A field guide to mesozoic birds and other winged dinosaurs

erately long rachides with long barbs (lacking barbules) along their lengths.
Stage III feathers may also have been present in ornithomimosaurians like
Dromiceiommus brevitertius, in which juveniles appear to have been covered
in Stage II or III down and adults appear to have had primitive winglike
arms with anchor points for pennaceous remiges and coverts, though
whether or not these were vaned (making them Stage IV) is currently unknown
(Zelenitsky & al. 2012).
Stage IV represents true, modern bird feathers. The primary barbs
and simple secondary barbules of the semiplume evolved another level of
branching, in the form of tertiary hooklets. These allow the barbules to hook
together and link barbs in a single, closed vane. Stage IV feathers are first
seen in the caenagnathiformes and basal eumaniraptorans. Vaned feathers
are aerodynamic and useful for flight, especially in some more advanced
derivations with asymmetrical vanes (where the barbs are longer on one
side of the quill than the other). Prum called asymmetrical vaned feathers
Stage V. Stage V, or flight feathers, are first found in ornithodesmids and
other eumaniraptorans, including Archaeopteryx.
The diagram on page 22 illustrates the approximate appearance of
the various feather stages in the archosaurian family tree. However, it should
be noted that new feather types often evolve from these basic structures,
and in many cases advanced feather types are lost in flightless groups. For
example, among modern ratites (ostriches, emu, kiwi, etc.), vaned feathers
have been lost. The remiges and rectrices of these birds have effectively returned
to Stage III semiplumes with central rachides but lacking barbules.
This is also true of the known feathers of hesperornitheans. In more recently
flightless birds, like the Kakapo (which still retains some gliding ability),
the wing feathers have reverted from Stage V to Stage IV, with symmetrical
vanes similar to caudipterids or Anchiornis. Given enough time, it is likely
that these feathers would lose their barbules entirely as gliding becomes less
and less an important part of the bird’s lifestyle. This degeneration of the
remiges appears to be a general trend in birds as they lose flight, probably
because vaned feathers are more costly to maintain than plumulaceous or
semiplume feathers.
Novel feather types that have evolved from the feather stages listed
above include the EBFFs of segnosaurians, comprised of long, flat, monofilament
quills; the ribbon-tail feathers of the so-called “opposite birds” (enantiornitheans),
likely produced by extending and flattening the rachis and
reducing the vane for most of its length; filoplumes, small feathers with the
barbs restricted to the tip of a thin rachis, which may act as sensory organs;
22
Opposite: Illustration of an Epidexipteryx hui, part of the Dauhugou Fauna.