Large theropod dinosaurs that quickly evolved to reach gigantic sizes were more likely than other, smaller dinosaurs to have developed bony skull ornaments – such as crests and horns – scientists have said.

Cranial ornaments are sexually selected morphological traits found in a range of animal species alive today – scientists have long hypothesised that they may act as socio-sexual signalling structures, seducing females and warning competitors to stay away.

Up to date, the most diverse and extreme array of cranial ornaments evolved in dinosaurs, but it is not known what purpose these features served and if they functioned as signal for potential mates or enemies. The consequences of large theropod dinosaurs, such as T-Rex, developing these traits is similarly not well-documented.

In the research published in the journal Nature Communications, scientists have investigated the relationship between possessing cranial ornaments and body size.

Large sizes and ornaments go hand in hand

The researchers, led by Terry Gates from North Carolina State University (US), analysed fossils and used statistical models to test whether species with cranial ornaments evolved larger sizes relative to species that lack such display features. They found out that large body masses were correlated with the presence of ornaments. Larger theropod dinosaurs were more likely to have cranial ornaments than smaller dinosaurs. The scientists even estimated the body mass below which bony ornaments are not predicted to have evolved – 36Kg.

More importantly perhaps, they say that an accelerated body mass evolution in dinosaurs went hand in hand with the possession of socio-sexual signalling structures like crests and horns. In other words, dinosaurs with ornaments grew in size more quickly than their peers.

However, these findings cannot be applied to all species of theropod dinosaurs. Those dinosaurs that are believed to have possessed pennaceous feathers covering their bodies appear to have achieved a wide range of body sizes without the presence of cranial ornaments. This suggests that in these species, feathers went on to play a similar role than cranial ornaments in other large theropod dinosaurs.

More research into the ecology of theropods will be needed to explain the observed relationship between the evolution of body size and bony skull structures – and how it contributed to communication and signalling between the dinosaurs.