The dinosaurs most closely related to today's birds were super egg producers. Known as maniraptorans, these bipedal creatures had two functional ovaries and produced a large number of eggs per clutch. Living species of birds, on the other hand, have only one functional ovary, typically on the left side of the body, and most produce only a few eggs at a time. When and why did today's flyers ditch a reproductive organ? Newly described fossils of early birds may hold the answer.
All the fossils in this study come from rocks laid down as sediments about 125 million years ago in northeastern China. Despite the poor bone preservation in two of the fossils, all include the well-preserved remains of ovaries and mature or nearly mature follicles, structures within the ovaries that contain developing eggs—the first such fossils of early birds known to do so, says Zhonghe Zhou, a paleontologist with the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing. One set of remains represents Jeholornis, a long-tailed bird about the size of a pigeon. Neither of the other fossils is preserved well enough to identify its species, Zhou notes, but each represents a bird somewhat larger than a sparrow. Each of the fossils included only one ovary—which, as in modern birds, appears to be located on the left side of the body.
Analyses by Zhou and his colleagues showed that the structures assumed to be follicles weren't seeds, because they weren't located in parts of the body where stomach contents would typically be found. The structures also weren't gastroliths, or stomach stones, because such stones typically retain their three-dimensional shape, whereas these structures appeared to have been soft tissue because they had been somewhat flattened before they were preserved.
"These are really incredible fossils," says Stephen Brusatte, a vertebrate paleontologist at the University of Edinburgh in the United Kingdom who wasn't involved in the study. Preservation of soft tissues is exceedingly rare, he notes, but "to have ovarian follicles preserved is just amazing."
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 says. That suggests that the creature wasn't an adult, hinting that females of its species became sexually mature before they were fully grown, he and his colleagues report online today in Nature.
One of the fossilized ovaries includes at least 20 mature or near-mature follicles. That's a sign that unlike modern birds, early birds probably laid a large number of relatively small eggs in a single clutch, Brusatte says. "We don't have any fossil nests of early birds, but this tells us that the reproductive biology of early birds was quite different than modern birds."
Some scientists have assumed that the evolutionary loss of one functional ovary—a weight-saving change that might have proved beneficial to flying birds—took place early in avian evolution. Until the new study, paleontologists hadn't unearthed any evidence for the notion that early birds, like their modern-day kin, had only one ovary.
The findings even provide hints about how early birds cared for their young, says Richard Prum, an evolutionary ornithologist at Yale University. Previous analyses of fossil dinosaur nests have suggested that in birds' closest dinosaur relatives, which laid large clutches of eggs, males sat on the nests and presumably cared for the hatchlings. That's also true of some of today's birds—including ratites, a group that includes ostriches and emus—which typically lay eggs in several nests and then leave the childcare to males. "So, large clutch size in early birds is strong evidence for male care," Prum contends.
The new findings "are very exciting," says 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, she notes. That development may even have helped some bird lineages survive the mass extinctions in the wake of an asteroid impact that claimed the dinosaurs 66 million years ago.