Early avians lost one of two ovaries to take flight.
17 March 2013
Palaeontologists have discovered the first fossilized
traces of developing egg cells in ancient fossil birds, showing a significant
trait that already 120 million years ago separate birds from their ancestors.
Like modern birds, these ancestors already had reduced their working ovaries to
one, setting them apart from their dinosaur cousins.
Zhonghe Zhou, a palaeontologist at the Chinese Academy of
Sciences in Beijing, and colleagues studied a fossil specimen
of Jeholornis — an early bird that retained archaic characteristics
such as a long bony tail — as well as a pair of fossils that belong to the
enantiornithines, another extinct group of birds. All three fossils, according
to Zhou and co-authors, contain preserved ovarian follicles, delicate
structures containing single egg cells that would have developed into eggs. The
researchers present their findings online today in Nature.
“It took us a while
to figure out what these strange circular structures actually represent,” says
Zhou. The small structures might possibly have been seeds or tiny stones the
birds had swallowed to grind food in their digestive system. But on the basis
of the size, shape, and position of the rounded structures, the team ruled out
the alternative explanations and interpreted them as ovarian follicles.
Continued:
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