Forget Everything You Know About Bird Ovaries

Lady birds have one ovary but their dinosaur ancestors had the usual two: Extraordinary fossil bird from 120 million years ago sheds light on the evolutionary change

Ruth Schuster
Ruth Schuster
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Cratoavis enantiornithine attacked by Mirischia
Cratoavis enantiornithine (looking horrified on the left) attacked by MirischiaCredit: Deverson Pepi, Wikimedia Commons
Ruth Schuster
Ruth Schuster

Can bird ovaries be preserved in the fossil record? Believe it or not, that has been a huge controversy in paleontological circles.

Now a team of Chinese scientists not only says that’s a yes, but their analysis of a bird that lived more than 120 million years old shows that avian ovarian structure differentiated itself from the dinosaurian condition very early in the birds’ evolution.

Preservation of bones going back millions of years is rare enough and requires special circumstances, let alone the preservation of soft tissues. That is incredibly rare and usually controversial – as in, are we actually seeing fossilized organs or an artifact of the mineralization process?

The new paper published in Nature’s journal Communications Biology by Alida Bailleul and colleagues from the Chinese Academy of Sciences meticulously analyzed a mysterious round structure inside one of the fossil enantiornithines found some years ago in the Jehol formation – and demonstrated that it is in fact an ovary. One ovary. Not “ingested seeds” as critics had suggested.

Today’s birds have one ovary. Yes, all those eggs we eat are the product of chickens and other birds’ single ovary. Respect! But scientists believe that the dinosaurs ancestral to birds had two ovaries, as animals in general do. So there is also a question when birds lost the second ovary, and it seems the answer is more than 120 million years ago.

Photograph of the fossil bird with ovarian follicles from the Jehol Biota of China and comparison with a chicken.
Photograph of the fossil bird with ovarian follicles from the Jehol Biota of China and comparison with a chicken.Credit: Alida Bailleul
Enantiornithine, with one ovary
Artist's impression of an enantiornithineCredit: Durbed

Of course there’s no proof that this enantiornithine is ancestral to the latter-day avian set, but it is telling.

Back to the study: In fact nine extraordinarily well-preserved bird fossils found in the  Jehol formation seem to present soft tissues, including ovarian follicles: one very early one, a Jeholornis; one Eoconfuciusornis; and seven enantiornithines. In fact taking the bigger picture, Jehol was a hotbed for soft tissue preservation: Previous studies have identified not only feathers in these early birds but actual skin, cartilage and ligaments, and lung tissue.

In the case of this enantiornithine, close inspection revealed that the round bit inside it, that some insisted was “ingested seeds,” possessed the tissue characteristics of the smooth muscles fibers intertwined with collagen fibers that exist in the ovarian membrane of today’s birds. “The tissues forming seeds (or any type of plant material) are completely different and their cells possess a distinct cell wall absent in all animal cells,” the team observes.

The team could even identify fossilized blood vessels, which ingested seeds would not have, and report finding no evidence of plant tissue.

A different fossil enantiornithine
A different fossil enantiornithine Credit: Yuguang Zhang / Jingmai OCon

And they concluded that not only can bird ovaries be preserved in the fossil record, but what they have in the nine Jehol birds is the left ovary (because it was on the left side of the body), and the right one was functionally lost early in avian evolution.

Why do we think the dinosaurs ancestral to our feathered friends had two ovaries? Because of fossils such as an oviraptorosaur (a toothless feathered group of the Cretaceous) with two eggs preserved dorsal to its pubic bones. And that, dear reader, suggests that the loss of function of the right ovary occurred close to the dinosaur-bird transition.

And why might birds have lost one ovary? What is the evolutionary advantage? The timing – close to the start of the avian lineage – suggests a link to the evolution of flight. In short, they reduced mass.

Apropos ovarian production, one thing birds apparently kept from their dinosaurian heritage is gaudy eggs, previous research has shown.

All this makes one wonder how many ovaries pterosaurs had. Answer: two. Like in the case of the oviraptorosaur, a pterosaur fossil with an egg was found to have a second egg inside the body cavity. In other words, losing an oviduct was not a prerequisite for developing powered flight – a 2015 paper (by a different team at the Chinese Academy of Sciences) concluded this. Who knows, maybe sheer efficiency contributed to the survival of birds, while the pterosaurs died out.

Artist's impression of an oviraptorosaur, Gigantoraptor
Artist's impression of an oviraptorosaur, Gigantoraptor Credit: Debivort / ArthurWeasley