Re: Swimmin' dinos, fish milk, avian polyphyly (was RE: Sheesh)

["T. Michael Keesey" <keesey@gmail.com>]

On 10/25/06, Thomas R. Holtz, Jr. <tholtz@geol.umd.edu> 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
> undulatory locomotion.
>         --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
> go through.

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
pelagic feeders?

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