Confuciusornis sanctus, a 125-million-year-old Mesozoic bird, had remarkable differences in plumage - some had long, almost body length ornamental tail feathers, others had none - features that have been interpreted as the earliest example of avian courtship.
However, the idea that male Confuciusornis birds had ornamental plumage, and females did not, has not been proven until now.
Dr. Luis Chiappe, Director of the Natural History Museum of Los Angeles County's (NHM) Dinosaur Institute, and colleagues studied hundreds of Confuciusornis fossils unearthed from rocks deposited at the bottom of ancient lakes in what is today northeastern China and found undisputed evidence of the gender difference: medullary bone.
"Our discovery provides the first case of sex identification in an ancient bird, an animal closely related to dinosaurs, such as the famous Velociraptor," said Chiappe.
"When people visit dinosaur exhibits, they often want to know if the skeletons are male or female. We have nicknames like Thomas and Sue, but of all the thousands of skeletons of dinosaurs or early birds found around the world, only the sex of a few has been determined," the researcher noted.
According to Anusuya Chinsamy of the Department of Biological Sciences, University of Cape Town, South Africa, the bone histologist on the team, "Just like modern hens, female Confuciusornis birds that lived 125 million years ago deposited this special bone inside their long bones, and then used it to make the calcium-rich eggshells."
Finding such tissue - present during a short period of time in reproductively active females - in a specimen that lacked long feathers proved that those birds without ornamental plumage are females.
"This now permits us to assess gender differences in growth and development of this Mesozoic bird," she said.
But while this discovery offers evidence that both early and modern female avian species were essentially using the same physiological strategy to reproduce, it also spotlights an important difference in when they sexually matured.
"In human terms, knowing the sex of these specimens sheds light on when these early birds begin puberty. Now we know that early birds began reproducing way before they were full grown, a pattern that contrasts with what we know of living birds, which typically begin reproducing after they reach full body size," said Chiappe.
In that way, ancient birds produced offspring like dinosaurs, which also began to reproduce before they were fully grown.
The research has been published in Nature Communications.
London: A team of paleontologists has discovered a way to determine the sex of an avian dinosaur species.
First Published: Wednesday, January 23, 2013, 15:49