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EARLY BIRD EVOLUTION
Concerning the discussion on the origins of birds, flight, and its early loss.
It has been suggested that Archaeoptyeryx shares with later birds derived
nonflight features not observed in potentially flightless dinosaurs. This is
true. But it is also true that the dinosaurs also share nonflight features
with birds not present in Archaeopteryx. For example, dromaeosaurs were more
avian in lacking the ectopterygoid process of the pterygoid, and having well
developed metotic struct posterior to the fenestra ovalis in the middle ear,
cervical ribs that do not overlap, a
more retrtoverted pubes, and a more reduced calcaneum. Of course alvarezsaurs
are packed full of avian features not seen in Archaeopteryx.
Also, the absence of such features is not as informative as we would like. For
a long time the presence of robust superior temporal and postorbital bars in
bird-like dinosaurs was cited as evidence against there being closer to modern
birds than Archaeopteryx, in which these bars are slender or in the case of
the postorbital bar possibly incomplete, and the jugal is no longer
triradiate. As Chiappe showed at SVP, Confuciusornis has robust bars that are
little different from those found on Permo-Triassic diapsids, with a fully
triradiate jugal. Obviously, either major reversals and/or parallelism is
Some of the birdy features of Archaeopteryx are minor and almost approached in
bird-like theropods. For example dromaeosaur palatines are almost triradiate,
the fourth process is vestigal. The jugal of troodonts is less triradiate tha
that of Confuciusornis.
Teeth are very plastic and readily modified. It is hardly likely that tiny
protobirds would retain the balded serrated teeth typical of larger predaceous
theropods. But if secondarily flightless theropods grew large again they could
be expected to reevolve bladed teeth to hunt big game. It is notable that none
of bird-lik etheropods retained typical theropod teeth, in which anterior and
posterior serrations were both fine and subequal. Tooth modification hints and
an evolutionary loss of basic theropod tooth design and its later partial
redevelopment after flight was lost.
The phylogentic problems posed by early birds and potentially bird-like
theropods are complex and intricate, and not subject to simple analysis.