Bony head ornaments signal some supersized dinosaurs

Dinosaur fashion, like that of humans, is subject to interpretation. Bony cranial crests, horns or bumps may have served to woo mates or help members of the same species identify one another. While the exact purpose of this skull decor is debated, the standout structures tended to come with an even more conspicuous trait: bigger bodies.

Terry Gates, a paleontologist at North Carolina State University in Raleigh, and colleagues noticed an interesting trend in the fossil record of theropods, a group of dinosaurs that includes Tyrannosaurus rex and the ancestors of birds. Bigger beasts often sported skeletal headgear.
Across the family tree, Gates and his team analyzed 111 fossils dating from 65 million to 210 million years ago, and the trend held true. It makes sense: “Dinosaur size matters in terms of how they will be visually talking to one another,” says Gates. “When you’re smaller, your means of visual communication would be different than when you’re giant.”

The researchers also calculated that over time, theropod lineages with head ornaments evolved giant bodies (larger than 1,000 kilograms) 20 times faster on average than those without. Ornaments might have supersized some dinos, but researchers aren’t sure. The analysis, which appeared September 27 in Nature Communications, suggests theropods had to reach at least 55 kilograms to grow the headgear.

But among big-boned relatives of modern birds, skull toppers weren’t in vogue. Many of these dinos grew heavier than 55 kilograms, but they instead sported feathers that resembled those used by modern birds for flight. That might be because bigger, bolder feathers and showy headwear served similar ends. Gates speculates: “Once you have a signaling device in the form of a feather, why grow a bony cranial crest?” For these plumed dinosaurs, feathers were in and bony ornaments were out.

Size matters
Many large theropods, a group of dinosaurs that includes Tyrannosaurus rex and the ancestors of birds, had bony head ornaments such as crests, horns and bumps. New research suggests theropods had to reach at least 55.2 kilograms to grow the cranial decor. But big-boned dinos related to modern birds lacked the ornaments. Instead, they were decked out in feathers resembling those used by modern birds for flight.