A field guide to mesozoic birds and other winged dinosaurs

Above: Basic diversity of tail structure in Mesozoic birds. A) frond-tailed (e.g. basal avialans);
B) ribbon-tailed (e.g. enantiornithes); C) fan-tailed (euornithes)
The Evolution of Flight
Modern birds are perhaps best characterized by their ability to
fly, that is, generate lift under their own power. Most bird groups today
are comprised of strong fliers, and researchers in the past have often been
tempted to conflate “bird” with “flight”. However, our current understanding
of the fossil record indicates the origin of birds (or at least of modernstyle,
Stage V feathers) must have predated powered flight.
Historically, there has been a debate over whether flight evolved
from the “ground up”, that is, from strictly ground-running ancestors, or
from the “trees down”, from arboreal, climbing ancestors. This dichotomy
has often been treated as a false one, as it is entirely possible that some combination
of the two ultimately resulted in the flight of modern birds. However,
both hypotheses do make testable predictions which can be compared
with the (admittedly limited) available fossil record. A strict trees-down
origin of flight would predict that bird ancestors were already arboreal creatures
that would show adaptations for climbing. It would also be likely under
this model that flight first passed through a gliding phase, with animals
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