Sexual selection is one of the driving forces of evolutionary biology. Specific features on certain animals have evolved specifically to attract mates, and although we can observe this in living species, trying to find these physical traits in extinct species proves far more challenging. A new paper published in the journal Palaeontologia Electronica claims to have found a physical example of this in a horned, frilled dinosaur after analyzing nearly 40 different specimens.

A 2010 paper notes that sexual dimorphism – major differences in appearance between male and female members of a species – has never been definitively demonstrated in non-avian dinosaurs. Nevertheless, several paleontologists have tried over the years.

Some have cited a difference between the sexes of the theropod dinosaurs Coelophysis as possible examples of sexual dimorphism. Two “morphs” of this particular dinosaur have been found, with one being more “robust,” with a more compact body and longer forelimbs, and the other being more “gracile,” with shorter forelimbs and a more elongated body. Some scientists believe these to be male and female versions of each other, respectively.

Another paper in 2012 notes that it may have been the case that, for many dinosaur species, both males and females had prominent features, and both sexes preferred mates with the most elaborate structures. If this was the case, there would be no need for one-sided sexual dimorphism among dinosaurs. This theory of mating behavior is known as mutual sexual selection.

Either way, the lack of complete fossils for many dinosaurs, and arguments over what specific physical features may have been for, means that the debate rages on in the paleontological community. It has been long assumed that non-avian dinosaurs would have sexually dimorphic features, though, as their modern relatives show prominent sexual dimorphism, with the males being particularly colorful and flamboyant in most cases. A classic example of this would be the peacock.

^{The female peahen is attracted to the quality of the train of feathers on a peacock. If living dinosaurs – i.e. birds – use sexually dimorphic features, did their ancestral, non-avian dinosaurs? Drop of Light/Shutterstock}

This new study claims that a more definitive example of sexual dimorphism has been found. Protoceratops, a herbivorous dinosaur from the Upper Cretaceous epoch (100 to 66 million years ago), lived in what is now modern-day Mongolia. A reassessment of their fossil record from paleontologists at the Queen Mary University of London, led by zoologist Dr. David Hone, has focused on their head crests.

Over 37 different species of this particular dinosaur, found in the Djadochta Formation in the Gobi Desert, were analyzed, along with others used in previous studies. Babies, juveniles and adults were all looked at as part of this investigation.

Protoceratops, both male and female, had a large, bony frill that extended from the back of the head and over its neck. These frills were found to be disproportionately sized in adults, in that they were much larger than their age would suggest. Additionally, these frills were absent in juveniles, and were observed to dramatically increase in size as they matured into adults.

This implied that they were a sexual selection feature that would only be required when they were able to reproduce. “The growth pattern we see in Protoceratops matches that seen for signaling structures in numerous different living species,” Hone said in a statement.

The authors hypothesize that the frill may have been used to attract suitable mates by asserting their dominance in a visual manner. Although this cannot be conclusively proven, as with any extinct species, this admittedly circumstantial evidence is certainly a strong contender.

^{As the adults make a show for each other, non-frilled juveniles can be seen roaming in the background. Artwork by Rebecca Gelernter/QMUL.}