[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: New Jeholornis specimen

Gregory S. Paul wrote:

Tim, like others, is making statements that expose a lack of knowledge about the issue of bird origins.

Ouch!! ;-)

Of course basal avepods were not flying - the extensive arm motion
was for predation - this was a true exaptation that allowed small avepods to develop a flapping wing without even trying.

IMHO, the above scenario lacks ecomorphological rigor. What advantage would an incipient flapping wing be to a small ground-running avepod? Of course, maybe these little avepods climbed trees (as you aver in DA) and this motion assisted in protracting the glide. However, there's no evidence for even rudimentary arboreal adaptations until the level of Paraves/Eumaniraptora - and DA claims that avepods had an extensive pedigree in trees long before scansorial critters like _Microraptor_ and _Archaeopteryx_ arrived on the scene.

Tim, like many others, continues to fail to understand the basic principles and methods at work here

I believe I do understand the principles and methods. It's just that I don't always agree with them. Two different things.

If characters are exaptations for flight, then they should be present in nonfliers or well developed or weak fliers.
If characters are flight characters then they should be present only in fliers.

Both make assumptions about the reasons behind the inception of flight-related characters. Thus, you conflate incumbent function with evolutionary genesis. Here's an example. You write:

The ribcage, sternal complex, hands and primary feathers of Jehol
dromaeosaurs are all far better adapted for flight than Archaeopteryx in numerous regards. <snip> For dromaeosaurs to have had less flight performance than Archaeopteryx they would have had to obstinately failed to use the full power of their larger
arm muscles to power fly, instead only gliding like dinosaurian coach potatoes. The improved wing feather attachments and stengthened sternal-ribcage complex would go to waste.

In modern birds, the uncinate processes reinforce the ribcage so it
will not deform during execution of the powerful flight stroke. However, although this is inarguably a flight-related feature in modern birds, uncinate processes are not present in _Archaeopteryx_. Thus, they were not essential for flight. They are present in Jehol dromies, as you note, as well as _Velociraptor_.

Another hypothesis may be advanced for the genesis of the "strengthened
sternal-ribcage complex": it was originally associated with big game predation. No dromie that habitually grappled with large prey (a _Protoceratops_, for example) wanted his or her lungs to be crushed during the process. This feature was co-opted in Aves for powered flight. (This "just-so" story is actually congruent with the most recent theropod phylogenies.)

To argue that the advanced flight adaptations of dromaeosaurs were basal exaptations for flight means that somehow these same features disappeared in Archaeopteryx, itself an animal fully capable of flying.

True, Archie was capable of flight. But no one disputes that its flight abilities were far inferior to those of most modern birds, or even Cretaceous fliers like _Sinornis_. And no one can state to a certainty that every component of _Archaeopteryx_'s flight apparatus represents the primitive avian character state rather than a secondary reduction or devolution. Sure, Archie was probably close to the direct ancestor of all other birds; but every aspect of its anatomy cannot be assumed to be an exemplar of the primitive avian condition.

Following on from this, the absence of uncinate processes from _Archaeopteryx_ might very well be secondary. Perhaps its weak flight abilities were secondary and attributable to its insular environment - an interpretation that you actually mention in DA.

In this strange scenario the big sternum of basal dromaeosaurs evolved for predation or something, along with the flattened finger, despite neither being known in any [non] flier.

This is circular. You assert that the Jehol dromaeosaurs could fly, and then use this assertion to argue that the enlarged sternum and flattened finger are therefore flight-related characters.

Then these classic avian flight features were lost in Archaeopteryx despite its fully developed wing array, only to reappear in
latter birds for purposes of flight.

The "reappearance" of this feature in later birds is an artifact of an imperfect fossil record, not of the evolutionary process.

Look, I'm willing to consider that deinonychosaurs and oviraptorosaurs, and perhaps therizinosauroids as well, are really secondarily flightless. What I baulk at is the assumption that every derived character shared by certain maniraptorans and all birds above the level of _Archaeopteryx_ was selected for improved flight performance. This list includes: enlarged sternum; uncinate processes on the ribs; elongated arms and hands; automatic elbow-wrist flexion; the "swivel-wrist" and arm-folding; flattened finger supporting the primaries; abbreviated tail skeleton; and vaned-and-barbed feathers.

Some among this cast of characters were no doubt engendered by improved aerial performance (e.g., pennaceous feathers). (BTW, this is not the same as saying pennaceous feathers evolved for powered flight.) But most of the osteological characters may have been originally brought about by a very different adaptive rationale - e.g., predation. I have seen no refutation of the hypothesis that many (most?) of these osteological characters originally had a non-aerodynamic function. Instead, DA posits that characters that are clearly flight-related in birds must have began as flight-related characters.



Add photos to your e-mail with MSN 8. Get 2 months FREE*. http://join.msn.com/?page=features/featuredemail