RE: no marine dinos/no viviparous dinos.

[Michael Habib <mbh3q@cms.mail.virginia.edu>]

> I beg for a third to be considered and would value your opinion:
embryonic
> rates/microenvironmental optima are so demanding for archosaurs that
> parental manipu lation is required (in the form of shading, clearing
CO2,
> brooding, and maintaining access to oxygen); that the only
viviparous
> creatures that have comparable embryonic rates are mammals--and they
have
> a supercharged delivery system (placenta)

While all known archosaurs seem to be (and have been) oviparous, they
do not all share a demanding metabolism.  Crocodilians, for example,
have low metabolic rate (in comparison to a mammal or bird).  At the
same time, some organisms with placental structures also have low
metabolic rates (some sharks and squamates).  In addition, parental
manipulation does occur in viviparous (and ovoviviparous) squamates.

> ...thi s system evolved under feet
> of dinos, in very small/rapidly reproducing (evolving)
organisms...this
> revolutionary organ could not evolve in large archosaurs due to
> (possibly) long generation times; 

Rapidly reproducing organisms do not automatically carry a faster rate
of change.  Selective pressure and variability, among other factors,
have to be taken into account.  Do lineages that have independently
developed viviparity tend to share a high reproductive rate?  If not,
I would be cautious in using estimates of generation time as
constraints on trait development, unless the generation times are
known to be vastly different, regardless of the margin of error.  (For
example, comparing a bacterium with a mammal.  Regardless of error
margin, the former can be reasonably assumed to have a vastly higher
reproductive rate under ideal conditions).

--Mike Habib
habib@virginia.edu