Stolen from this New Scientist article.
“Triceratops had three facial horns and a short, thick neck-frill with a saw-toothed edge. Torosaurus also had three horns, though at different angles, and a much longer, thinner, smooth-edged frill with two large holes in it. So it’s not surprising that Othniel Marsh, who discovered both in the late 1800s, considered them to be separate species.
Now Scannella and Horner say that triceratops is merely the juvenile form of torosaurus. As the animal aged, its horns changed shape and orientation and its frill became longer, thinner and less jagged.”

