More than 75 million years ago, a pair of oviraptors nicknamed Romeo and Juliet died next to each other, buried alive by catastrophic dune collapses triggered by heavy rains. Now, researchers examining the fossils of the star-crossed lovers have discovered that their tails are different: The male had tail bones that helped him woo the female like a peacock. The findings were published in Scientific Reports last month. 

Peacocks have colorful, extravagant fans, roosters have tall crests, and male birds of paradise have extraordinary tails—these help the dudes attract and court the lady birds. Determining the sex of a dinosaur, however, is much harder. “Because soft anatomy seldom fossilizes, a dinosaur fossil usually provides no direct evidence of whether it was a male or a female," University of Alberta’s Scott Persons says in a news release.

In previous work, Persons (pictured) and colleagues suggested that an early oviraptor ("egg thief") species had tail fans composed of long feathers used to enhance courtship displays—just like peacocks, turkeys, and prairie chickens. "My analysis of the tail skeletons supported this theory, because the skeletons showed adaptations for both high tail flexibility and enlarged tail musculature," he says, "traits that would have helped an oviraptor to flaunt its tail fan in a mating dance." And if that’s the case, oviraptor tail bones would be sexually dimorphic. 

Well, enter Romeo and Juliet, or sometimes Sid and Nancy, two Khaan mckennai oviraptors excavated within 20 centimeters of each other in the 1990s at an Upper Cretaceous site in the Djadokhta Formation of the Gobi Desert in Mongolia. Both are adults, and one is slightly larger than the other. Importantly, their first four chevrons—the bones jutting out from underneath the first five caudal (tail) vertebrae—were different. Chevrons are attachment sites for muscles, and while one had finger-like chevrons, the other had larger, spearhead-shaped ones. 

Pictured here, the anterior caudal sequence of MPC-D 100/1127 (A, Juliet) and MPC-D 100/1002 (B, Romeo).

The differences don’t seem to be due to gaps in age, pathologies, or the fossilization process. Rather, the team thinks the larger chevrons, Nature explains, helped anchor muscles that control the flexible, feather-tipped tails of males.

"We discovered that, although both oviraptors were roughly the same size, the same age and otherwise identical in all anatomical regards, ‘Romeo’ had larger and specially shaped tail bones," Persons says. "This indicates that it had a greater capacity for courtship displays and was likely a male." Juliet’s shorter, simpler tail bones indicate a lesser capacity for peacocking—suggestive of a female. 

Images: Sydney Mohr (top), W. Scott Persons IV et al., Scientific Reports 2015 (middle), University of Alberta (bottom)