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Re: archosaur origins

> Allow me to throw some cold water on this scenario. Practically all known
> terrestrial vertebrates that are adapting to a predominantly aquatic
> lifestyle begin to acquire fishlike characteristics. The body becomes more
> sinuous and streamlined, the tail often acquires a propulsive function, the
> fore and hind feet >shorten< and tend to become webbed or even paddlelike
> (pinniform). Think of otters, seals, alligators and crocodiles, mosasaurs,
> turtles, plesiosaurs, archaeocetes, penguins, and so forth. On this basis I'd
> strongly question any assertion that hind limbs evolved longer and stronger
> as an adaptation to an aquatic lifestyle. Au contraire: if anything, I'd say
> the long and strong hind limbs came first and, if found in an aquatic animal,
> were in the process of evolving into shorter and less intrusive swimming
> appendages--as in the scenario described below:

I fully agree that hindlimbs tend to become smaller and more paddle 
like in aquatic animals, but could it be possible that strong 
hind-limbs are an interesting anatomical design for quick propulsion 
in very shallow, horsetail choken near shore water? Perhaps it is a 
bit out-of-topic or irrelevant to mention, but recent studies of the 
very early tetrapod Acanthostega show this animal had strong hind 
limbs and rather feeble fore limbs (in contrast with its 
osteolepiform and panderichthyd forerunners where the pectoral fin is 
more strongly developed); the same studies strongly favor a very 
shallow water environment for these animals (See Coates and Clack's 
1990 Nature paper (vol347) and Per Ahlberg's review about early 
tetrapods in Nature 1994). Per Ahlberg even suggests that all 
tetrapods are essential principally "rear-wheel driven" animals (i.e. 
a strong hindlimb is perhaps a primitive tetrapod feature, that was 
acquired in the very beginning of tetrapod evolution, in the extensive shallow 
lagoons and deltas where lay their cradle.)
But perhaps this has little to do with proterosuchians.

>  Second, first archosaurs were agile terrestrial small carnivores such 
>  as the euparkeriids, which descended from tiny "eusuchians" as Robert 
> Carroll's Heleosuchus and Heleosaurus, which 
>  could already atteign some degree of bipedalism the large semi-aquatic 
> Proterosuchia were an offshoot that
> secondarily  invaded the water. >>

Am I correct in assuming that this is the "scenario" now largely accepted?

Pieter Depuydt
Department of Respiratory Diseases
University of Ghent