Protoceratops andrewsi made football-shaped heads cool more than 100 million years before Hey Arnoldfirst aired.

This relative of Triceratops had an unusual bony frill on top of its head / over its neck, and scientists have inferred that the frill was for sexual display purposes. It is very difficult to reconstruct the behavior of extinct animals, because we only have physical remains (for a recent example, see coverage of the study hypothesizing that big grooves in the ground were evidence of dinosaur mating dances). But Protoceratops is a very commonly preserved dinosaur, so paleontologists were able to look at frills in many individuals ranging in age from hatchling to full adult. They found that the frill changed both in size and shape over time, with a specific growth pattern that suggests it was relevant to age-specific behavioural changes and was likely used to attract mates.

This is interesting, because much dinosaur ornamentation has been explained through functional hypotheses such as predator defense. The frill may represent an example of the power of sexual selection to drive morphological change. We see the power of sexual selection in thousands of living species, such as the elaborately ornamented birds of paradise with their peculiar mating rituals-- both characteristics that seem unquestionably undesirable from a perspective of direct fitness enhancement. 

More and more, scientists are recognizing that sexual selection is an extremely powerful evolutionary force-- and Protoceratops' football-shaped frill is a good example of this theroetical shift.