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Theropod dinosaurs were smooth movers 100 million years ago, performing special mating dances to attract females, scientists have discovered. Huge scratch marks discovered in Dakota Sandstone dating from the Cretaceous period (around 66-145 million years ago) appear to be very similar to nest scrape displays seen in modern birds.
Over 60 marks – some the size of bath tubs – were found at several sites, with the largest spanning over 750m2. The scrapes are parallel double troughs, with well-defined marks separated by a raised central ridge.
These markings are very much like those made by Atlantic puffins and ostriches during the mating season, with males scraping at the ground (a nest-building technique) to show they would make a good mate. Places they perform this dance are known as display arenas or leks.
Previously, different types of mating displays have been found among dinosaurs, with flamboyant head gear, horns, frills and crests. Indeed, 2013 research suggested dinosaurs put on plumage displays, shaking their tail feathers to attract mates.
Published in Scientific Reports, Martin Lockley and colleagues from University of Colorado Denver found larger areas where tracks of theropod dinosaurs were found, along with these huge scrape marks. Previously, the idea of dinosaurs putting on such displays were speculative based on comparisons with modern bird behaviours – the scrape marks are the first physical evidence of these rituals actually taking place.
Lockley told IBTimes UK they were not expecting to find the marks: "We found many tracks in Western Colorado before we found these scrapes. So it was a surprise. Birds show many types of display behaviour. Scraping (or nest scrape display) is only one type. All previous speculation about theropod display has been about how crest and feathers would be attractive. So no - no one expected to find these big scraped display arenas.
"Crests and feathers are not proof of mating behaviour. But they are a fair basis for speculation. Previously that was all palaeontologists had to go on as a basis for speculation. The scrapes seem to be proof."
He said males were probably the dominant display artists, just as with modern birds, adding that the behaviour has remained almost completely unchanged for 100 million years in some bird species – they are just much smaller now. And like with birds that perform these dances, the researchers believe dinosaur nesting sites would have been located nearby.
Further to this, the researchers believe the site could have been used by a number of different theropod dinosaur species. Some scratch marks analysed were likely made by Acrocanthosaurus – a large dinosaur that was 25ft long with 18 inch feet. But different scrapes were different sizes, suggesting a different species. The team suggested the site would have been a bit like "Spring Break" for the dinosaurs.
"Today some colonial birds breed with other species in suitable areas," Lockley said. "We know from the track evidence that there were dinosaurs of different sizes. However, since most breeding dinosaurs of any one species were likely to have been adult sized, it does seem reasonable to conclude that the different-sized tracks and scrapes represented different species.
"The question from our point of view is will we find more display arena sites with such compelling evidence of a lot of frenzied activity in the breeding season. I think the answer is yes, we will likely find many more sites now that we know what to look for."