Flashy Boy or Drab Girl? Sex of Ancient Bird Determined

How do you tell if an ancient bird is male or female? Check the plumage, of course. Researchers from the Natural History museum have determined the sex of a now-extinct bird.

The fossil in question is of Confuciusornis sanctus, a 125-million-year-old Mesozoic bird. It was about the size of a crow, and lived in a seasonal forest surrounded by small lakes. Researchers note that the area in which it lived was a very rich ecosystem with a large variety of both plants and animals. Hundreds of these fossils were found in lake deposits in modern-day northeastern China. Previously, scientists had noted that some of these birds had ornamental tail feathers while others did not. But it was only recently that researchers proved that those with the ornamental plumage were male, while those without it were female.

How did they come to their conclusion? Instead of relying on the fact that most modern male bird species are flashier, Luis Chiape and his colleagues unearthed more definitive evidence. They studied hundreds of the bird fossils until they found their proof: medullary bone. This spongy tissue is unique to reproductively active female birds; it helps them manufacture eggshells. Since the feathers on the fossil with medullary bone were not flashy, the scientists were able to conclude that the males were the ones that liked to put on a show.

These new findings show that like modern male birds, ancient male birds also possessed extravagant plumage. Researchers also found medullary bone in some fossils of birds that weren't fully grown, which suggests that ancient birds matured sexually long before their skeletons matured. The findings could also help scientists determine the sex of others birds that lived millions of years ago.