Gouges in the ground are best evidence yet of dinosaur courtship

by Terri Cook
Thursday, January 14, 2016

Dinosaurs might not have been lonely in love, according to new research published in Scientific Reports. Scientists recently discovered the first tangible evidence that dinosaurs engaged in courtship behaviors: parallel scrape marks up to about 1.8 meters long and 40 centimeters deep that were gouged in the ground during the Cretaceous.

Based on comparisons with modern birds, especially ground-nesting species like Spoonbilled sandpipers and plovers, the researchers think that large theropod dinosaurs — the group to which Tyrannosaurus rex belongs — created the gouges by scratching the ground during so-called “nest scrape displays.”

The scientists identified more than 70 such scrapes at several sites in Colorado, including Dominguez-Escalante National Conservation Area, Gunnison Gorge National Conservation Area and Dinosaur Ridge.

“These things are the size of bathtubs,” says Martin Lockley, a professor emeritus at the University of Colorado at Denver who led the study.

About half of the scrapes occur as pairs, with each having been carved by a separate foot. Some have clear clawmarks or even a complete footprint associated with them, and the best ones, according to Lockley, “have a left footprint on the left side and a right footprint on the right side.”

Because modern birds are thought to be descendants of a subgroup of theropod dinosaurs, scientists have suspected that theropods may have displayed mating behaviors similar to those of their avian cousins. But since most courtship behaviors — like certain vocalizations and ritual “dances” — don’t leave tangible evidence preserved in the rock record, the existence of such behaviors in dinosaurs has, until now, merely been the subject of academic speculation.

The scrapes are “a very strong piece of evidence that theropod dinosaur behavior was similar to certain modern birds,” Lockley says. “What we recognize as avian behavior goes back at least 100 million years.” The scrapes may have been made by groups of males assembled together during mating seasons to engage in competitive displays aimed at attracting female attention. Such behavior is common among birds, which often display a frenzy of excitement prior to mating as part of their breeding cycle. Dinosaurs might have been the “original lovebirds,” he jokes.

The scrape marks documented by the team are randomly distributed and vary greatly in both size and depth. The team concluded that they are not remnants of nests, Lockley says, because the marks are still evident, indicating that the animals didn’t sit or lie down in them, which would have smudged them.

All of the scrapes were found in the Dakota Sandstone, a roughly 100-million-year-old formation deposited near the eastern edge of the Western Interior Seaway during the Late Cretaceous. Since the scrapes were presumably associated with territorial activity during the spring breeding season, Lockley says they probably occur near as-yet undiscovered nesting sites; the scrapes may therefore provide clues to the environments where theropods chose to nest. “If the analogy with birds is anything to go by,” Lockley says, “it’s almost certain that … they were nesting very nearby the scrape areas.”

The diversity of the scrapes, and the fact that they’re associated with footprints of different sizes, suggest dinosaurs of different sizes made the scrapes. This is interesting, Lockley says, because it could mean that both adults and juveniles may have been present — or perhaps even that different species may have been mating together, as is observed among many birds that nest in colonies.

This discovery may also revise how we think about dinosaurs' social behavior. “We’ve often thought of theropod dinosaurs as solitary animals,” Lockley says, but it seems this view may need to be updated.