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Re: Swimmin' dinos, fish milk, avian polyphyly (was RE: Sheesh)
On 10/25/06, Thomas R. Holtz, Jr. <email@example.com> wrote:
2) Mechanical constraints. The limbs of most dinosaurs were pretty tightly
locked into parasagittal motions: great for striding, but
not terribly good for swimming. Additionally, the major clades had mechanisms
that tended to tighten up the flexibility of the
dorsal column (e.g., epaxial ossified tendons or hyposphene-hypantrum
articulations): again, great for striding, not so good for
--In this case, the reason that advanced birds might have been able to
colonize the seas is that they were able to exapt their
already-transformed fore- and hindlimb orientations, bypassing as it were the
phases that a stegosaur or compsognathid would have to
There's also the character that the clade with marine lineages,
Ornithurae, is actually named for: extremely short tails. Perhaps long
tails would have gotten in the way of parasagittal kicking. Then
again, this would fail to explain why enantiornitheans (except for
long-pygostyled forms) and pterodactyloids (whose limbs were perhaps
not as parasagitally-oriented as those of dinosaurs, anyway) never
evolved aquatic lineages.
I like the ecological constraint idea, at least for explaining why
other avialans never went aquatic. Ornithurans (sensu stricto) are
supposed to have had better adaptations for long-distance flying, no?
Are there any basal hesperornitheans that could have been flying
I suppose it's also possible that there's some undetected difference
in the feathers of ornithurans vs. non-ornithuran avifiloplumosans,
something about ornithuran feathers that makes them more waterproof.
Of course, that wouldn't help in explaing the lack of marine lineages
in non-avifiloplumosan pan-avians....
T. Michael Keesey
The Dinosauricon: http://dino.lm.com
Parry & Carney: http://parryandcarney.com