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

[Sim Koning <simkoning@msn.com>]

> From: twilliams_alpha@hotmail.com
> To: simkoning@msn.com; dinsoaur@usc.edu; twilliams_alpah@hotmail.com
> Subject: RE: Campbell's even crazier than a MANIAC? (archeopteryx
> Date: Wed, 24 Sep 2008 00:37:59 -0500
>
>
>
>
> 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 know you've seen this comparison 100 times, but how heavily does 
dromaeosaurid anatomy differ from that of a hoatzin chick 
http://en.wikipedia.org/wiki/Image:Hoatzin_chick.jpg

Also, the greatly elogated digits of an Archaeopteryx manus could have been 
entirely an adaption for an arboreal lifstyle. Iguanas have greatly elogated 
toes for exactly same reason, and they lack pretty much any other adaptation 
for arboreality, yet they are habitual tree climbers. In fact the greatly 
elogated 3rd digit of an Epidendrosaurus manus closely resembles the elongated 
toes of an iguana's foot. Given real world examples such as iguanas, which have 
virtually no morphological differences from ground dwellling lizards (except 
for their elongated digits) I don't think it's easy to say Archaeopterx was not 
a habitual climber simply because it doesn't have more than one adaptation for 
climbing. The fact that they seem to be well adapted for running along the 
ground could simply be evidence of Archaeopterx striking a balance between an 
arboreal and cursorial lifestyle.

>> 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.

It IS easier to become airborne from an elevated position and you can also 
travel much further. Even if theropod could jump really high, it would still 
only be falling from a few feet, which would only give it a tiny fraction of 
the air time it would get from jumping out of a 50 foot tree. Plus after 
jumping it would immediately begin to slow down, and in the end it would have 
been better off simply running. 

> 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.

I think it does, simply because there has to be a transition between a simple 
fall and powered flight, and gliding is much less energy consuming than 
flapping your wings to achieve what is basically a prolonged fall. Yes I guess 
an animal could flap its wings to prolong its fall, but why waste the energy 
when it could achieve the same thing by evolving a larger wing surface to 
simply glide.


> 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.

I completely agree.

>> 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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