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The massive horns of Triceratops evolved over more than a million years, a study shows.

A study of 50 skulls dug up at the Hell Creek Formation - a famous fossil site in Montana - found the beast underwent change over a very long period of time.

Older fossils lying deeper in the rock had shorter horns than the younger specimens nearer the surface, say scientists.

This Triceratops fossil came from the middle of the Hell Creek Formation. It is displayed at MSU's Museum of the Rockies. Older fossils lying deeper in the rock had shorter horns than the younger specimens nearer the surface, say scientists.

The findings published in Proceedings of the National Academy of Sciences suggest Triceratops horridus took between one and two million years to evolve into Triceratops prorsus.

In the past it was believed the two species might have existed at the same time.

Triceratops, which lived 65 million years ago, was a giant plant-eating creature best known for its great bony head frill and three horns - one above each eye and a small stubby one above its beak-like mouth.

An adult could grow to about 30ft long and weigh five tons.

Dr John Scanella, of the Museum of the Rockies, and colleagues said: 'The placement of over 50 skulls of the well-known horned dinosaur Triceraptops within a stratigraphic framework for the Upper Cretaceous Hell Creek Formation reveals the evolutionary transformation of this genus.'

Triceratops - Greek for 3-horned face - has been described as a reptilian knight that carried two lances and a shield on top of its enormous head.

A Triceratopcs prorsus fossil came from the top of the Hell Creek Formation. It has a long nasal horn and shorter beak is a Triceratops prorsus.

The horns grew up to 3 feet long and the forward curvature marked their owners as sexually mature.

The bony frill changed with age as well.

In juveniles the edge was adorned with triangular bones resembling arrowheads.

These flattened as the animal got older and were barely visible in adults. It had a huge, barrel-like body and short tail.

It's believed Triceratops used its horns as weapons against predators like T rex and also for mate attraction - much like colourful feathers in birds.

Some Triceratops specimens show healed wounds in their skull or frill - showing they fought among themselves. They may have fought over females, territory or leadership.

This image shows the evolution of the Triceratops in the Hell Creek Formation of Eastern Montana

It had a very long and powerful beak.

Each jaw had closely packed teeth with a broad grinding surface. With its scissor-like beak and grinding teeth Triceratops was able to bite off and chew even the toughest plants.

Dr Scanella said: 'Our research indicates the currently recognised species of Triceraptops - T horridus and T prorsus - are stratigraphically separated and the evolution of this genus likely incorporated transformational change.

'These findings impact interpretations of dinosaur diversity at the end of the Cretaceous and illuminate potential modes of evolution in the Dinosauria.'

Over the course of the project, the researchers found that Triceratops is the most common dinosaur in the Hell Creek Formation.

The Museum of the Rockies now houses one of the largest collections of Triceratops in the world.

'Most dinosaurs are only known from one or a handful of specimens,' Scannella said.

'Some dinosaurs are known from a large number of specimens, but they're often found all in one place – on a single stratigraphic horizon.

'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.

'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.'