The dinosaur known as triceratops didn't always have the distinctive triple horns that gave it its name, researchers say; in fact, they've determined, those horns took millions of years to evolve.

Studying more than 50 triceratops skulls unearthed from a well-known fossil site located in Montana, researchers from Montana State University discovered older skulls from deeper layers of the site has much shorter horns that found on skulls from later periods.

The findings show triceratops -- the name is from the Greek for 3-horned face -- evolved their horns slowly over a long period of time, the paleontologists said, taking one to two millions years to evolve from Triceratops horridus -- the older form -- to Triceratops prorsus, the variety known for is well-developed triple horns.

Adult triceratops could be 30 feet long and weight 5 tons, displaying their iconic bony head decoration that covered it neck and their horns, one over each eye and a smaller one over its parrot-like mouth.

The researchers said finding the large number of skulls at the Hell Creek site in Montana allowed them to track the development of the giant creatures' facial anatomy.

"Over one to two million years at the end of the Cretaceous Period, Triceratops went from having a small nasal horn and long beak to having a long nasal horn and shorter beak," they reported in the Proceedings of the National Academy of Sciences.

As individual triceratops matured, the structure of its skull and horns changed, with horns growing to 3 feet and becoming curved as the creatures reached sexual maturity, the researchers also reported.

"The new study finds evidence that not only did Triceratops change shape over the lifetime of an individual, but that the genus transformed over the course of the end of the age of dinosaurs," study leader John Scannela said.

Researchers say they believe the horns were used as defensive weapons in encounters with predators such as T. Rex while the hard, bony head frill would protect the triceratop's vulnerable neck area.

Healed wounds displayed in some fossil skulls and frills also suggest triceratops may have fought among themselves in competition for territory, leadership or mates, they say.

Much is known about triceratops because of the large number of fossils uncovered since the first one was discovered in 1887,, unlike some other dinosaurs that are known from just a few fossils or even a single example, Scannella said.

'The great thing about Triceratops is that there are a lot of them, and they were found at different levels of the Hell Creek Formation," he said.