Living archosaurs crocodilians and birds differ greatly in their reproductive strategy; these specializations in Aves evolved during their evolution from dinosaurs. Crocodilians have two ovaries, as do all known dinosaurs, while living birds have only one. All fossil bird specimens preserve only the left ovary, as in living birds. Jeholornis is one of the most basal birds known, indicating that the right ovary was lost very close to the dinosaur-avian transition. This confirms hypotheses that the right ovary was lost in order to reduce weight in flight.
The difference in size of the ovarian follicles in each specimen records an evolutionary gradient from the large clutches of smaller eggs present in crocodilians and non-avian dinosaurs to the smaller clutches of large eggs present in living birds. The morphology of the ovarian follicles indicates that the yolks grew more slowly than those in living birds, consistent with the lower metabolic rate determined for Mesozoic birds through histological analyses that shows bone was deposited more slowly. Living birds grow very quickly and reach skeletal maturity before they become reproductively active. Histological analyses of these new specimens show that, like non-avian dinosaurs, basal birds became sexually mature before adulthood.
"It took us a while to figure out what these strange circular structures actually represent," said Dr. ZHOU Zhonghe, project leader of the IVPP . The small structures might possibly have been seeds or tiny stones the birds had swallowed to grind food in their digestive system. But on the basis of the size, shape, and position of the rounded structures, the team ruled out the alternative explanations and interpreted them as ovarian follicles.
"These fossils are incredible, we may never have another discovery like it," said author Dr. Jingmai O'Connor from the IVPP, "They answer many important questions regarding the evolution modern avian reproduction. The biological insights provided by fossils from the Jehol are unparalleled."
The fossils furnish information about the reproductive habits of these animals. In one of the birds, some of the bones in the creature's wing weren't fully fused, Zhou said. That suggests that the creature wasn't an adult, hinting that females of its species became sexually mature before they were fully grown.
The new findings "are very exciting," said Frankie Jackson, a vertebrate paleontologist at Montana State University, Bozeman. Besides revealing that even early birds had reproductive biology significantly different from that of their closest dinosaur kin, they provide a new approach to estimate the brood size and the onset of sexual maturity in adults. These reproductive traits—including the loss of one of the ovaries, which probably would have rendered egg-laden females significantly lighter—likely had a substantial impact on the evolution of flight. That development may even have helped some bird lineages survive the mass extinctions in the wake of an asteroid impact that claimed the dinosaurs 65 million years ago.
Provided by Institute of Vertebrate Paleontology and Paleoanthropology
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