Paleontologists discover fossilized ovarian follicles in three birds from Early Cretaceous

March 18, 2013 by Bob Yirka, report

Fig.1 One of two enantiornithine with fossilized ovarian follicles. Credit: ZHOU Zhonghe
( —A team of paleontologists with the Chinese Academy of Sciences in Beijing has discovered what are believed to be fossilized ovarian follicles in three birds from approximately 125 million years ago. As they describe in their paper published in the journal Nature, one of the birds appears to be a known type—Jeholornis—a bird from the Early Cretaceous that retained some non-avian features. The other two specimens were not well enough preserved to identify.

Scientists have for many years been interested in learning more about how it was that birds evolved avian traits distinct from non-avian dinosaurs—one of which was the change from two ovaries to just one. Modern crocodiles for example, have two, whereas all modern birds have just one (on the left side). It is believed this change came about to help reduce weight for flying females. Researchers have found fossilized pre-avian bird samples, called maniraptorans, which are in essence, flying dinosaurs, but evidence of birds from the time when the changeover was occurring has been scant. Fossilized bird specimens with traits retained from their forebears, such as the bony tail found with Jeholornis, are rare as are those where the single ovary can be seen. In this , all three birds have what appear to be not just single ovaries, but follicles (structures within ovaries that hold developing eggs) inside of them.

Fig.2 Close-up of fossilized ovarian follicles in one of two enantiornithine. Credit: ZHOU Zhonghe

The researchers have deduced that the structures they've discovered are follicles by eliminating the possibility that they are something else, such as seeds or rocks, rather than by proving it through analysis—their placement in the body (on the left side), for example, their shape and size, and the fact that they were compressed flat along with the rest of the body as they fossilized (indicating they were of ), all point to follicles similar to those found in modern birds. Also their numbers indicate that early birds produced far more eggs than is common with birds today—one of the specimens had 20 follicles—a sign of not just a large clutch, but of small eggs and possibly protection and care given by males. They also noted that some of the bones of one of the birds had not fused, indicating that were likely able to reproduce at a younger age than modern birds as well.

First discovery of fossilized ovaries reveals early evolution of avian reproduction
Fig.3 The evolutionary gradient from crocodilians to living birds. Credit: SHI Aijuan

Explore further: Tree resin captures evolution of feathers on dinosaurs and birds

More information: Preservation of ovarian follicles reveals early evolution of avian reproductive behaviour, Nature (2013) doi:10.1038/nature11985

The two groups of archosaurs, crocodilians and birds, form an extant phylogenetic bracket for understanding the reproductive behaviour of dinosaurs. This behaviour is inferred from preserved nests and eggs, and even gravid individuals. Data indicate that many 'avian' traits were already present in Paraves—the clade that includes birds and their close relatives—and that the early evolution of the modern avian form of reproduction was already well on its way. Like living neornithine birds, non-avian maniraptorans had daily oviposition and asymmetrical eggs with complex shell microstructure, and were known to protect their clutches. However, like crocodilians, non-avian maniraptorans had two active oviducts (one present in living birds), relatively smaller eggs, and may not have turned their eggs in the way that living birds do. Here we report on the first discovery of fossilized mature or nearly mature ovarian follicles, revealing a previously undocumented stage in dinosaur reproduction: reproductively active females near ovulation. Preserved in a specimen of the long bony-tailed Jeholornis and two enantiornithine birds from the Early Cretaceous period lacustrine Jehol Biota in northeastern China, these discoveries indicate that basal birds only had one functional ovary, but retained primitive morphologies as a result of their lower metabolic rate relative to living birds. They also indicate that basal birds reached sexual maturity before skeletal maturity, as in crocodiles and paravian dinosaurs. Differences in follicular morphology between Jeholornis and the enantiornithines are interpreted as forming an evolutionary gradient from the reproductive condition in paravian dinosaurs towards neornithine birds. Furthermore, differences between the two enantiornithines indicate that this lineage might also have evolved advanced reproductive traits in parallel to the neornithine lineage.

Press release

Related Stories

Findings show ancient birds died in flash flood

November 14, 2011

( -- During a presentation at the Society of Vertebrate Paleontology's 71st annual Meeting in Las Vegas, researchers Gareth Dyke and Darren Naish from the University of Southampton presented their findings of ...

Sex of early birds suggests dinosaur reproductive style

January 22, 2013

In a paper published in Nature Communications on January 22, 2013, a team of paleontologists including Dr. Luis Chiappe, Director of the Natural History Museum of Los Angeles County's (NHM) Dinosaur Institute, has discovered ...

Fossil egg links dinosaurs to modern birds

July 12, 2012

Researchers from Spain identified in Lleida a series of dinosaur eggs with a unique characteristic: They are oval in shape. The discovery represents proof in favor of the hypothesis that birds and non avian theropods, dinosaurs ...

Early birds had an old-school version of wings

November 21, 2012

In comparison to modern birds, the prehistoric Archaeopteryx and bird-like dinosaurs before them had a more primitive version of a wing. The findings, reported on November 21 in Current Biology, lend support to the notion ...

Primitive birds shared dinosaurs' fate

September 19, 2011

A new study puts an end to the longstanding debate about how archaic birds went extinct, suggesting they were virtually wiped out by the same meteorite impact that put an end to dinosaurs 65 million years ago.

Recommended for you

Archaeologists discover Incan tomb in Peru

February 16, 2019

Peruvian archaeologists discovered an Incan tomb in the north of the country where an elite member of the pre-Columbian empire was buried, one of the investigators announced Friday.

Where is the universe hiding its missing mass?

February 15, 2019

Astronomers have spent decades looking for something that sounds like it would be hard to miss: about a third of the "normal" matter in the Universe. New results from NASA's Chandra X-ray Observatory may have helped them ...

What rising seas mean for local economies

February 15, 2019

Impacts from climate change are not always easy to see. But for many local businesses in coastal communities across the United States, the evidence is right outside their doors—or in their parking lots.

The friendly extortioner takes it all

February 15, 2019

Cooperating with other people makes many things easier. However, competition is also a characteristic aspect of our society. In their struggle for contracts and positions, people have to be more successful than their competitors ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.