Horned dinosaurs' bony headpieces change during their lifespans - which makes identifying species difficult.
Now a study of skulls and skull fragments has shown that there are two distinct species - triceratops and torosaurus, which both lived in the same period 65 to 68 million years ago.
Skulls of Triceratops (top) and Torosaurus (bottom). Scientists had remained divided over whether skulls were older and younger dinosaurs, or two separate species
Triceratops in an African environment: Scientists now believe that Triceratops and Torosaurus were two distinct species of dinosaur
Both animals had three horns, but at different angles and the torosaurus’ neck-frill was thinner, smoother and had two holes in it.
Earlier research by John Scannella and Jack Horner at the Museum of the Rockies in Bozeman, had claimed that the triceratops was just a young version of a torosaurus and its horns changed shape as the dinosaur aged.
Such discoveries aren't out of the ordinary - in 1903 it was discovered that the brontosaurus was actually a juvenile version of an Apatosaurus, a species that had already been discovered.
The new study analysed the skull structures of triceratops and torosaurus remains - and found both young and old individuals of both species.
The research, published today in the journal PLoS One by Daniel Longrich of Yale, used numerical analysis to organise the remains into 'growth series', showing how the dinosaurs grew up.
Map showing where Torosaurus and Triceratops remains have been found in continental America
Male torosaurus drinking from a river: Researchers now believe that there are two distinct species, rather than torosaurus and triceratops being different stages in the development of the same animal
'We found that a well-defined sequence of changes exists in horned dinosaurs. Using this scheme, we identified mature and immature individuals of both Torosaurus and Triceratops.'