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A pair of turkey-sized 'egg thief lizards' dubbed Romeo and Juliet were found lying next to each other in a 75-million-year-old rock in the mid-90s, but it has taken until now for experts to determine the sex of the 'lovers'.

Now, researchers believe they have come up with a way to tell fossils of small feathered male dinosaurs from those of females.

They say that the key differences between the sexes lie in bones near the base of the tail. 

A pair of turkey-sized 'egg thief lizards' dubbed Romeo and Juliet were found lying next to each other in a 75-million-year-old rock (pictured) in the mid-90s. Palaeontologists led by Scott Persons (pictured) believe they can tell males and females apart from certain tail bones

Paleontologistsat the University of Alberta examined a pair of fossils found in Mongolia, which were first described in 2001.

One is a complete skeleton, while the other is missing the middle and end of its tail.

'Determining a dinosaur's gender is really hard,' said graduate student Scott Persons, who is lead author of the study, published in Scientific Reports.

'Because soft anatomy seldom fossilises, a dinosaur fossil usually provides no direct evidence of whether it was a male or a female.'

So instead, the researchers looked at indirect evidence of sexually dimorphic display structures, which are evident today in birds.

Examples include the fans of peacocks, the tall crests of roosters or the long tail feathers of some birds of paradise, which are used to attract and court mates. 

They are almost always much larger in males who do the courting, than in females, who do the choosing.

They suggest that male oviraptors had broader and longer chevron bones in their tails so they could shake their feather-tipped tail to woo potential mates (illustrated), much like modern peacocks

The team theorised that like birds, small feathered dinosaurs such as oviraptors used long feathers on their tails to attract mates, rather than for flying. 

'My analysis of the tail skeletons supported this theory, because the skeletons showed adaptations for both high tail flexibility and enlarged tail musculature - both traits that would have helped an oviraptor to flaunt its tail fan in a mating dance,' Mr Persons said.

After studying the fossils carefully, his team found signs of sexual dimorphism in the length and shape of blade-like bones called chevrons, which point downwards from the vertebrae near the base of the tail.

A number of chevrons in one of the fossils were longer and had broader tips than those in the other specimen, indicating the creature is male, the experts said.

They suggested that males had larger chevrons to anchor the muscles that controlled their flexible, feather-tipped tails.

The researchers suspect that male oviraptorosaurs shook their tail feathers in intricate displays to woo potential mates, akin the behaviour of modern-day peacocks.

They suspect females had shorter chevrons to make laying eggs easier.

Scientists have recently shown that peacocks shake their tails to make a noise that is too low for us to hear. Their mating ritual may bear some resemblance to that of small feathered dinosaurs

PEACOCK FEATHERS MAKE INFRASOUND NOISES TO TALK TO OTHER BIRDS 

Scientists have shown that peacocks (stock image) shake their tails to make a noise that is too low for us to hear

Scientists have shown that peacocks shake their tails to make a noise that is too low for us to hear.

Male birds may make the infrared rumble to scare off an approaching rival or to attract a mate.

Researchers at the University of Manitoba in Winnipeg, Canada, have shown that the birds make infrasound noises - below 20 hertz - using their tails and that other peacocks respond to them.

Humans hear a rustling sound when this happens, but the experts found that the birds hear a loud sound of between 70 and 108 decibels, according to the study published in the journal Animal Behaviour.

Male and female peacocks became more alert and moved around when the low noises were played back to them by researchers, with males shrieking out in response.

Experts are unsure how the birds detect infrasound, but know that some can feel vibration through their legs. 

Graduate student Angela Freeman observed that male peacocks were most likely to make the low sounds when they showed off their train feathers, producing two displays which she called the 'shiver train' and the 'pulse train'.

The shiver train is produced by vibration of the train feathers from the centre feathers to the outside of the array and is used by males when females are far away.

The pulse train is produced by a vibration emanating from the base of all the train feathers and it is used when the female is nearby.

Peacocks' tails are a sign of strength and good genes, so a strong noise may reinforce a message of virility, Ms Freeman said.

This could be especially useful as the birds live in scrubby woodland, where high noises can only travel over short distances and birds sometimes can't see each other.

'We think that possibly these signals could signal to females and males who are out of sight behind a bush or over the hill,' she added.

The two raptors were discovered in the Gobi Desert of Mongolia.

Both died and were buried next to each other when a large sand dune collapsed on top of them, creating the interesting fossil.

When they were first unearthed, the two oviraptors were given the nicknames Romeo and Juliet because they seemed reminiscent of Shakespeare's famously doomed lovers.

'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,' Mr Persons said.

'This indicates that it had a greater capacity for courtship displays and was likely a male.'

By comparison, the second specimen, Juliet, had shorter and simpler tail bones, suggesting a lesser capacity for peacocking, and has been interpreted as a female.

According to Persons, the two may very well have been a mated pair, making for a romantic story, as the dinosaur couple was preserved side by side for more than 75 million years.

While the theory is compelling, Thomas Holtz, a vertebrate paleontologist at the University of Maryland in College Park told Nature that it's not yet completely convincing, because only one pair of oviraptor specimens have been studied.

Therefore, there is a possibility that the difference in Chevron shape was a variation, rather than a sign of sexual dismorphism.