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In a message dated 96-09-29 15:46:13 EDT, znc14@ttacs1.ttu.edu (Jonathan R.
Wagner) writes (concerning _Megalancosaurus_):

>        And this makes it a good protobird?!?!?!  A prehensile tail?  Just
> where is there any evidence for THAT in bird evolution.  I bet it has a long
> trunk too.  that really makes it look like a bird!  And sprawling limbs?  I
> don't quite get how everyone thinks an arboreal animal is going to develop
> an erect gait on it's own.  The illustrations for George's BCF article in
> Omni say it all:  there is no reason for an arboreal animal to develop an
> erect gait, and there is no expalantion for a quadrapedal animal developing
> a flight surface on its front limbs only.  And don't bring up primates.  No
> potential bird progentior could have loooked like a narrow-tailed
> flexible-limbed, weird thing like a monkey. 

The prehensile tail, which (at least in _Drepanosaurus_, a close relative of
_Megalancosaurus_ from the same locality, even has a specialized hook at the
tip), makes it a good tree-dweller. This is all we need of the ancestral bird
at this point in avian evolution. And yes, it had a long trunk and sprawling
limbs. And the ankles seem to be crurotarsal, not mesotarsal as in dinosaurs.
But there was a sharp cervical-dorsal shape-change, as to be expected in a
good ornithodiran.

Don't forget that _Megalancosaurus_ was not >the< bird ancestor; it only
shared a common ancestor with birds farther back in time. Like _Longisquama_,
it exhibits a host of derived features that completely eliminate it from the
>central< dino-bird lineage. It's just another side-branch, like the later
theropods were.

And I have news for you: the very earliest-known dinosaurs, specifically
_Pisanosaurus_, _Herrerasaurus_, and even large prosauropods such as
_Euskelosaurus_, were not >fully< erect. Their hind limbs spread outward
somewhat, and their feet were "pigeon-toed." Several authors have commented
on this over the years, notably van Heerden on _Euskelosaurus_. This means
that a >fully< erect gait developed convergently in sauropodomorphs,
ornithischians, and theropods. Not much of a stretch, however, since they
were almost there before their divergence.

If arboreal animals cannot develop an erect posture, then just how >did<
monkeys do it? Ditto kangaroos, whose present-day smaller, arboreal relatives
are pretty much erect.

Your comments about acquiring a fully erect gait address a point that I've
been pondering for some time myself. One way to overcome this little
difficulty is to suggest that dino-birds (like other thecodontians) were
already semi-erect quadrupeds when they became arboreal. This also makes it
easier to understand where phytodinosaurs came from and why most of them
remained quadrupedal.