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Re: The dino-bird connection revisited



In a message dated 96-02-19 17:38:47 EST, nking.ucs@smtp.usi.edu (King, Norm)
writes:

>These questions combine cladistics and BADD/BCF (oh no!).
>
>Among archosaurs, did birds originate the kinetic skull (the front of the 
>skull can be bent upward)?

Yes, this is strictly within post-_Archaeopteryx_ lineages.

>Does _Protoavis_ really have a kinetic skull?  (Chatterjee says yes.  Is 
>that a part of his analysis that is in question?)

Yes, this is part of his analysis that has been questioned. The skull seems
too fragmentary to tell positively that it was kinetic.

>Does _Archeopteryx_ have a kinetic skull?  (I think not--yes?)

Not as far as I know.

>If _Protoavis_ did, and _Archeopteryx_ did not, might that suggest that 
>_Archeopteryx_ is a flying theropod rather than a flying bird?  That 
>would surely explain why _Archeopteryx_ plots out very close to 
>theropods.

The akinetic skull is a vertebrate plesiomorphy, so by itself all it tells
you is that _Archaeopteryx_ was some kind of vertebrate. Other characters are
what bring _Archaeopteryx_ into a close relationship with theropods. Enlarged
cranium with lost temporal bar, as described by Larry Martin, are among the
features that unite _Archaeopteryx_ with later birds.

>What is a bird, really?  Can skeletal features provide a 
>characterization, since feathers normally won't be preserved?  I mean 
>without assuming a particular phylogeny first, since I'm not sure we know 
>whether BCF or BADD.

In the present version of BCF, birds (infraclass Aves) are all dinosaurs more
closely related to _Megalosaurus_ than to _Iguanodon_. This includes all
dinosaurs with furculae and/or feathers, for example, as well as a bunch of
other groups in which either or both of these features are secondarily lost.

What you're probably more willing to accept as birds are those _flying_
vertebrates in which the manus had only three fingers, together with
secondarily flightless forms descended from such flying ancestors. This
diagnosis includes _Archaeopteryx_ and all other fossils commonly accepted as
birds. BCF asserts that all cursorial tetanuran theropods (at least), being
secondarily flightless forms, also qualify for "birdhood" under this
diagnosis; BADD asserts that cursorial tetanuran theropods were not
secondarily flightless forms, and that any features observed common to
cursorial and flying tetanuran theropods were exapted for flight. And that's
whence any debate between those two hypotheses takes off.

>Of course, if we assume that _Archaeopteryx_ is a bird, that really stacks 
>the deck in favor of maniraptoran ancestors for birds.  What should I 
>know that would show me why the claim of theropod ancestry for birds is 
>not this transparently circular?

Chiappe 1995 and Martin 1995 describe several (rather minor) anatomical
characters that unite _Archaeopteryx_ with later birds in a clade they call
Aves. For me the strongest of these is the loss of the temporal bar--the
bridge of bone that separates the two diapsid temporal fenestrae on each side
of the skull--which occurred as part of the general enlargement of the
braincase and orbits. Not to mention flight feathers (a non-skeletal
character), of course. _Archaeopteryx_ skull material is crushed, however,
and some authorities assert that the bar was present but was lost before
fossilization.

As Ostrom has pointed out, any phylogeny of Aves _must_ deal with
_Archaeopteryx_ somewhere along the line.