January 21, 2011
Fossil Find Solves Dinosaur Sex Riddle
The discovery of an ancient fossil, nicknamed "ËMrs. T', found with a fossilized egg in a Jurassic rock bed dating back 160 million years, has allowed scientists for the first time to sex pterodactyls.
The fossil was found in the Jurassic sedimentary rocks of China's northeastern Liaoning Province and shows a nearly complete skeleton of a heavy-hipped female Darwinopterus and her egg. The find provides scientists with the first direct evidence for gender in these extinct reptilian fliers, showing that females were crestless, solving a long debated issue of what some pterosaurs did with their spectacular head crests. It suggests that the males used the crest as a showy display.
Pterosaurs were warm-blooded, winged creatures that flew among other dinosaurs between 220 and 65 million years ago, and resembled a cross between a stork and a bat.
The researchers said that the hawk-sized Mrs. T displays wider hips than males, and likely lacked the same distinct head crest that could be seen in males.
David Unwin, a paleobiologist in the Department of Museum Studies at the University of Leicester, said the discovery of a mother together with her egg is "incredibly rare," and a first for pterosaurs.
"Certainly if somebody had said to me a few years ago, 'What do you think about the chance of finding a pterosaur preserved together with an egg?' I just would have said 'You're crazy,'" Unwin told AFP.
The discovery will help researchers better understand and identify previously found fossils as either male or female, especially in Darwinopterus, where both sexes were about the same size.
"She has relatively large hips, to accommodate the passage of eggs, but no head crest," Unwin said in a statement. "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."
Many of her other traits match those of 10 similar specimens found in the same rock formation -- an elongated neck and tail, a protruding beak-like mouth lined with slender teeth, and an extended fifth toe.
The researchers who studied the fossil, which was first identified in 2009, gathered clues that showed how the young creature's life came to a sudden violent end.
Judging by the visible bone break near her wing, some kind of sudden accident, such as a volcanic eruption or a severe storm, must have fractured the forearm, making it impossible for her to fly, they said.
Evidence shows that she plunged into a body of water and drowned. As her body grew waterlogged, it sank to the bottom. The egg, which looked like it was days from being laid, was pushed out of her decaying body.
"The association of the egg with the skeleton, at least in pterosaurs, is unique. We have no other similar sort of specimen," Christopher Bennett, an expert at Fort Hays State University in Kansas who reviewed the research, told AFP.
Bennett said the creature might have had some sort of head markings, though it was likely not the same flashy display grown by males. "I am not entirely convinced that this individual didn't have any crest. It is possible that it was a young adult and a bony crest had not yet grown," he added.
Nonetheless, the fossil is a "nice specimen" and shows that "females had a pelvis that was deeper and had a larger opening," said Bennett.
Unwin, writing in the journal Science, said: "Mrs. T's egg is relatively small and had a soft shell. 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 meter wingspan Quetzalcoatlus."
"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. 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 behavior. We can also play matchmaker for pterosaurs bringing back together long separated males and females in the single species to which they both belong," he concluded.
Image 1: This graphic shows the sex-related features of Darwinopterus. The male (right) has a large head crest, but this is absent in the female (left). Credit: Mark Witton ([email protected])
Image 2: "Mrs. T" is a female Darwinopterus (wingspan 0.78 m) preserved together with her egg. Credit: L Junchang, Institute of Geology, Beijing ([email protected])
Image 3: This is a close up of the egg (20 by 28 mm) preserved together with Mrs. T, a female Darwinopterus. Credit: L Junchang, Institute of Geology, Beijing ([email protected])
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