RE: Campbell's even crazier than a MANIAC? (archeopteryx

[Tim Williams <twilliams_alpha@hotmail.com>]

Sim Koning wrote:


> Just to be clear, I'm not trying to say dromaeosaurid claws were adapted 
> primarily for climbing. I'm 
> simply trying to say that it is a little unrealistic to say these creatures 
> weren't fully capable of climbing 
> up trees. We humans don't even have claws on our feet, yet we have no problem 
> using them to climb 
> trees. 


Nobody (myself included) is arguing that dromaeosaurs could NEVER EVER climb 
trees.  Rather, IMHO the best approach is to assess the tree-climbing abilities 
of dromaeosaurs scientifically.  Sure, lots of animals can climb trees given 
the right motivation: dogs; goats; even humans in search of coconuts.  But 
there's nothing in the anatomy of these species (dog, goat, human) to suggest 
that they climb trees on a habitual basis.  


For vertebrates, there are certain morphological traits associated with a 
scansorial or arboreal ecology.  These include: opposable digits on the hands 
and/or feet; highly mobile wrist and/or ankle joints: prehensile tail; elongate 
penultimate phalanges; curved digits (for suspensory activity); certain 
shape/curvature of the claws; and so on.  You don't have to have all of these 
features to be a scansorial/arboreal animal.  But the presence of one or more 
of these features in an animal is a good indication of scansorial/arboreal 
behavior.


_Archaeopteryx_ and some dromaeosaurs (like microraptorines) show some of these 
features - but these features are weak or incipient at best (e.g., elongation 
of the penultimate phalanges; certain modifications of the hallux; claw 
curvature).  Overall, the morphology of these theropods is consistent with 
animals that ran on two legs, and had forelimbs with minimal prehensile 
ability.  Some features indicate that they may have had some scansorial 
ability, which helped them venture into trees.  But not much.  Certain features 
indicate that climbing might have been quite difficult for _Archaeopteryx_ and 
relatives (e.g., long & stiff fingers; arcuate motion at the wrist; hinge-like 
motion at the ankle; long hindlimbs designed for running).


> I also think it's pretty obvious that basal avians such as archaeopterx must 
> have evolved from highly
>  arboreal acestors, simply becasue they could fly. 


No.  This is NOT 'pretty obvious' at all.  


In fact, there's no reason at all to ASSUME that the immediate ancestors of 
_Archaeopteryx_ must have been 'highly arboreal'.  There are people who make 
this claim, but their evidence is weak and based almost entirely on a belief 
that because it's (supposedly) "easier" for flight to evolve from a gliding 
ancestor, then it must be correct.  I've never understood this reasoning.


There are alternative models for the origin of avian flight that propose a 
terrestrial runner or leaper as the ancestor of birds.  These "ground-up" 
models often get shoved aside, because of the claim that it's "easier" to 
become airborne from an elevated position (like a tree), because the animal 
doesn't have to fight against gravity.  But what tends to get forgotten is that 
the hindlimbs of theropods were capable of considerable strength, which may 
have enabled the ancestors of birds to launch into the air without the need to 
climb up a tree to gain elevation.


Sorry to be so severe on this point, but for the longest time the science 
surrounding the origin of avian flight was held back by the assumption that 
birds must have evolved from an arboreal glider.  In doing so, the morphologies 
of _Archaeopteryx_ and its closest relatives (e.g., deinonychosaurs) were 
completely ignored.



> The intermediary stage proceeding flight has to be gliding 


No, the intermediary stage proceeding flight DOES NOT have to be gliding.  It 
might have been; but this cannot be assumed.


Personally, I think that the ancestors of birds might have spent some time in 
trees, and used their proto-wings during aerial descents.  But rather than the 
classical "trees-down" and "ground-up" behaviors being mutually exclusive, I 
suspect both arboreal and cursorial behaviors may have played a role in the 
construction of the flight apparatus of birds.  There have been quite a few 
recent papers on this, mentioned in the DML archives.


> and I don't know of any gliding animals 
> (execpt flying fish) that are not highly arboreal.


This is irrelevent.  We know of four examples of powered flight: insects, 
pterosaurs, birds, and bats.  A gliding ("trees-down") ancestry is probable for 
bats, based solely on the anatomy of modern bats (especially the incorporation 
of the hindlimbs into the wings).  But there's no reason why this should be 
assumed for birds.  (Or even pterosaurs, for that matter.)


The ancestral stages of insect flight might have worked *against* gravity.  
There's one highly plausible hypothesis which holds that the ancestors of 
flying insects used their proto-wings to skim across the surface of water, in 
the manner of some modern stoneflies.  Now, I'm not suggesting the ancestors of 
birds did this (!).  But there's no reason to assume that a passive gliding 
stage preceded powered flight in birds, any more than this has been assumed for 
insects.



Cheers

Tim


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