Geoscientist Online

A fossil pterosaur, preserved with her eggs still inside her, has finally proved that these giant flying reptiles - which lived 220-65 million years ago - were sexually dimorphic.  Ted Nield reports.

Geoscientist Online 21 January 2011

The discovery in China of a pterosaur (perhaps more familiarly known by the older name “pterodactyl”), fossilised together with her egg in 160 million year-old Jurassic rocks, provides the first direct evidence of gender in these extinct flyers. Thanks to this unequivocal find, palaeontologists now know that females were crestless. This also strongly suggests that the function of the prominent head-crest was as part of showy male displays – to attract mates or intimidate rivals. The puzzle over the crest’s function has worried palaeontologists for over a century.

The find of this specimen of genus Darwinopterus was made by an international team of researchers from the Universities of Leicester, Lincoln and the Geological Institute, Beijing – the same team responsible for discovering and naming the genus back in 2009. Details of this unique new find are published today in the journal Science.

David Unwin, (University of Leicester), who was part of the research team, says: “Many pterosaurs have head crests. These can reach five times the height of the skull. Scientists have long suspected that they were used for display or signalling, and may have been confined to males. But in the absence of any direct evidence for gender, this idea remained speculative.”

As a result, crested and crestless forms were often separated into completely different species, Unwin says. “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 specimen, christened “Mrs T” (a contraction of “Mrs Pterodactyl”) by the research team, was made in Liaoning Province in NE China, and may represent the result of a tragic accident. The well-developed shell shows that Mrs T was just about ready to lay her egg when she broke her left forearm and was killed - possibly in a storm, or even a volcanic eruption, which were common in this part of China just then.

The dimorphism is not confined to the head. Unwin says: “Mrs T shows two features that distinguish her from male individuals. She also has relatively large hips, to accommodate the passage of eggs. Males, on the other hand, have relatively small hips.”

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

“Gender is one of the most fundamental of biological attributes, but extremely difficult to pinpoint with any certainty in the fossil record. Being able to sex pterosaurs is a major step forward. Now, we can exploit our knowledge of pterosaur gender to research entirely new areas, such as population structure and behaviour.” Unwin says they can also reconcile misclassified dimorphs and “play matchmaker”, bringing back together long separated males and females into the single species to which they both belong.

The find also has much to tell about pterosaur reproduction. “Mrs T’s egg is relatively small and had a soft shell” Unwin says. “This is typical of reptiles, but completely different from birds, which 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 Quetzalcoatlus.”