Looooooong time passing....
Seems like some of them were never here to begin with. Over the years, scientists have named about 700 different species of dinosaurs. But a recent study indicates that perhaps as many as a third of these were phantoms—specimens that were given distinct names despite actually belonging to another, well-known species.
For example, Torosaurus is now thought to be just a fully mature version of Triceratops. At the other end of the age scale, Nanotyrannus is considered by some to be just a juvenile form of the famous Tyrannosaurus rex.
Why the changes? Well, identifying species is hard, even under the best of circumstances. With fossils, it’s especially tricky. You often only have one specimen to study, not dozens or hundreds as with living creatures. You can only see the fossil’s bones, not the full creature. And, most important, you only have the dead body—you can’t watch the living creature to see how it changes as it grows. (Dinosaur bones, it seems, are extremely malleable and prone to change shape as the creature matures.)
But don’t be too hard on the poor paleontologists. Other scientists have this same problem. Last year, it was reported that over 30% of all living marine creatures had been misidentified, and for the same reasons. An individual (or small group) was slightly larger than normal, or slightly smaller, or a slightly different color, or came from a different location—enough to lead the scientist to classify it as a new species, when in fact it was already a member of an established species. If taxonomists can make that many mistakes with living creatures, we shouldn’t be surprised that the dinosaur family tree will need a little pruning.