Re: Tiktaalik

[Tim Williams <twilliams_alpha@hotmail.com>]

Mike Keesey wrote:

> > This experimentation lasts until an evolutionary "tipping point" is 
> reached,
> > when the body plan becomes committed to a given ecomorphology 
> (locomotory
> > style, in these cases), and there's no way back.
>
> Plesiosaurs, placodonts, icthyosaurs, spheniscids, choristoderans,
> mosasauroids, chelonids, cetaceans, pinnipeds, sirenians ...

Haha!  Therein lies the rub.  Although all of these groups returned to the 
water, none of them reverted back to the exact morphology of a fish, or even 
a basal sarcopterygian.  Too much has happened for that, in terms of 
morphological change within these lineages.

> Or, for avialans: patagopterygids, phorusrhachids, diatrymatids, ratites 
> ...

Again, although these birds are all flightless and cursorial, they never 
returned to the body plan of a non-ornithothoracine theropod.  These birds 
all retained the ornithothoracine body plan (including the peculiar method 
of stride generation), and adapted it for cursoriality.  The femur remains 
in its typically ornithothoracine orientation (near-horizontal, when walking 
or standing) and the tail remains
absurdly short, as the typically ornithoracine pygostyle.

Ever since birds threw their lot with the ornithothoracine body plan, 
they've been stuck with it ever since.  This body plan is superb for powered 
aerial flight, but is not exactly optimal for rapid bipedal cursorial 
locomotion (though most ratites manage okay).  No ornithothoracine bird has 
re-evolved a long tail or the primitive dinosaurian method of stride 
generation (executed at the hip, not the knee).  They're all "boxed in" to a 
post-_Archaeopteryx_ body plan, so cursorial flightless birds re-invented 
the ornithothoracine body plan, rather than revert to the primitive 
cursorial theropod body plan.

> Better examples would be pterosaurs and bats, where there really
> wasn't any locomotory reversion  (that we know of, anyway).

I see locomotory reversion as more than just land-to-air (or vice versa) or 
water-to-land (or vice versa).  I'm referring more to the exact body plans 
and strategies involved.  Such "reversions" are usually "re-inventions" 
rather than strict reversions, since these animals cannot simply un-do the 
changes that have occurred over many millions of years.  So, while it is 
true that ichthyosaurs and cetaceans have adopted a superficially fish-like 
body, insofar as the body plan is hydrodynamic, neither group can breathe 
underwater, which imposes constraints not present in most fishes.  Many 
extinct sea reptiles bred out of water.  I've heard whales must leap out of 
the water in order for sperm exchange to occur.

Cheers

Tim