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Re: live birth



Date:          Wed, 8 Mar 1995 09:04:43 -0500
Reply-to:      bill.adlam@st-peters.oxford.ac.uk
>From:          Bill Adlam <bill.adlam@st-peters.oxford.ac.uk>
To:            Multiple recipients of list <dinosaur@lepomis.psych.upenn.edu>
Subject:       Re: live birth

On Wed, 8 Mar 1995 AnmlPeople@aol.com wrote:

> Since apparently no sauropod eggs have been found,  is it reasonable to
> postulate that perhaps sauropods gave live birth?
And Bill Adlam continued:
>This has been proposed.  I find it hard to imagine it evolving: it would be
necessary to go from big, calcified air-breathing eggs to much smaller ones
before ovoviviparity and then viviparity could be possible.  Only then could
large size at birth re-evolve.

I know its hard to imagine, but in some ways I find the reverse even 
harder.  Firstly, its hard to say that no sauropod eggs have been 
found - 'identified' is a better word. Unless one of the eggs in your 
clutch is hatching, or contains identifiable chick remains (as 
opposed to embryonic tissue), you have no firm case for claiming a 
genus for your eggs.  It is very hard to say that egg X comes from dinosaur Y, 
simply because the same horizon, close 
at hand, contains remains (however abundant) of dinosaur Y.  Witness 
the problems over the parenthood of Roy Chapman Andrews Gobi Desert 
eggs. So we MAY have sauropod eggs.   Some choose to allocate the largest of 
eggs to sauropods, on 
the basis that 'they were the biggest dinosaurs, so must have had the 
biggest eggs' - a maxim that is conspicuously absent from the rest of 
the animal kingdom.

But what I have problems with, regarding the sauropods, and even 
allowing that they may have laid the largest eggs that we currently 
know about, is the sheer practical problems of being a sauropod 
parent and NOT squishing your offspring that you have just expended 
so much genetic energy producing.  Their eyesight would need to be 
exceptionally good, given the size of the newly-hatched, to avoid 
this as a very real problem. [Anybody got anything re endocasts of 
sauropod optic lobes?]  

So, you could say that maybe the sauropods just laid very big eggs, 
producing large offspring that were above the 'Critical Size 
Threshold'.  We just haven't found any trace of them - yet.  It is 
conceivable, that, unlike Jack Horner's fauna, sauropods did not 
choose to lay their eggs in floodplain/ usefully fossilising terrain. 
 But, hold on, there are problems with that line of thinking.  You 
cannot simply enlarge eggs until they are a large enough size for 
this argument.  There are structural limitations - the larger the 
egg, the thicker the eggshell needs to be for the structure to be 
competent.  And the thicker the eggshell needs to be, the more it 
becomes impossible for the embryo to breath.  The eggshells in our 
13.5 cm 'smartie' shaped eggs at the Hunterian are already close to 
their maximum thickness without endangering the embryo with 
asphyxiation.

Its a bit of a long way around, but it is what encourages me to 
seriously consider live birth as a viable theory.

Hope this has helped.  Anybody else?

J.J.Liston
Hunterian Museum
University of Glasgow
SCOTLAND