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Re: Mesozoic biosphere depth

>Has it been possible to determine how "deep" the
biosphere was during the Mesozoic, that is, how far
under the earth's surface it "penetrated?"<  

It might have been a little deeper or shallower.  More likely shallower, if
geothermal grdients were steeper.  These really deep microbes are just
that--microbes.  They aren't higher life forms, and have probably been
around since far back in the Precambrian.

>If so, was it substantially different than the present
earth biosphere?  And if so, what impact would this
have had on the animals occupying the surface?<

I would expect the "deep (subsurface) biosphere" to have had about the same
impact then as it does now.  Like, we hardly know it's there.