Sexual selection in action. Tyrannosaurids engage in a courtship ritual (an azhdarchoid and neornithine make guest appearances). It's not a great picture, but I've seen worse (it's by Darren Naish).

Sexual selection the phenomenon in which organisms choose mates on the basis of desirable traits is one of the fundamental driving processes of evolution. It s all around us, seemingly explains an enormous amount of the morphological and behavioural variation observed in the natural world, and has been shown to be tightly linked with speciation, adaptation, and even the susceptibility of a species to extinction. Accordingly, it is inherently logical to assume that sexual selection shaped the evolution of extinct animals as well as modern ones.