Dinosaurs engaged in showy displays to attract mates 100 million years ago by clawing huge troughs in the ground with their feet, according to a study released Thursday.
Scientists have long speculated that theropods -- which included mega-carnivores like T. Rex and more nimble dinos that eventually evolved into birds -- engaged in courtship rituals of some kind.

One of the main drivers of evolution is sexual selection, in which males compete for the attention and affections of potential partners.

But in the absence of physical evidence, these theories remained just that: speculation.

The study, published in Scientific Reports, provides the first hard proof of foreplay in the Cretaceous, a period that ended with the abrupt extinction of non-avian dinosaurs some 66 million years ago.

Exhibit A: several dozen double furrows -- some the size of a bathtub -- spread across four sites in the Dakota Sandstone, a rock formation in the western US state of Colorado.

"These huge scrape displays fill in a missing gap in our understanding of dinosaur behaviour," lead author Martin Lockley, a researcher at the University of Colorado, said in a statement.

The fossilised marks are similar to those of some modern birds, reinforcing recent studies establishing an evolutionary link, the study said.

Atlantic puffins, for example, scratch out similar troughs in the run up to breeding, while the ostrich -- much closer in size to some small dinosaurs -- do the same, and then use the shallow troughs to nest.

"Modern birds using scrape ceremony courtship usually do so near their final nesting sites," Lockley commented.

"So the fossil scrap evidence offers a tantalising clue that dinosaurs in 'heat' may have gathered here millions of years ago to breed and then nest nearby."

Bird specialists describe such behaviour as "nest scrape display," or "pseudo-nest building", he added.

From the time of their discovery, there was no doubt that the deep furrows found in Colorado had been produced by dinosaurs.

But the team led by Lockley -- a well-known expert on dino footprints -- also went through a list of other possible explanations.

One was that, rather than ritual markings, the scrapes were actual nesting grounds. They might also have been evidence of dinosaurs digging for food or water.

A third hypothesis was that they had been made to mark territory, the way many mammals today do in the wild by spaying urine or pungent chemicals, or gnawing on tree bark.

In each case, however, the researchers pointed to evidence showing why none of these scenarios was possible or likely.