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



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