CLEVELAND — A new dinosaur species was a plant-eater with yard-long horns over its eyebrows, suggesting an evolutionary middle step between older dinosaurs with even larger horns and the small-horned creatures that followed, experts said.

The dinosaur’s horns, thick as a human arm, are like those of triceratops — which came 10 million years later. However, this animal belonged to a subfamily that usually had bony nubbins a few inches long above their eyes.

Michael Ryan, curator of vertebrate paleontology for the Cleveland Museum of Natural History, published the discovery in this month’s Journal of Paleontology. He dug up the fossil six years ago in southern Alberta, Canada, while a graduate student for the University of Calgary.

“Unquestionably, it’s an important find,” said Peter Dodson, a University of Pennsylvania paleontologist. “It was sort of the grandfather or great-uncle of the really diverse horned dinosaurs that came after it.”

Mr. Ryan named the new dinosaur Albertaceratops nesmoi, after the region and Cecil Nesmo, a rancher near Manyberries, Alberta, who has helped fossil hunters.

The creature was about 20 feet long and lived 78 million years ago.

The oldest known horned dinosaur in North America is called Zuniceratops. It lived 12 million years before Mr. Ryan’s find and also had large horns.

That makes the newly found creature an intermediate between older forms with large horns and later small-horned relatives, said State of Utah paleontologist Jim Kirkland, who with Douglas Wolfe identified Zuniceratops in New Mexico in 1998. He predicted then that something like Mr. Ryan’s find would turn up.