Did meat-eating dinosaurs like Tyrannosaurus rex do a chicken dance to woo their mates? That’s the implication of a new study, which points to fossilized foot scrapes as evidence that a variety of dinosaurs pranced like parrots and puffins to impress the opposite sex. The finding adds further evidence that ancient dinos shared many of the same behaviors as their modern bird relatives.
“Most or perhaps all of the behaviors present in birds today originated in nonbird dinosaurs,” says Darren Naish, a paleontologist at the University of Southampton in the United Kingdom. “If these scrape marks are really what the authors say they are, this study is pretty compelling support for that contention.”
The scrape marks were uncovered at four sites in Colorado by a team led by Martin Lockley, a paleontologist at the University of Colorado, Denver. Three of the sites are in western Colorado, and one is in the east of the state. All contain sediments of the Dakota Sandstone, a geological formation laid down during the Cretaceous period, which stretched from about 145 million to 66 million years ago. The largest of the four sites, in western Colorado, features some 60 scrapes on a sandstone surface measuring about 50 meters long and 15 meters wide. Most of the scrapes are made up of parallel double gouges in the ground, and some also include the clear outlines of three-toed footprints.
From their size and shape, Lockley thinks the traces were probably made by the meat-eating dinosaur Acrocanthosaurus, which lived about 110 million years ago and was one of the largest carnivores in the area at that time. It weighed as much as 6 metric tons, was up to 11 meters long, and sported a long, narrow skull and bony spines poking up from its vertebrae that probably supported a ridge made of tough muscle.
Writing online today in Scientific Reports, the team argues that the scrapes closely resemble those made by ground nesting birds—such as the Atlantic puffin and some species of plovers and parrots—that engage in “very energetic” courtship displays, which include lots of prancing around and scratching the ground.
“Birds seem to get in a frenzy of prenuptial, premating activity,” Lockley says, adding that the “bird literature actually speaks about peaks of emotional activity. It seems that [carnivorous] dinosaurs did the same.”
Co-authors Martin Lockley (right) and Ken Cart pose next to scrape marks made by meat-eating dinosaurs. These included the possibility that the scrape marks were made while dinos were digging for food or water, while marking territory, or even while building nests or gathering in colonies. Only the courtship display hypothesis, the researchers concluded, was consistent with all of the features of the scrape marks, such as their abundance, spacing, and density on the ground.
The traces “do not seem connected to nest building, burrowmaking, or finding food and water,” agrees Anthony Martin, a paleontologist at Emory University in Atlanta. “So wooing traces are a good explanation, especially when compared to modern bird traces. They also tell us how much energy dinosaurs put into wooing a potential mate, which they apparently did by feeling the earth move under their feet.”
Naish adds that although he was “initially extremely skeptical” when he first started reading the paper, he now thinks that “they may well be on to something.” But he says he still wonders “if there’s an explanation the authors might have missed.”
For example, the scrape marks could have been made when meat-eating predators, such as the mighty Acrocanthosaurus, scraped their powerful hind legs on the ground to make noise and warn other males—of either the same or a different species—to stay away, says Timothy Isles, a paleobiologist at the University of Portsmouth in the United Kingdom. An even bigger problem, he says, is that competitive displays by male birds seeking to mate with females are “only observed in a small number of species around the world.” That makes it less likely, he says, that the behavior was “widespread and ubiquitous” in their extinct dinosaur ancestors.