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