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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 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.



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