From Geotimes:
The spikes and plates of the Jurassic Stegosaurus may look like armor that could have staved off intrepid predators, but defense most likely was not their main purpose. According to new research, these bony growths on the back and tail were actually meant for species recognition — so that one Stegosaurus could pick his friends out of a crowd. “Paleontologists have been trying to determine what the plates and spikes of stegosaurs were for, for over a century,” says Russell Main, lead author of a new study published in Paleobiology this month. “The hypotheses have included defense, thermoregulation and display, [for] either sexual or species recognition,” he says.
In previous studies, scientists ruled out the defense mechanism as the primary function, as did Main, who is now a graduate student at Harvard University, and his co-authors Kevin Padian of the University of California, Berkeley, Armand de Ricqles of the Collège de France, Paris, and John Horner from Montana State University in Bozeman, Mont. Although their fearsome appearance may have played an accessory role in protecting the large herbivores, upon examination of the bone structure of the plates and spikes, the researchers determined that the relatively light construction was not robust enough to act as a deterrent to predators, Main says. “
More here.