When is a triceratops not a triceratops?
The answer is not when it is a Torosaurus! It was once thought that triceratops and torosaurs were different creatures, but research published last year (2010) is causing a re-think.
The research published by Scannella and Horner (2010) shows that the skull of the triceratops changed shape quite dramatically as it grew. What were once thought to be two different types (genera) of dinosaur are in fact the same species. As triceratops got older the shape of its skull changed to be that of what was until now called a torosaur. Although at first glance they are very different. Triceratops had three facial horns and a short, thick neck-frill with a saw-toothed edge. Torosaurs also had three horns, though at different angles, and a much longer, thinner, smooth-edged frill with two large holes in it.
How did they work out that these creatures are the same species? Dinosaur bones have growth lines (something like tree rings) and by counting these lines it is possible to estimate the age of the dinosaur when it died. By studying 29 triceratops skulls and nine torosaur skulls and counting the growth lines on each they were able to show that the skulls changed from triceratops to torosaur as they got older. So in fact torosaur is an old triceratops and the number of dinosaur species just got smaller.
And this is not just an isolated case. As it appears that a similar thing happens with pachycephalosaurus, the juveniles of which were previously called Dracorex and Stygimoloch.
This is also the case for that most famous of dinosaurs, T. Rex, which is an "older version" of Nanotyrannus.
This work has possible implications for evolutionary accounts of dinosaurs. Indeed it is possible that evolutionary transitions are nothing of the sort, they could represent different stages in the life of one species rather than an evolutionary divergence.
We would suggest that this new evidence means that the case for dinosaur evolution just got weaker.
John B. Scannella and John R. Horner, Journal of Vertebrate Paleontology, Volume 30, Issue 4, July 2010 , pages 1157 – 1168.