"These are the first sites with evidence of dinosaur mating display rituals ever discovered, and the first physical evidence of courtship behavior," study co-author Martin Lockley of the University of Colorado at Denver said in a statement. "These huge scrape displays fill in a missing gap in our understanding of dinosaur behavior."

The 50 or so grooves, which averaged just over six and a half feet long, were found in 100-million-year-old rock — and in places where there are already signs of dinosaur foot traffic. "Despite this abundance of footprints, display scrapes represent a previously-unrecognised, entirely new category of vertebrate trace fossil," the authors wrote in the study. In other words, no one has ever seen these markings and associated them with dinosaurs before.

The size, shape and time period suggest that theropod dinosaurs could have been responsible for making them. In fact, the researchers report, some of gouges include complete outlines of three-toed theropod tracks.

If the scrapes really did come from dinosaurs, then paleontologists have to ask what they were made for. The researchers write that the gouges are the wrong size and shape to have been made as part of the construction of a nest, at least based on what we think we know about such things. The lack of evidence of fossilized eggs also makes that explanation unlikely. And if dinosaurs made these grooves while digging for food or water, it's unusual that none of the grooves show signs of success — a destroyed animal burrow, for example, or places where water had washed away the scratches.

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But we know that birds are descended from a few select dinosaur lineages — and that dinosaurs may have been a lot more bird-like than we give them credit for. And when you factor that in, the scratch marks have a clear modern equivalent: The "nest scrape display" or "scrape ceremonies" that birds use to show off for potential mates. During these rituals, male birds scratch and kick at the dirt in a sort of artful interpretation of the act of nest building, showing female birds that they have the right stuff to provide for a family.

"It's dinosaurs and sex," Lockley told the Denver Post. "What a combo."

Several outside experts expressed cautious support for the theory to Science Magazine. But some pointed out that it's difficult to draw conclusions about dinosaur behavior from gouges in the ground, and it's entirely possible that the researchers have missed the true meaning of the grooves.

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