, a daughter, said she once saw his college transcript. ''When he was a pre-med student, his grades were only average,'' she said. ''After the change, he made straight A's.''
After he earned a doctorate in geology and paleontology at Columbia in 1960, he joined the faculty at Yale, where he remained for the rest of his career. He retired in 1992 but continued his research and writing there, mainly on theories of the origin of flight, until his health failed.
He organized and led fossil-hunting expeditions to Wyoming and Montana throughout the 1960's. He was a longtime editor of The American Journal of Science, organizer of an international conference on Archaeopteryx, and fellow of the American Academy of Arts and Sciences.
Dr. Ostrom's wife, the former Nancy Hartman, died three years ago. In addition to his daughter Karen, of Goshen, Conn., his survivors include another daughter, Alicia Linstead of Larkspur, Calif., and three grandchildren.
He is also survived by a generation of former students and other paleontologists influenced by his discoveries and interpretations of dinosaurs, birds and early flight. At the Peabody Museum, on the Yale campus, stands one of his prized legacies: the reconstructed skeleton and a fleshed-out model of Deinonychus. The museum boasts that the creature Michael Crichton called Velociraptor, the terror of the book and movie ''Jurassic Park,'' is ''really our own Deinonychus parading around under an assumed name.''
In 1999, Dr. Ostrom presided over a symposium in his honor at Yale. Former students and other scholars spoke of feathered dinosaurs and concluded with a tribute to their teacher and colleague. They hailed Deinonychus as ''one of the most famous dinosaurs of all time'' and told Dr. Ostrom, ''You have led the renaissance in thinking about dinosaurs and have revolutionized our concept of them.''
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