RE: no marine dinos/no viviparous dinos.

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

On Fri, 4 Jan 2002, Tyrberg Tommy wrote:

> Since viviparity has evolved a large number of times and in essentially
> every major vertebrate group except birds...

...and crocs, providing extant phylogenetic bracket which, then, includes
dinosaurs...

> Two main explanations have
> been suggested:

I beg for a third to be considered and would value your opinion: embryonic
rates/microenvironmental optima are so demanding for archosaurs that
parental manipulation 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)...this 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; nor could it evolve in small archosaurs
(birds) perhaps for the reasons you suggest, but also because of flight
requirements, i.e., baby-on-board puts flighted animals at competitive
disadvantage (general absence of bats in day niche supports this--though I
know there are other hypotheses).
Thanks.