Fossil by fossil, paleontologists have traced birds back to their dinosaur ancestors. As they’ve done so, traits once thought to be distinct characteristics of avians – such as feathers and air sacs – have turned out to be dinosaurian features that evolved long before the first true birds. But for all their shared characteristics, a significant reproductive detail separates birds from their dinosaurian ancestors. Whereas non-avian dinosaurs had two functioning oviducts, modern birds only have one. Now a set of three fossils suggests that the system of having one active oviduct evolved very early in bird evolution, about 120 million years ago.

Paleontologist Zhonghe Zhou of the Chinese Academy of Sciences in Beijing and colleagues detail the intricately-preserved fossil birds online today in Nature [1]. The fossils in question are a specimen of Jeholornis, an early bird that retained archaic characteristics such as a long bony tail, and a pair of birds that belonged to an entirely extinct group called enantiornithines. All three fossils, according to Zhou and coauthors, contain delicately-preserved ovarian follicles that could have turned into eggs.

“It took us a while to figure out what these strange circular structures actually represent,” says Zhou. The small, rounded structures seemed like they could be seeds or tiny stones the birds swallowed to grind food in their digestive system. But based on the size, shape, and position of the rounded structures, Zhou and coauthors ruled out the alternate explanations and interpreted the endofossils as follicles. Furthermore, the researchers point out, the follicles appear to be preserved on the left side of all three birds – just as in their still-living relatives, but unlike non-avian dinosaurs. Indeed, the fossilized hips of an oviraptorosaur – a feathered, beaked theropod dinosaur – contained two eggs, hinting that one egg developed in each oviduct and showing that non-avian dinosaurs retained two functioning oviducts, similar to modern crocodiles [2]. Birds shut down right oviduct, which the new fossils indicate occurred by the Early Cretaceous.

“We are pretty sure non-avian dinosaurs have a more crocodilian style of reproduction, with two functional ovaries and oviducts while avian dinosaurs have only one ovary and oviduct like most living birds,” Zhou says. Yet this doesn’t mean that early fossil birds were exactly like their living kin. Even though the fossil birds apparently had only one active oviduct, they still produced a greater number of eggs at a time. “The ovarian follicles in Jeholornis are more like that of a crocodile,” Zhou says, pointing out that the follicles in the archaic bird were “more numerous and relatively smaller, and the variation in size among the follicles is lower, compared to the enantiornithine birds.” Had the follicles developed into eggs, Jeholornis might have laid as many as twenty eggs in a clutch, while the enantornithes would have laid five and twelve, respectively.

But why did the reproductive plumbing change in birds? Traditionally, biologists have suspected that the switch from two active oviducts to one was tied to the evolution of flight. Since Jeholornis and the two enantornithes were early birds that were relatively close to the transition from non-avian to avian dinosaur, Zhou and colleagues say, the switch to one oviduct indeed seems to be correlated with the evolution of flight.

Of course, the reproductive implications of the find hinge upon the identity of the fossils as true ovarian follicles. University of Maryland paleontologist Thomas Holtz, Jr. agrees that the size, shape, and position of the rounded bodies are consistent with the interpretation of the fossils as follicles, but notes that alternative hypotheses – such as arthropod, amphibian egg, or plant fossils – can’t be ruled out just yet. “High quality SEM scans of the objects might help resolve this,” Holtz says. Still, provided that the objects really are follicles, Holtz agrees that the fossils show an intermediate state between non-avian dinosaurs and modern birds, and that the presence of one active oviduct would be “loss of a redundant organ to save weight.” If Zhou and coauthors are right, Holtz says, “this is really cool! If it isn’t, we’ll have to see what they really are.”