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More Holtz on the origin of birds



Now somebody's REALLY set me up for a lecture! :-)

Feel free to go on to the next article, if you're not into dromaeosaurids.

>       Some weird stuff about Dromaeosaurs:
>      --Reversed pubes. Why do they have these? One possibility is that 
>they evolved among flying animals to offset balance problems caused by 
>wings and large arms, or that they allowed the chest to become 
>shorter and more compact (as it tends to in flying animals). They evolved
>in herbivores, but to allow for a bigger gut. Dromaeosaurs did not evolve
>them for this reason.

Dromaeosaurids, therizinosauroids, and birds are all theropods with a
retroverted pubis.  This is associated with the decoupling of the tail from
locomotory use (the tail is a vital part of locomotion in most diapsids).

All these three groups are also characterized primitively by shortened,
thicker tibiae and metatarsi (later reversed in many bird groups).

In therizinosauroids, I would support herbivory as a reason, but you never
know..

All this points to a reorganization of the hindlimb system from the
primitive system (with long M. caudofemoralis longus providing very powerful
thrust for forward motion) to a more compact system.  Some have suggested
the new system was hopping-based.  I would suggest, alternatively, that the
new system sacrificed speed for agility (see the next point).

The long arm argument doesn't hold, since some theropods with very long arms
(oviraptorosaurs, troodontids, ornithomimosaurs) have more primitive pubes.

>      --Stiffened tail. Not uncommon among dinosaurs, but closely 
>resembles that of Archies, and less so, Rhamphorhynchs. It's possible 
>that this was evolved to serve as a feathered elevator/rudder.

The proximally mobile, distally stiffened tails of dromaeosaurids and birds
are pretty rare among dinosaurs.  Ostrom argued that the dromaeosaurid tail
was a balancing pole during combat, allowing the raptor to balance on one
foot while slashing with the other.  I would add that this tail morphology
would also allow dromaeosaurids and Archie to make rapid turns while
running, much as cats do (see the cheetah in a recent car commercial, if
nothing else).

>      --Those funny bones that lie in rows on the dorsal side of the rib 
>cage- "ulcinaries" Paul calls them. These might have served to stiffen the
>rib cage, against the contraction of powerful arm and shoulder muscles.
> Well developed on modern birds, these are absent in Archaeopterygians.

Unciates.  They may have been cartilaginous in Archaeopteryx.  Stiffening
the rib cage, as you say, would be useful for grasping predators as well as
for fliers.

>      --the folding arms. Are we seriously supposed to buy that folding arms
>were more efficient for some running theropods, and coincidentally,
> a few million years later, they happen to be useful for birds trying 
>to keep their flight feathers out of the way and out of harms way? Yeah, 
>right.         

Apparently, you are unfamilial with a large body of literature on evolution
(Darwin's 1859 book might be a good start). :-)  Evolution works by exapting
previously existing structures for new uses.

I would suggest that the arms of these theropods were NOT folded to increase
speed, but were instead folded for use in a praying mantis-like grasping
system.

>      --big breastplate. I'll bet you the furcula is unusually large as 
>well among the Dromaeosaurs. (George?)

A big breastplate only tells you that the forelimbs were powerfully
muscled.  I thing all of us agree that dromaeosaurids (and most
coelurosaurs) had strong arms.

Furculae are not yet officially reported in dromaeosaurids, but their
presence elsewhere in Tetanurae strongly suggests that they were there.

>      --big arms. Theropods in general, from Tyrannosaurs to Carnotaurs, 
>have evolved smaller arms. Why the big reversal in Dromaeosaurs? 

[Brace yourself, everyone :-) ]

AAARRRRRRRRRRRRRRRGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGHHHHHHHHHHHHHHH!!!

Would someone PLEASE kill this bugaboo of an "argument" for me?  Thank you!

Despite endless usage in ornithological textbooks everywhere, there is NO
(count them, No) major trend towards arm reduction within theropod
evolution.  A few lineages (advanced megalosauroids, neoceratosaurs,
tyrannosaurids, and Compsognathus) had reduced arms (shortened fore arms in
the first two, reduced as a whole in the second).  Among the remaining
groups of theropods, arms were of moderate (Coelophysoidea, Carnosauria) or
long (Coelurosauria) length.

There is NO reversal of arm length in Dromaeosauridae.  All of the out
groups (Oviraptorosauria, Therizinosauroidea, Arctometatarsalia, etc.) are
characterized primtively by long arms.

>       The point is not that Archaeopterygians and Dromaeosaurs share a
>lot of features. That only proves that they were related, not how. The 
>point is a lot of Dromaeosaur features seem to make little sense 
>outside the context of a flying ancestor. 

No, they DO make a lot of sense in the following scenario.  Here goes some
speculation, based on a peer reviewed phylogeny of the theropods and a good
deal of functional work, in progress:

Among advanced coelurosaurs, there evolved a clade specializing in
grapple-and-slash predation (as found today in big cats).  This clade
retained the primitive features of long, grasping arms, furculae, unciates
(maybe?).  This group was selected for reduced speed but increased agility,
taking the tetanurine trend of a distally stiffened tail to an extreme and
selecting for a reorganization of the pelvic girdle.

One branch of this clade, composed of the smaller members, were scansorial
predators.  This lineage includes Archaeopteryx.  The adaptations, already
acquired for a particular form of predation, were exapted for flight.

The other branch were larger terrestrial (scansorial as small forms?)
predators.  These became the dromaeosaurids.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist
Dept. of Geology
University of Maryland
College Park, MD  20742
Email:Thomas_R_HOLTZ@umail.umd.edu (th81)
Fax: 301-314-9661
Phone:301-405-4084