UK: Dinosaur-era sex riddle solved

Killed and preserved with her egg, a fossil of a flying reptile shows for the first time how hips and crests can be used to sex pterodactyls.

Discovery of an ancient fossil, nicknamed 'Mrs T', has allowed scientists for the first time to sex pterodactyls - flying reptiles that lived alongside dinosaurs between 220 to 65 million years ago.

Pterodactyls featured prominently in Steven Spielberg's Jurassic Park III and are a classic feature of many dinosaur movies where they are often depicted as giant flying reptiles with a crest.

The discovery of a flying reptile fossilised together with an egg in Jurassic rocks (about 160 million years old) in China provides the first direct evidence for gender in these extinct fliers.

This fossil shows that females were crestless, solving the long-standing problem of what some pterosaurs did with their spectacular head crests that were used as showy displays by males.

The find was made by an international team of researchers from the universities of Leicester, Lincoln and the Geological Institute in Beijing. Details of the unique new find were published last month in the journal Science.

David Unwin, a palaeobiologist at Leicester's department of museum studies, was part of the research team that studied the fossil.

"Pterosaurs, flying reptiles, also known as pterodactyls, dominated the skies in the Mesozoic Era, the age of dinosaurs, 220-65 million years ago. Many pterosaurs have headcrests," Unwin said.

In the most spectacular cases these can reach five times the height of the skull. Scientists have long suspected the crests were used for some kind of display or signalling and may have been confined to males while females were crestless.

But, in the absence of any direct evidence for gender, the idea remained speculative and crested and crestless forms were often separated into completely different species.

"The fossil we have discovered, an individual of Darwinopterus (a pterosaur first described by the same team of scientists in 2009) is preserved together with an egg showing that it must be female," Unwin said.

This type of discovery, in which gender can be determined with certainty, is extremely rare in the fossil record, and the first to be reported for pterosaurs.

The fossil, christened 'Mrs T' (a contraction of 'Mrs Pterodactyl') by the research team, was made in Jurassic rocks of Liaoning province in north-east China and seems to represent a tragic accident.

The well-developed shell shows that Mrs T was just about ready to lay her egg when she was killed in an accident that broke her left forearm, possibly the result of a storm, or perhaps even a volcanic eruption as these were common in this part of China around 160 million years ago, Unwin said.

"Mrs T shows two features that distinguish her from male individuals of Darwinopterus. She has relatively large hips, to accommodate the passage of eggs, but no head crest.

"Males, on the other hand, have relatively small hips and a well developed head crest. Presumably they used this crest to intimidate rivals or to attract mates such as Mrs T.

"Mrs T is a once in 10 lifetimes discovery. As long as the skull or hips are preserved we can now confidently identify males and females and, even more importantly, we can use this technique to sex other pterosaurs because they often show differences in head crests and hips just as in Darwinopterus."

Gender was one of the most fundamental of biological attributes but extremely difficult to pinpoint with any certainty in the fossil record, Unwin said. Being able to sex pterosaurs is a major step forward.

"Finally, we have a good explanation for pterosaur head crests, a problem that has puzzled scientists for more than 100 years. Now, we can exploit our knowledge of pterosaur gender to research entirely new areas such as population structure and behaviour.

"We can also play matchmaker for pterosaurs bringing back together long separated males and females in the single species to which they both belong."

Apart from gender, Unwin said the new find also had much to tell about pterosaur reproduction. Mrs T's egg was relatively small and had a soft shell which was typical of reptiles but completely different from birds that lay relatively large hard-shelled eggs.

"This discovery is not surprising though, because a small egg would require less investment in terms of materials and energy - a distinct evolutionary advantage for active energetic fliers such as pterosaurs and perhaps an important factor in the evolution of gigantic species such as the 10-metre wingspan of Quetzalcoatlus."