New Fossil Evidence Shows Reproductive Changes During Transition From Dinosaurs To Birds
May 30, 2014

New Fossil Evidence Shows Reproductive Changes During Transition From Dinosaurs To Birds

Brett Smith for - Your Universe Online

Over the course of millions of years, some dinosaurs evolved into the modern birds we see today – and that transition included the shift to a single-ovary reproductive system, according to a new report in the National Science Review journal.

The study authors said this change was probably advantageous to flying animals that evolved toward lighter and lighter weights.

"The most widely accepted hypothesis for the presence of a single functional ovary in living birds is that the right ovary … was lost to reduce body mass in gravid females during flight,” the authors of the new study wrote.

The research team, based in China, added that this change to the reproductive system probably took place "gradually during the evolution of dinosaurs and basal birds," a change that the fossil record provided only scattered evidence for until very recently.

Birds, including the 150 million-year-old Archaeopteryx, are unique to egg-laying animals in that they have a single-ovary system. A feathered dinosaur thought to be tightly related to birds, the oviraptorosaurian maniraptoran theropod, had two working ovaries, the study team noted.

"Ovarian follicles shed new light on dinosaur reproduction during the transition towards birds,” said study author Zhonghe Zhou, director of the Institute of Vertebrate Paleontology and Paleoanthropology, at the Chinese Academy of Sciences.

In the new study, the scientists report that the amount of ovary follicles uncovered in bird specimens tended to decline throughout the course of evolution. They pointed out that the newly discovered Jeholornis, which had the lighter-weight single ovary system, featured 20 eggs and some fossils of enantiornithine birds, which became extinct at the end of the Cretaceous 66 million years ago, had only 5 to 6 follicles.

"Jeholornis," the team wrote, "with its long dinosaurian boney tail, is only slightly more derived than Archaeopteryx, indicating that even the most basal birds were already modern in this aspect."

The team went on to say the transition "occurred at — or very near — the dinosaur–avian transition, supporting the hypothesis that birds lost the use of [one] ovary due to the energetic pressures of flight."

Zhou said the new report builds on previous work published in the journal Nature by adding new specimens to the research.

"It is also notable," he said, "that we are looking for more fossil evidence of follicles in birds and hopefully more information will be revealed in the future."

Another study published this week found that ancient bird populations living 125 million years ago were much less diverse than they are today.

“There were no swans, no swallows, no herons, nothing like that. They were pretty much all between a sparrow and a crow,” said Jonathan Mitchell, an author of that study, which was published in the journal Proceedings of the Royal Society B.

The study team theorized that the lack of diversity among Cretaceous birds was due to their newness on the scene and inadequate time to diversify.

“It looks like they just hadn’t evolved the crazy diversity of ecologies that we see in modern birds,” Mitchell said.