Basal Birds Were Reproductively Modern

As winged dinosaurs underwent a series of evolutionary changes during the transition into Aves, or birds, one pivotal transformation was the appearance of a single-ovary reproductive system. "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," report a team of Chinese scientists who are adding new details to the mosaic of understanding how terrestrial dinosaurs gave rise to birds and powered flight.

These scientists, led by the director of the prestigious Institute of Vertebrate Paleontology and Paleoanthropology in Beijing, posit that this adaptation of the reproductive system likely occurred "gradually during the evolution of dinosaurs and basal birds," yet add that until recently, the fossil record provided only scattered evidence on the timeline of these phylogenetic changes.

But the discovery of a series of fossils that include evidence of single ovaries in the basal bird Jeholornis and in an array of more derived enantiornithine birds from the Cretaceous, in what is now eastern China, is helping them pinpoint the timing of the pared-down reproductive system of avian dinosaurs.

Members of Aves, which includes the common ancestor of the 150 million-year-old Archaeopteryx and all living birds, are unique among amniotes in terms of featuring a single-ovary system, notes Zhonghe Zhou, director of the IVPP and lead author of the study "Ovarian follicles shed new light on dinosaur reproduction during the transition towards birds."

One feathered dinosaur considered closely related to birds, the oviraptorosaurian maniraptoran theropod, had two functional ovaries, according to Zhou and co-authors of the paper.

In contrast, they add, the fossil of the primitive bird Jeholornis that was recently uncovered in the northeastern Chinese province of Liaoning featured the lighter-weight single ovary system. "Jeholornis," they explain, "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."

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