"Animals today use signals other than size to communicate with one another," Gates explained, offering the rhinoceros as a living example. "Rhinos are large, but use their horns for maintaining social structure."
He and his colleagues are not sure when ornaments first evolved on carnivorous dinosaurs, and what initial function they might have held. They suspect that the features started out small before becoming larger and more varied across different dinosaur species.
Oviraptors -- a type of theropod that had claws, a toothless muscular jaw, slender limbs and other bird-like characteristics -- might have initially used their head ornaments for heat control. Later they were probably co-opted for visual communication, which is critical for today's birds.
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As some dinosaurs evolved feathers, the appendages became more important to visual displays. Feathers, more so than bony skull ornaments, allow for impressive flexibility.
"For instance, feathers can be lifted or depressed; feather color can be changed by rubbing them; and animals with feathers get new ones every molt," Gates said. "This is in contrast to bony ornaments that have much less flexibility by being permanently on display."
Today's birds have not completely gone off bony cranial ornaments, though. Hornbills, cassowaries, helmeted guineafowls and helmeted currasows, among others, all have flashy head ornaments along with their feathers.
Paleobiologist Gregory Erickson of Florida State University told DNews that the new study is thought-provoking.
"I don't think anyone (previously) suspected there was a relationship between cranial ornamentation and gigantism in dinosaurs," Erickson said, adding that he was most intrigued by the finding that feathered maniraptors -- dinosaurs closely related to today's birds -- generally did not evolve much, if any, cranial ornamentation.
Peter Makovicky, associate curator of dinosaurs at the Field Museum of Natural History, said that the authors showed there is evolutionary and biological significance behind "some of the amazing structures that make predatory dinosaurs look 'cool.'"
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