A field guide to mesozoic birds and other winged dinosaurs

Basal Avialans
The lineage leading to modern birds probably diverged from the
deinonychosaurian lineage during the mid- or early Jurassic. All birds closer
to Aves than to deinonychosaurians are called avialans (“winged birds”),
though that name has also been used for a group based on a physical character
(the presence of wings used for flight) and so might be replaced by the
PhyloCode.
Successive groups of avialans acquired more and more characteristics
of modern birds, including the fusing together of the bones in the wing,
lengthening the forearm relative to the hind limbs, and shortening of the
tail. Only a few of these more advanced long-tailed birds are known, including
the Chinese forms Jeholornis and Yandangornis. The most advanced
known bird fossil that can reasonably be considered “long-tailed” is the one
named “Zhongornis haoae”, which has a very shortened tail, though not
as short as Confuciousornis, and lacking a pygostyle or any caudal fusion.
Interestingly, it is likely that the “Zhongornis” specimen is simply a juvenile
confuciousornithid, which would have implications for the evolution
of short tails: a short-tailed bird that retained a long tail as a chick, almost
as tadpoles reduce their tails as they mature into frogs.
The avebrevicaudans, “short-tailed birds”, represent the first birds
to shorten their tails to the point that they possessed ten or fewer vertebrae.
The last few vertebrae were usually fused together into a single solid
structure, similar to the pygostyles of true birds (euornitheans). However,
there is no evidence that these tails possessed mobile feather fans--on the
contrary, most known primitive avebrevicaudans appear to have had only a
few pairs of streamer-like rectrices, if any.
Confuciusornithids and other primitive short-tailed birds generally
had long, large wings, though they lacked well-developed breast muscles
and could not lift their wings very far above their backs, preventing strong
flapping or ground-based takeoffs. The low, inward and forward-facing
halluces and large, strong wing claws imply that they instead climbed tree
trunks to reach gliding or flying height. Many species had toothless beaks,
which evolved independently of those found in modern birds.
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