Size and Weight:
You might be excused for thinking Styracosaurus was all head: this dinosaur's broad, flat face extended upward into a massive frill, which was itself topped by no less than six dangerous-looking spikes. As if all that weren't enough, the three-ton Styracosaurus also had a two-foot-long horn extending from its snout, two smaller horns jutting out from its cheeks, and (for some obscure reason, most likely related to its sense of smell) unusually large nostrils.
Why would nature have allowed this otherwise gentle ceratopsian to evolve such an elaborate (and fearsome) cranial display? There are three possible explanations. First, it may have been a sexual adaptation (that is, male Styracosaurus adults with bigger frills and sharper spikes had a better chance of mating with females); and second, the ornamentation may have evolved as a means of defense, either to actively gore hungry raptors and tyrannosaurs or make Styracosaurus look larger and more threatening, and hence a less appetizing meal.
There's also an intriguing third possibility. The enormous frill of Styracosaurus may have helped to dissipate heat from its body, much like the floppy ears of an African elephant. If true, this would lend support to the theory that dinosaurs had warm-blooded metabolisms and needed to occasionally shed excess heat. (However, it's extremely unlikely that that the ornithischian Styracosaurus had a classic endothermic metabolism, compared to saurischian theropods for which the evidence is much more compelling.)