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Re: origin of birds



Mickey Rowe writes:
 > HORTON@BCRSSU.AGR.CA (Scott Horton) writes:
 >
 > >How do we know that dinosaurs did not "live" in trees? Perhaps
 small
 > >carnivores like Deinonychus could climb short distances into trees
 ...
 >
 > Yes, in fact (and I'm saying these things only to see if Stan will
 > shoot them down :-) couldn't it be possible that a) the semi-lunate
 > bone in the wrist of _Deinonychus_ would allow it more mobility in
 the
 > hand in order to allow it to grab onto branches at odd angles and b)
 > the "terrible claw" on the toe of _Deinonycchus_ and other theropods
 > could have been used to grab onto tree trunks like the spikes in a
 > telephone repairman's boots?  How difficult would it be to reconcile
 > such a scenario with the orientation of the back-bone and tail as
 the
 > animal(s) attempted to climb?

I think this one is fairly unlikely, though I am, perhaps, not
the best person to judge.

Deinonychus, and most other known theropods, are clearly non-
arboreal.  Deinonychus is more so than most, in my opinion,
since its large claw is not really suited to gripping, only
to ripping, and its stiff tail is more designed to balance a
runner than a percher.  I strongly suspect that a repeat of
Feduccia'a analysis on D. would show it NOT to be a percher
or climber, but rather a runner.

[The orientation of D.'s big claws is wrong for the repairman's
boots scenario - the boot spikes project laterally, D.'s claws
are oriented forward and downward].


However, I do not think it is a very big step from the smaller
coelurosaurs, like Compsognathus, to a tree-climber.  As can be
shown with living mammals, even forms not especially specialized
in that way can climb trees at need, provided they are small
enough and have small, sharp claws.  Once the habit is established
for any reason, selection will tend to enhance it (other things
being equal), since climbing trees is a good way to escape larger
non-arboreal predators (like Deinonychus and kin, which were
around 100 kg or so in weight).

That is, I suspect that Compsognathus might actually have been,
just barely, able to scramble up a tree when chased.


Amoung the non-dinosaurs, there are several good candidates for
this sort of transition, for instance Euparkeria is even more
likely to have been at least minimally capable of climbing trees.
Similar arguments can be made for Longisquama and the Lagerpetonids
and Lagosuchids.  (Actually, I suspect that the pterosaurs derived
from a stock close to the last two mentioned).


In analysing the evidence, one important piece is the clear
evidence of the under-representation of birds, pterosaurs and
small theropods in the fossil record.  By and large only aquatic
or near-shore birds and pterosaurs are found in the fossil record.
This can be easily verified for birds, and becomes fairly obvious
for pterosaurs once you analyse the pattern of occurance.
[Quetzalcoatlus may be a conspicuous exception].  Since small
theropods had a similar bone structure to birds (light, hollow
bones with pneumatic spaces), it is essentially certain that
"coelurosaurs" are lmost as poorly recorded as birds.
[By my estimate, less than 1% of all extinct bird species
have been found - I would be surprised if as many as 5%
of the "coelurosaurs" have been found].


Unfortunately, this leaves us in the nasty position of arguing
from ignorance.  We simply do not have a sufficient sample of
the possible ancestors and basal avian stock to determine the
actual ancestry with any certainty.

Now, in this regard, I consider the recent find called
Mononykus to be quite revealing.  Even though it is seems
to be carinate (that is it has an avian sternal keel), and
may thus be a bird, it gives an idea of what an arboreal
"coelurosaur" might have been like.

swf@elsegundoca.ncr.com         sarima@netcom.com

The peace of God be with you.