[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
Re: New alvarezsaurid
>Archeoptegerids, hesperornithiformes, enantiornithines, etc.. All
>extinct lines of avian evolution, though impressive in that each group
>came up with something none of the others did, such as the fully
>reversed hallux, or fused carpometacarpus, keeled breastbones, loss of
>teeth, and so on. While very similar in appearance to monder avians,
>these had teeth, clawed hands (*Eoalulavis* got the first alula,
>and lost the thumb claw in the bargain) none of these have that
All ornithirines had a prokinetic snout as shown by Witmer and
Martin. The new enantiornithine described by Sanz et al. had a very
ornithurine-like quadrate with a prominent orbital process; the jugal is
also reduced to a jugal bar in that enantiornithine. Cathayornis seems
to have all of the features of the new enantiornithine with another
feature: an ossified mesethmoid, which marks the development of a
bird-like nasofrontal hinge in enantiornithines. Archaeopteryx lost the
ascending process of the jugal and squmosal-quadratojugal contact and
had a relatively more adavanced quadrate, all of which suggest a
primitive system of kinesis.
>Alvarezsaurids came up with the keel and opisthopubic pelvis, it seems,
>independent of the hesperornithiforms or whoever else developed the
>back before the K-T boundary.
Why does it seem that they evolved it independly of other birds?
>What we have here is a lot of "avian" groups producing one, two, or
>several characters thought to be distinctly bird-ish. BCF would say
>supports their case, and indeed it does, but what if it means that
>all these groups were playing with the various characters but not
>getting the right number of them to turn them POOF! into birds, as per
>Fedducia? Yes, they could do that, and more. One line, not so
>such as avetheropods (as opposed to maniraptorans) such as
>oviraptorosaurs, who have a variety of characters unique to birds,
>including the palate structure, who are morphologically similar to
>alvarezsaurids like *Mononykus*, *Patagopteryx*, or *Shuvuuia*, who all
>have avian characters in the sternum and skull. And these in turn could
>give rise to smaller, longer-armed creatures who continued to reduce
>tail and eventually formed a pygostyle, and the skull, of course,
>to change only slightly. So, BADD has its merits.
Alvarezsaurids are oviraptorids that turned into birds? First of
all, I don't think that oviraptorids had a triradiate palatine or any
other of the palatal features of birds.
>So birds could have arrived from theropods, and have turned into
>theropods, all at the same time, and the group we commonly think of as
>birds would have to be reconsidered. The fact is, all three theories
>have their salient (and equivocal) points, and what we may actually
>today is two different lines of evolution that have horrible converged
>upon each other, or one line that arrived from a hitherto-unknown line.
This conclusion is based on misinterpreted evidence. We can trace the
"gathering" of neornithine traits quite clearly. All birds had a
reversed hallux, it was lost in hesperorithiformes just as it was in
loons and grebes. The keeled sternum was also lost in
hesperornithiformes because it did not use its forelimbs for anything.
Enantiornithines have a carina, though it is in a posterior position as
opposed to the ornithurine anterior. The similiarity of the
scapulacoracoid in oviraptorosaurs and alvarezsaurs is actually just a
similiarity that is brought on by the flightless nature of alvarezsaurs
( compare a Diatryma scapulacoracoid to a Tyrannosaurus and see the
convergence ). Aves is clearly a natural group.
Get Your Private, Free Email at http://www.hotmail.com