Carnivorous dinosaurs may have engaged in courtship ceremonies in display arenas that resemble those of birds today. Researchers examining the traces of large, Cretaceous-age scraping patterns reveal that they’re the remnants of some fancy footwork. The findings are published in Scientific Reports this week.
Nests and eggs are evidence for the later stages in the reproductive cycles of both birds and theropods, a group of carnivorous dinosaurs that includes the Velociraptor. But until now, it’s been impossible to show direct evidence of earlier stages – namely, courtship or mating displays – in extinct theropods, though this behavior is well-known among birds today. Some ground-nesting species, for example, engage in a mating ritual called nest scrape display or pseudo nest-building (where males advertise their ability to provide by digging mock nests).
A team led by Martin Lockley from the University of Colorado Denver examined physical evidence of large scrapes made by the left and right feet of theropod dinosaurs uncovered at four sites from a single Cretaceous rock unit in Colorado called Dakota Sandstone. The largest site, with an area of about 750 square meters (8,000 square feet), contained at least 60 traces of scrapes: parallel double troughs with scratch marks separated by a raised central ridge. The team created 3D images of the digging traces, as well as a latex mold and fiberglass replica. The reconstruction above is based on Acrocanthosaurus as a best educated guess, Lockley tells IFLScience.
The large scrapes, some up to two meters (6.6 feet) long, resemble those made during nest scrape displays of ground-nesting birds today: in particular, the marks made by Atlantic puffins during breeding season and shallow scratches made by ostriches. The marks in these display arenas (called leks) aren't the result of digging for food, water, or shelter, and based on the various sizes and depths of the scrapes, different species of theropods had used the sites during breeding season, probably in the springtime. The actual nests were likely established nearby.
The team named these fossilized traces Ostenichnus bilobatus or "bilobed display trace," and these represent a stereotypical avian behavior that has previously never been documented for Cretaceous theropods. "These huge scrape displays fill in a missing gap in our understanding of dinosaur behavior," Lockley adds in a statement. "These are the first sites with evidence of dinosaur mating display rituals ever discovered, and the first physical evidence of courtship behavior," which he also describes as "pre-historic foreplay" for "dinosaurs in 'heat.'"
Martin Lockley (right) and co-author Ken Cart next to two large Cretaceous scrapes from western Colorado. M. Lockley