mosasaur babies/no marine dinos

[John Bois <jbois@umd5.umd.edu>]

> Viviparity in early medium-sized amphibious
> aigialosaurs may have freed them from the need to return
> to land to deposit eggs, and permitted the subsequent
> evolution of gigantic totally marine mosasaurs.

Couldn't agree more with Caldwell, Michael W.& Michael S. Y. Lee and thanks
to Ben Creisler for the refs.  I would like ask the question this inference
suggests: why no archosaur marine creatures?  Specifically, if mosasaurs
could develop vivparity in the sea (in response, perhaps, to selective
pressure in the form of predation, or even interspecific competition for
predator-less nest sites), why was this not possible for archosaurs?  And,
did their inability to become viviparous--if true--contribute to their being
denied this body plan/niche.

Assumption: Dinosaurs were endothermic.  This is a more expensive metabolic
mode
and requires a more precise/demanding incubation regime or period than that
of ectothermic
reptiles.  Indeed, this lifestyle _requires_ parental investment-to provide
greater access to oxygen (i.e., eggs could not be buried and forgotten), and
to hold temperature closer to the optimum (e.g., by shading, maintaining a
vegetation mound, active brooding).  Slow metabolic rates of lizard babies
probably required less oxygen...and could handle the slow delivery rate in
placenta-less lizard mommy's tummy.

Hypothesis:  Dinosaurs could not be fully aquatic because their embryos
require more oxygen than is available in aqueous environment without a
delivery system (e.g., a
placenta).  There is more oxygen in air than water.  Penguins demonstrate
the effectiveness of the body plan as a fishing machine.  But, the oxygen
requirement means that eggs must be placed on land.  This limits
size-inasmuch as birds designed for swimming are clumsy on land (imagine an
elephant seal incubating an egg).  So, ectotherms and placentals are the
only large size animals to rule the waves
To summarize: archosaurs are very efficient fish hunters but are
size-limited in the marine environment.  This is due to a) high oxygen
demand for developing egg/embryo; b) inability to incubate a land egg with a
fish body plan.

A related phenomenon is the distribution of penguins.  Given their great
fishing ability, they should be competitive in all parts of the world.  But
their distribution is primarily southern.  I believe nest sites are the
limiting factor here; that is, nest sites and predator abundance--which
amounts to the same thing--nest sites without predators are much more common
in the Southern H. than the Northern.
Comments welcome.