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Re: Dinosaur Books and Movies



Brian wrote-


> Seriously though, what anatomical features qualify a candidate for bird
> status? Where do dinosaurs end and birds begin? I remember GSP pointing
out
> that there are no great anatomical differences, but what minor ones exist
to
> define Aves?

Well, Aves is officially defined as all descendants of the most recent
common ancestor of Archaeopteryx and neornithines, so the following
characters might apply.
- reduced olfactory lobes
- less than nine caudal vertebrae with transverse processes
- very short anterior chevrons, none taller than anteroposteriorly long
- manual digit I doesn't extend past manual phalanx II-1
- phalanges on third manual digit reduced (III-1 and III-2 sutured, not
jointed; possibly ancestral to the pygostylian condition of having two
non-ungual phalanges on digit III)
- proximodorsal ischial process
- tibia subequal in width and length in proximal view
- reversed hallux
These characters are present in Archaeopteryx and pygostylians, but not in
most non-avian dinosaurs.  However, forms like Rahonavis, Microraptor,
Sinornithosaurus and Bambiraptor have uncertain positions compared to
Archaeopteryx and show some of the above characters.  Rahonavis has 2, 6, 7
and 8, but lacks 3.  Microraptor might have 8, but lacks 6.
Sinornithosaurus lacks 5 and 8, but has 4 and 6.  Bambiraptor lacks 4 and 5,
but has 6.  Unenlagia is very fragmentary, but lacks 3 and has 6.  Thus, the
exact set of characters that define Aves is uncertain due to the possible
membership of some basal taxa.  Then there's the distinct possibility
Archaeopteryx is closer to dromaeosaurids sensu stricto than it is to
pygostylians, which would suggest the above characters reversed in
dromaeosaurids.  Finally, some non-avian dinosaurs do exhibit a few of the
characters above-
- reduced olfactory lobes (oviraptorids, Avimimus)
- less than nine caudal vertebrae with transverse processes (compsognathids)
- tibia subequal in width and length in proximal view (Bagaraatan,
Mononykus)
- reversed hallux (Caudipteryx)
Still, parsimony would suggest these evolved in parallel.  As usual, a
complex and incomplete answer to a simple question. :-)

Mickey Mortimer