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