Mesozoic Mating Rituals

The mid Cretaceous site in the Dakota sandstone where the leks were found

Birds are well known for the lengths they will go to attract a mate. Some, like the birds of paradise invest heavily in beautiful displays of feathers or elaborate dances and songs. Others take a different tack.

Weaver birds construct complex nests from twigs to attract a mate. Bower birds do much the same, but then fill their nests with objects of a particular bright colour, carefully arranging collections to highlight personal favourites. Dinosaurs, as the ancestors of birds, may well have engaged in similarly extravagant activities to attract a partner; recent discoveries of courtship rituals unearthed in Colorado appear to support this view.

The 3D digital reconstructions of the leks

The evidence, discovered by researchers led by Martin Lockley from the University of Colorado Denver, comes from the mid Cretaceous Dakota sandstone. Dinosaur fossils are very rare, but trace fossils of their existence is more abundant, including nests and footprints.

The new trace fossils, some the size of bath tubs, represent scrape marks created during ritual mating displays. Known as leks, modern examples are created by some birds, their association with dinosaurs in the fossil record makes sense, considering the phylogeny of the two clades.

The lack of nests and eggshell around the leks suggests that mating rituals occurred in a different place to nesting itself. This is not to say, however, that the two activities took place far away from one another. 'The scrape evidence has significant implications,' said Lockley. 'This is physical evidence of prehistoric foreplay that is very similar to birds today. Modern birds using scrape ceremony courtship usually do so near their final nesting sites. So the fossil scrape evidence offers a tantalizing clue that dinosaurs in 'heat' may have gathered here millions of years ago to breed and then nest nearby.'

An artist's impression of the mating rituals in progress

The fossils were part of large bedding planes and were too large to be removed physically. Latex and fibreglass casts were made instead and then an imaging technique, known as photogrammetry, employed to create 3D digital models of the leks.

Analysis of the models showed that some of the leks still preserved the three toed marks of their creators, indicating that they were made by theropod dinosaurs. Again this makes sense as the theropods are the direct ancestors of the birds. Non-theropod dinosaurs on the other hand may have had different mating rituals.

By examining the size of the leks, the researchers were able to show that individuals ranging in size from lengths of 2.5 to 5 metres and hip heights of 1 to 2 metres, were responsible for creating them. This suggests either a mix of species in the same mating area or co-occurrence of adults and sub-adults in the same mating season. In turn this provides physical evidence to support the influence of sexual selection pressures in dinosaurs, as is seen in birds today.

'These are the first sites with evidence of dinosaur mating display rituals ever discovered, and the first physical evidence of courtship behaviour,' concluded Lockley. 'These huge scrape displays fill in a missing gap in our understanding of dinosaur behaviour.'