When 2 dinosaurs become 1

Prepare to have your mind blown.
Certain dinosaurs—physically disparate enough that we've always thought of them as different species—may actually be the same animal at different stages of its life cycle. Also: Those big, protective-looking bone formations surrounding some dinos' heads and necks probably weren't all that useful as a defense against predators.
Case in point, triceratops. Or, maybe we should be calling it torosaurus now, I'm not sure. See, according to research done by scientists at Montana's Museum of the Rockies, the familiar triceratops is really just the juvenile form of the more-elaborately be-frilled and be-horned torosaurus.

This extreme shape-shifting was possible because the bone tissue in the frill and horns stayed immature, spongy and riddled with blood vessels, never fully hardening into solid bone as happens in most animals during early adulthood. The only modern animal known to do anything similar is the cassowary, descended from the dinosaurs, which develops a large spongy crest when its skull is about 80 per cent fully grown.
Scannella and Horner examined 29 triceratops skulls and nine torosaurus skulls, mostly from the late-Cretaceous Hell Creek formation in Montana. The triceratops skulls were between 0.5 and 2 metres long. By counting growth lines in the bones, not unlike tree rings, they have shown clearly that the skulls come from animals of different ages, from juveniles to young adults. Torosaurus fossils are much rarer, 2 to 3 meters long and, crucially, only adult specimens have ever been found. The duo say there is a clear transition from triceratops into torosaurus as the animals grow older. For example, the oldest specimens of triceratops show a marked thinning of the bone where torosaurus has holes, suggesting they are in the process of becoming fenestrated.

There are other species this might apply to, as well. Some with even bigger shifts in appearance.
While this is a Big Hairy Deal for dinosaur science, it also elicits a little bit of a "duh" moment when you go back and look at the animals in question. What you should really be getting out of this story is an illustration of how difficult it is to study a creature that's been extinct for millions of years.
After all—as my husband pointed out—nobody would be shocked to learn that a baby chick, an adult chicken, and plate of parmigiana were all the same animal. But that's because we've experienced chickens. Were an alien to drop in on Earth for one afternoon, they might be just as amazed at the life cycle of poultry as we are now at the triceratops/torosaurus'. Paleontologists are tasked with reconstructing the lives of animals nobody has ever seen alive. And that creates a world where the obvious just isn't.

New Scientist: Morph-o-saurs: How shape-shifting dinosaurs deceived us