A field guide to mesozoic birds and other winged dinosaurs

ers are anchored to a single ploughshare-shaped bone, called the pygostyle,
and so form a fan that the bird can often fold and unfold with the help of
the attendant musculature of what is known as the rectrical bulb.
However, this distinction is not cut and dry among Mesozoic birds.
The advent of the true fan tail seems to have come later than previously
thought. Many early short-tailed birds, like the long-winged, beaked confuciusornithids,
only had a pair of specialized ribbon-like feathers on the
tail, and then only in one sex. More than half of known Confuciousornis
sanctus specimens lack rectrices; their tails are merely fluffy stumps, covered
in contour feathers. The stump-tailed forms may represent females, or
alternately, individuals undergoing a molt. The ribbon-like feathers of (presumably)
males are also found in many specimens of enantiornitheans (the
“opposite birds”), usually in pairs of two or four feathers, but up to eight in
some species. All have fused, shortened tail vertebrae at the tail tip, similar
to the modern pygostyle. However, in these birds the fused tail structure is
simple and rod- or dagger-shaped, not the complex pygostyle with attendant
musculature of modern birds, and so the tail feathers could not have
been opened or closed to aid in landing or turning in mid-flight. These intermediate
short-tailed birds are perhaps best termed “ribbon-tailed birds”
after this unique configuration.
True fan-tailed birds seem to be more advanced, and consist mainly
of modern birds (Aves) and their closest relatives (collectively the euornitheans,
“true birds”). These are distinguished by the presence of a fan of
graduated rectrices, with at least the most posterior two attached directly
to a ploughshare-shaped pygostyle. The pygostyle is also associated with
a muscular organ called the bulbi rectricium (rectrical bulb), to which all
the rectrices anchor. The bulb is what allows the feathers to expand and
contract in a fan-like manner, allowing for far greater maneuverability than
the forebears of these species could achieve. In birds without this structure,
the tail feathers would have been largely immobile and in most cases were
purely for display.
Frond-tailed birds, ribbon-tailed birds and fan-tailed birds all coexisted
during the rapid diversification of avialans in the early Cretaceous.
Most flying birds of the late Cretaceous were members of the two primary
short-tailed lineages: Enantiornithes (ribbon-tailed) and Euornithes (fantailed).
Enantiornithes were the dominant flying birds of the Mesozoic, but
all became extinct along with most other remaining dinosaur lineages 66
million years ago. Only a few lineages of euornitheans survived, and these
were all members of the groups that would eventually produce ratites, fowl,
and shorebirds, and which diversified into all of the other modern bird
groups during the Cenozoic era.
26