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Monday, October 21, 2013

A tale of two tails

What’s a bird to do if his flight-approved, aerodynamic tail isn’t showy enough to attract the ladies? Have two tails of course. Anyway, that seems to be how the Early Cretaceous bird Jeholornis solved this problem. Scientists led by Jingmai O’Connor of the Chinese Academy of Sciences discovered that Jeholornis had two functional tails.

Before I continue, I want to make one thing clear. We’re not talking about two separate strings of vertebrae. If you remove all the feathers, Jeholornis had a normal-looking skeleton with a single long vertebral tail. However, once you put the feathers back on, you get two distinct tufts of feathers at each end of that vertebral trail that most likely served very different purposes.

Reconstruction of the plumage of Jeholornis. (Scale bar: 5 cm.)
DOI: 10.1073/pnas.1316979110.

In modern birds, the last few vertebrae are fused into a solid structure called the ‘pygostyle’. The flight feathers of a bird’s tail (rectrices) are attached to the pygostyle. Jeholornis had a broad fan of feathers at the base of its backbone, similar to that seen on modern birds, but unlike all known birds, those feathers are not attached to a pygostyle. The authors suspect that this may be a feature that is unique to this species. This first ‘tail’ no doubt assisted the animal in flight. 

The second tail is composed of a feathery frond at the end of the vertebral column. This showy spread of feathers might not have hindered the bird in flight, but it was probably primarily ornamental. The researchers do not know whether it was present in both sexes or only one.

A prehistoric bird with two tails.
A reconstruction of a two-tailed 120-million-year-old Jeholornis. Illustration courtesy Aijuan Shi.

At first, I thought it was slightly misleading to say that Jeholornis had two tails, rather than a single tail containing two distinct features. However, these two parts of its anatomy not only served different functions, but were also attached to the body in different ways. So, I’m sticking with my title. 

Jingmai O’Connor, Xiaoli Wang, Corwin Sullivan, Xiaoting Zheng, Pablo Tubaro, Xiaomei Zhang, & Zhonghe Zhou (2013). Unique caudal plumage of Jeholornis and complex tail evolution in early birds Proceedings of the National Academy of Sciences of the United States of America DOI: 10.1073/pnas.1316979110.