Re: [asa] Did dinosaurs become chickens?

Don:

>
> My general reading in recent years leaves me with the impression
> that not all expert paleontogists buy into the bird-from-dinosaur
> idea. I'd appreciate a comment on the status of that controversy.

Yes there are a few paleontologists that dissent from the consensus.
There are also some ornithologists who also dissent from the dinosaur-
to-bird model. The alternative model supported by these dissenters
is the thecodont-to-bird model. In this model dinosaurs evolved from
the group of reptiles from which dinosaurs and crocodiles also
evolved. In this way dinosaurs and birds are sister groups (having a
common ancestor in the thecodonts), rather than birds being a
subgroup of specialized dinosaurs. The thecodont model was the
dominant one until fairly recently, and until the many new
discoveries of the last couple decades it was an entirely reasonable
one. It is very common with the rise of any new scientific theory
that the previously held view will persist for some time. This is a
good thing as it forces the new theory to continue to address
challenges posed by supporters of the earlier theory.

What has been exciting to observe over the last number of years is to
see how the objections of the dissenters have been met by new
discoveries. Nearly every major objection has been quite
convincingly answered, and (in my view and that of the great majority
of paleontologists) the dissent is becoming more and more forced.
Some of the objections that have been answered include:
1) Maniraptoran dinosaurs do not possess a furcula ("wishbone") that
is a critical feature of birds -- furcula have now been discovered in
maniraptoran dinosaurs that closely resemble those in Archaeopteryx;
2) The pelvis of birds is different from that of theropod dinosaurs
(the group to which maniraptorans belong) -- the most bird-like
maniraptorans have a pelvis very similar to that of Archaeopteryx;
3) Maniraptoran dinosaurs appear in the fossil record much later
(late Cretaceous) than their presumed descendants (late Jurassic) --
Dromaeosaurid dinosaurs are now known from the middle and late Jurassic;
4) No dinosaur possessed true feathers -- nearly all maniraptoran
dinosaurs are now known to have been covered by various filamentous
body coverings. Some species have true non-flight feathers on their
arms and tails, and a couple have modern asymmetrical flight feathers.
5) Flight must have evolved from the "tree down" through gliding, but
dinosaurs were fast running ground animals -- very small arboreal
dromaeosaurids are now known from the late Jurassic and early
Cretaceous. At least one of these appears to have been a glider with
asymmetrical flight feathers on its arms and legs.

There remains one argument of the dissenters that has still not been
satisfactorily answered. The dissenters argue that in modern birds,
the three fused fingers of the wing represent the digits numbered
2,3, and 4. This is supported by the embryology of birds in which
the wings begin with five finger buds. The three fingers of
maniraptoran dinosaurs, however, are recognized as being digits 1,2,
and 3 based on the fossil trends in finger reduction. The current
response is that this change may have resulted later in bird
evolution by a "frame shift" in which the embryological identity of
the digits is shifted. At any rate, the fingers in Archaeopteryx and
dromaeosaurid dinosaurs are virtually identical.

This summary does not address all of the issues being debated, but it
hopefully gives you the basic idea.

Keith

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Received on Sat Jan 31 12:13:05 2009