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

[Tim Williams <twilliams_alpha@hotmail.com>]

Tom Holtz wrote:

> 2) Mechanical constraints. [snip]	--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.

This is my favorite, and it was examined both quantitatively and 
qualitatively by Gatesy and Middleton (1997).  Here's an excerpt...

"Basal theropods, which were terrestrial bipeds, had a single locomotor 
module composed of the hind limb and tail. In birds, aerial locomotion was 
acquired with the origination of the wing module and a decoupling of the 
hind limb and tail into separate pelvic and caudal modules. This increase in 
modularity is thought to have granted birds more locomotor "options" than 
non-avian theropods. More specifically, an aerial locomotor system could 
have eased constraints on the hind limb and allowed specialization for 
habitats and lifestyles unavailable to non-birds."

Although the study was criticized on statistical grounds (Carrano and Sidor, 
1999), it still carries a lot of weight.

Gatesy S. M., K. M. Middleton. (1997). Bipedalism, flight, and the evolution 
of theropod locomotor diversity. J. Vert. Paleontol. 17: 308?329.

Carrano M. T., C. A. Sidor. (1999). Theropod hind limb disparity revisited: 
Comments on Gatesy and Middleton (1997). J. Vert. Paleontol. 19: 602?605.

> 4) Ecological constraings. Not particularly happy with this one, [snip] 
> Again, the reason that derived birds might have made it as sea dinos is 
> that they were able to bypass the near-shore life habit stage by having a 
> novel method (i.e., diving from above) to make it as pelagic creatures. We 
> already know of other Cretaceous derived avialians (such as _Ichthyornis_) 
> who may have been pelagic feeders.

I'm quite happy with this hypothesis, for certain 'swimming dinosaurs' (but 
not all).  This is just the route by which penguins are thought to have 
evolved - from pelagic divers very similar to auks.  I suspect the same is 
true for plotopterids.

However, this scenario doesn't explain the origin of hesperornithiforms, 
which are foot-propelled divers and so more like loons than penguins.  
Penguins, auks and diving petrels use their wings to propel themselves 
underwater.  Hesperornithiforms lost their wings, which suggests that they 
did not evolve from pelagic divers.  Hesperornithiforms probably evolved in 
a near-shore environment, like loons.  Or, given their association with 
epicontinental seaways, this near-shore ecology could be secondary, and 
hesperornithiforms evolved from pelagic divers after all.  But this sounds 
highly non-parsimonious to me, so it's safe to assume that 
hesperornithiforms began as near-shore foot-propelled divers rather than 
pelagic wing-propelled divers.


Jeff Hecht wrote:

> Basal crocodiliformes were quite similar to basal archosaurs, and some 
> crocs did evolve to live in marine environments, so looking at what 
> happened during their divergence might give some clues.

This is an excellent point, especially considering that basal 
crocodylomorphs ('sphenosuchians') included erect-limbed, cursorial and 
perhaps facultatively bipedal forms.  Aquatic and marine forms did arise 
from this stock, which required a reversal of many basal crocodylomorph 
traits.

Cheers

Tim

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