Modern birds (Neornithes) have a reproductive system that is highly modified compared to other amniotes. They are the only amniotes with a single functional ovary and oviduct (although within Aves there are a few exceptions, such as the Kiwi bird). I described nine fossil birds from the Jehol preserving remnants of the ovary. These specimens indicate that the loss of the right ovary occurred very early in avian evolution.
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