Paleontologists John Scannella and Jack Horner from the Museum of the Rockies in Bozeman, Montana have determined that the triceratops and torosaurus were not separate species, but the same species at different maturity levels. They examined several skulls of both, and found that the triceratops skulls, by order of age, transformed into torosaurus skulls. The shield-like frills on juvenile triceratops were not defensive bony structures, but were spongy radiators that also served as a display of maturity, eventually thinning and spreading out into the frill of the adult torosaurus. Because of their similarity, some paleontologists in the 1970s had proposed that they were the same species, but there weren't enough fossils to confirm it then.
New Scientist article.