When they want to get their love on, theropod dinosaurs used to engage in dancing displays, new evidence suggests.
A team of researchers from the University of Colorado in Denver has identified fossilised marks in Cretaceous-age sandstone rock in Colorado as similar to those made by modern birds performing ground-based mating displays. Their research was published last week in the journal Nature.
These marks were found in four different locations, the largest of which had 60. These mostly consisted of parallel double troughs, and adjacent mounds, indicating that the earth had been scooped out. Many of the troughs also had scratch marks, indicating the use of claws.
While it's possible that the scrapes could have been used for nesting, they lack features commonly found in fossilised dinosaur nests. Nests usually have a circular, flat-bottomed shape, rim-like edge and fragments of egg shell and hatchling remains. Nesting sites usually have regular spacing, too, and this was not consistent with the site, the researchers asserted.
The scrape marks were also inconsistent with the manner in which dinosaurs might dig for prey or water; they bore no resemblance to other marks associated with digging for food, and water would have washed away the marks.
Moreover, the scrapes are consistent with scrapes made by modern ground-nesting birds conducting "nest scrape displays," mating displays that involve clawing at the ground in a similar manner to how they would build a nest. Ostriches, for example, do this, as do seven species of shorebirds and the New Zealand ground parrot, the Kakapo. These arenas in which male birds conduct their mating display are called leks.
"The Colorado evidence points to a longevity of behaviour suggesting that theropod leks were integral to the social structure of at least some Cretaceous species," the team wrote.
"More importantly, these scrapes can be interpreted as the missing physical evidence which indicates that non-avian theropods engaged in stereotypical avian courtship and lek-like behaviours, which were previously only a matter of speculation among paleobiologists."
The hypothesis adds to the growing body of evidence that supports the notion that not only were theropods bird-like in anatomy, they were also bird-like in behaviour, too.