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Re: Sheesh



On 10/24/06, Andreas Johansson <andreasj@gmail.com> wrote:
On 10/24/06, Mike Taylor <mike@indexdata.com> wrote:
> ROBERT NICHOLLS writes:
>  > ...humm, well, I've just finished reading "The Ancestor's Tale" by
>  > Richard Dawkins and in it he asked "why were there no
>  > dinosaur-moles?"  I would be very interested to hear some views
>  > from the DML on this...
>
> ... presumably meaning no fossorial dinosaurs.
>
> More suprising still, to me, is the absence of secondarily aquatic
> dinosaurs.  So far as we can tell, they didn't make it back to the sea
> once in 150 million years,

One subclade has done so repeatedly....

> whereas we mammals have done it repeatedly
> in the last 65 million.  Why would that be?

Just a reflection; the sirenians nonewithstanding, most secondarily
aquatic amniotes I can think of have been carnivores. Carnivorous
dinosaurs were pretty much all bipedal. Now, birds excepted, most
secondarily aquatic amniotes were, I believe, quadrupeds before
returning to water.

Good points. Another question to ask: why is it that only ornithurans (sensu stricto), out of all pan-avians/avemetatarsalians*, were able to return to the water (and repeatedly so)? Other archosaurs (pan-crocodylians/crurotarsans) spawned many aquatic and/or amphibious lineages (including all extant forms).

* Remember, there are no aquatic pterosauromorphs or non-dinosaurian
dinosauromorphs either. And it isn't "birds" (whichever sense) that
are the only ones to have become aquatic; it's specifically (as far as
we know) ornithurans (node-based sense: hesperornitheans + carinates
sensu stricto).
--
T. Michael Keesey
The Dinosauricon: http://dino.lm.com
Parry & Carney: http://parryandcarney.com