Re: Pro(to)avis

[Tim Williams <twilliams_alpha@hotmail.com>]

Michael Mortimer wrote:

> > Depending on whether you are a ground-up believer or a tree-down believer, 
> > the creature would have been either a good runner (cursorial-style legs) 
> > or a good climber (well-developed manus claws and shorter legs).
>
> Er... why?  The taxa around the base of Avialae seem to be okay runners and 
> okay climbers (Archaeopteryx, Rahonavis, Microraptor, Jinfengopteryx, Mei).

I agree with Mickey.  One of the encouraging trends in recent years is that, 
when it comes to the origin of avian flight, we have steered away from the 
traditional 'trees-down' versus 'ground-up' dichotomy.  Rather, the first 
birds (and their immediate forbears, and their sister taxa) may have divide 
dtheir time between the ground and the trees (or other elevated substrates, 
like boulders as suggested by the "Pouncing Proavis" hypothesis).  
Furthermore, there is no reason why both terrestriality and arboreality may 
not have both played a role in the evolution of powered flight.  Certainly, 
_Archaeopteryx_ and many near-avian maniraptorans show a suite of features 
consistent with both climbing and running.

Another question is whether pre-_Archaeopteryx_ ancestors of birds passed 
through a four-winged ("tetrapteryx") gliding stage on the way to evolving 
flight.  Some non-avian maniraptorans and primitive birds show long hindlimb 
feathers, called "hindwings" or (more imaginatively) "butt-fans".  In 
_Microraptor_ and _Cryptovolans_ (which are probably the same) there are 
long feathers on the tibia and even longer feathers on the metatarsus.  Long 
metatarsal feathers are also seen in the putative non-avian _Pedopenna_.  
One enantiornithine specimen (IVPP V13939) exhibits long feathers on the 
tibiotarsus.  _Archaeopteryx_ shows short feathers on the tibiotarsus, but 
we cannot know (yet) whether this character in _Archaeopteryx_ is incipient 
or represents a secondary reversal from a four-winged ancestor that had 
_Microraptor_/_Pedopenna_-like hindwings.

> > At any rate, this taxon probably lived sometime in the late Triassic or 
> > earliest Jurassic.
>
> This I have to disagree with.  We don't even have tetanurines from that 
> early.

I wonder... if the Ceratosauria is paraphyletic, and coelophysoids and 
ceratosauroids (=ceratosaurids, abelisaurids, etc) are successive outgroups 
to the Tetanurae, would this mean that tetanurines might not have evolved 
until the Jurassic?

> Falcarius killed the segnosaurian identity of Eshanosaurus.

Any takers on what _Eshanosaurus_ really is?  I still suspect that it's 
theropodan rather than a sauropodomorph.

> I say Middle Jurassic, where we have taxa slightly less derived than 
> Archaeopteryx (Epidendrosaurus), and basal paravians (Pedopenna).

The age of _Epidendrosaurus_ and _Pedopenna_ depends upon the dating of the 
Daohugou Beds, and a Middle Jurassic age is still controversial.  It would 
be nice though...

Cheers

Tim