The structure illustrated in the recent National Geographic article as occurring on the "unnamed ovoraptor=Nomingis brevicauda" is certainly open to an alternative interpretation. Note that the fusion of bones occurs distally on the elongated tail, not at the very base as in modern birds. It could be that this is simply a pathological condition of bone fusion. Recall that the entire posterior portion of the avian skeleton is subject to fusion (e.g. distal limb bones, synsacrum, pygostyle). The condition illustrated double page spread (p 104?), might be only a fusion of tail vertebrate. In the figure caption the structure is refereed to as"an incipient version of the pygostyle, which provides a foundation for tail feathers.
This is boat a cautions and ambiguous statement. Fusion of the most distal tail vertebrate may or may not be related to the development of a pygostyle. The structure may not develop by fusion of the tip to the base, but as a process related to the fusion of the sacral vertebrate. It is also possible that the author was implying that the pygostyle in modern birds "provides a foundation for tail feathers". There is, of course, no evidence for the existence of tail feather in this particular specimen. The reconstructed figure clearly has a fan of tail feathers as do most modern birds.
The only other direct evidence bearing on this issue is Archaeopteryx which had a lengthy tail with modern style rectrices attached in series along its length and perhaps Protarchaeopteryx. However the mechanism that relates the reptilian tail and the pygostyle is still unknown. While there are some good guesses from development, the evolutionary course is still speculative.
Alan H. Brush
92 High St.
Mystic, CT 06355