New research is to be published which claims that the plates and spikes of the Stegosaurs, the horns of triceratops, the helmet-like domes of the pachycephalosaurs, and the crests of the duck-billed hadrosaurs were just for show.
Previously in the case of the Stegosaurs, it's been claimed that the plates and spikes were defensive, or that they were for sexual display at mating time, or even that they were a cooling mechanism for the creatures. In the latter case, the evidence put forward to support it was that the plates contained large blood vessels- in fact, it turns out that these vessels are dead ends- there was no way the blood could cool as it passed through the plate and then return to the body. It was also noted that modern species with antlers possess similarly large blood vessels- these are required for the growth of the antler. As regards their use for sexual display, researchers found that there was no sexual dimorphism- that is, the shape of the plates does not change between the sexes, probably ruling out their use in this manner.
The idea now is that stegosaur plates were merely for species identification. The comparison is with the many modern species of antelope in Africa which have different shaped horns- they serve the purpose of differentiating one species from another. It seems like a very mundane explanation but it's also quite plausible- though it must be noted that when it comes to dinosaurs, theories are not always long lasted or by any means certain- in fact, this battle of ideas is one of the most intriguing things about this area of study. Species identification they may be for, but it seems odd to me that pachycephalosaurs, triceratops and hadrosaurs are included in the lead-in- especially since none of these are mentioned in the article.
The dome-heads of pachycephs are very thick- a two foot long head topped with eight inches of solid bone- with neck muscles designed to lock the head at right angles to the body. This is surely not "species identification", as these features were most likely used for "butting" in mating contests (think deer locking antlers) and for defence against predators.
Triceratops had a very sturdy skull- seven feet long, four feet wide and solidly constructed. Their horns could grow up to four feet in length and their frill was essentially armour covered with tough, horny skin and tipped with small horn-covered spikes. This obviously lends itself to defence and offence and appears to be a case of overkill for mere species identification. As for the duck-billed hadrosaurs, their elaborate crests contained hollowed structures which seem perfectly suited for use as resonating chambers- no doubt these could be used to make a variety of calls with which to attract a mate. Again, more than simple species identification.