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Re: Feduccia (was: polarity of bipedality in dinosaurs)
In a message dated 96-09-26 17:29:23 EDT, firstname.lastname@example.org (Jonathan R.
> At 02:46 PM 9/26/96 -0500, Dinogeorge wrote:
>>How flying birds might have evolved from terrestrial, cursorial
>>forms is nearly impossible to determine, but it must have happened,
>>to account for all those synapomorphies.
> No, actually, plenty of theories as to how have been proposed.
> Perhaps you are confusing lively scientific debate with an inability to
> determine the *truth*. Seems to me that, with all your lecturing on how
> science works, you of all people should realize that it's perfectly ok for
> science to not have all the answers. Sometimes professionals are befuddled,
> not because of some flaw in their paradigm, but simply because the best
> solution hasn't presented itself.
I'm not as confused as you might think, my man.
I'm not presenting my own theory here, just an argument as it is presented by
BADD cladists. According to them, since the cladograms say birds are dinosaur
descendants, they must have evolved flight from the ground upward, because
the earliest known dinosaurs were cursorial ground-dwellers. The cladists
generally leave the details of just how this might have happened to others
whose interests lie more in that direction. This strikes me as rather
arrogant, but that's how some cladists are these days.
The paleornithologists scoff at the idea of avian flight evolving from the
ground upward (rightly, in my opinion), which in turn leads them to scoff at
the cladistics (wrongly, in my opinion).
> Also, I don't believe that BCF explains how these little
>"lizardlike" arboreal animals began incorporating more vertebrae
>into their sacra and fusing them together. The animals George
>describes would not need this kind of extra support until they
>returned to a terrestrial, cursorial niche. Yet, all birds show
>these adaptations... Maybe the BCF birds are secondarily flighted?
The earliest ground-dwelling dinosaurs had only 2 or 3 sacral vertebrae
(prosauropods weighing tons generally had 3), the primitive number for all
Dinosauria, so an increased theropod sacral count to, say, 4 or 5 may have
had little to do with cursoriality or body support at first. As Gatesy has
pointed out, an increase in the sacral count could be related to the changes
in the way the caudal and upper hind limb musculature worked, perhaps for
balance during perching. In small dino-birds, the caudal musculature
certainly became exapted for controlling the tail airfoil, and one can
account for an increased sacral count and elongate ilium on this basis. The
long ilium and increased sacral count of later theropods (ceratosaurs up)
were well suited for further evolution and exaptation into a cursorial
lifestyle, as you note: Another item making the transition from tree-dwelling
to ground-dwelling easy for certain dino-bird descendants.
In later birds, the sacral count increased drastically with the shortening of
the tail, even in small Mesozoic birds that weighed but a few ounces. Thus,
considered by itself, a sacral-count increase is not necessarily an
exclusively cursorial adaptation, and may have very little to do with
supporting a ground-dwelling animal.